Double tap transformer?

[JWallstrop]

Across the pond in the US we allow utility-owned transformers to be tapped up to six times but an identically set up customer-owned transformer to only be tapped once unless following the feeder tap rules, see: Feeder and Secondary Tap Rules (mikeholt.com)

Tapping of the secondary on a customer-owned transformer is governed by NEC 240.21 (C) Location in Circuit | UpCodes
Tapping of the secondary on a utility-owned transformer is governed by NEC 230.71 Chapter 2: Wiring and Protection, National Electrical Code 2020 | UpCodes

I'm curious how it's done in the UK?

 

[Julie.]

Totally differently.

If you are in a major residential area, then large transformers feed out at 400v three phase +n , and each property is usualy single phase connected off this feeder (could be 5-8 feeders out from a sub), the number of allowed properties is dependent upon the transformer size, and the local utility's guidelines, often these feeders loop all the way around to another substation and another transformer, however the "ring" would be run open at some point.

When it's more remote then the transformer would be sized for the number of properties within easy reach, this would also supply local street lighting etc just the same as the above example.

Different utilities have slightly different policies, but it's generally in line with the above.

Large industrial customers may take in at MV (6.6 -33kV) and have their own transformers, whatever is downstream of the connection point is up to them, however there is usually operational restrictions such that they cannot interconnect multiple connection points (neither directly or via the LV network).

Often for medium sized customers the utility will provide a dedicated transformer, or supply at MV and the customer provides their own transformer(s), again downstream of the transformer is up to the customer.

Bit of a simplistic overview.

Remember, we supply everything at 230V single phase or 400V three phase, so the current is substantially lower than at 120V

(Other than MV connections)

Forgot to add, each connection point to a utility has an agreed maximum demand, so the utility may supply a 1MVA transformer, but yet only agree to a 500kVA MD, the restriction may be due to network issues within the utility network, but use an oversize transformer for future proofing.


This is a typical 5 way feed open pillar used in a substation

 

[pc1966]

Interesting to read the USA details, though I'm not sure I grasp all of the details.

Looking at references to "Taps Not Over 3 m (10 ft) Long" and similar it looks a bit like our regulations for conductors not in themselves protected at source for over-current.

The closest I can think of in my own limited experience are the likes of a bus-bar chamber where you can have multiple 'taps' taken off to fused-switches that in turn feed some device or DB elsewhere. The UK regulations allow this form of protection at the far end of the reduced-section conductor provided the length of the vulnerable conductor is less than 3m and there are additional measures to avoid damage / short circuit (such as enclosed in the bus bar chamber).

So if you have, for example, an 800A supply you can run off 25mm (approx #3 AWG) conductors to a 100A fused-switch or similar that protects it from down-stream faults (such as the outgoing cable or DB end short, or even overload from too much DB load where the sum of breakers exceeds the supply capacity).

 

[pc1966]

I also get the impression that the UK likes HRC fuses more than the USA. I personally like fuses, reliable and very good at limiting fault energy which can help reduce just how bad your day can get.

In the picture posted by @Julie. you can see 4 sets of 3 fuses for the 4 of the possible 5 feeds from that pillar that I guess were planned for use. However, the idea of removing or inserting such fuses live, as the DNO folks do, gives me the willies!

 

[Julie.]

I also get the impression that the UK likes HRC fuses more than the USA. I personally like fuses, reliable and very good at limiting fault energy which can help reduce just how bad your day can get.

In the picture posted by @Julie. you can see 4 sets of 3 fuses for the 4 of the possible 5 feeds from that pillar that I guess were planned for use. However, the idea of removing or inserting such fuses live, as the DNO folks do, gives me the willies!

Yeah, I think quite often they distribute to customers with no fuses at all within the LV dist network.

 

[Julie.]

Totally differently.

If you are in a major residential area, then large transformers feed out at 400v three phase +n , and each property is usualy single phase connected off this feeder (could be 5-8 feeders out from a sub), the number of allowed properties is dependent upon the transformer size, and the local utility's guidelines, often these feeders loop all the way around to another substation and another transformer, however the "ring" would be run open at some point.

When it's more remote then the transformer would be sized for the number of properties within easy reach, this would also supply local street lighting etc just the same as the above example.

Different utilities have slightly different policies, but it's generally in line with the above.

Large industrial customers may take in at MV (6.6 -33kV) and have their own transformers, whatever is downstream of the connection point is up to them, however there is usually operational restrictions such that they cannot interconnect multiple connection points (neither directly or via the LV network).

Often for medium sized customers the utility will provide a dedicated transformer, or supply at MV and the customer provides their own transformer(s), again downstream of the transformer is up to the customer.

Bit of a simplistic overview.

Remember, we supply everything at 230V single phase or 400V three phase, so the current is substantially lower than at 120V

(Other than MV connections)

Forgot to add, each connection point to a utility has an agreed maximum demand, so the utility may supply a 1MVA transformer, but yet only agree to a 500kVA MD, the restriction may be due to network issues within the utility network, but use an oversize transformer for future proofing.


This is a typical 5 way feed open pillar used in a substation

View attachment 93738

Replying to myself???

This is an example LV network showing the interconnections etc and general dist cabling - obviously only a small section - the big arrows are the NO points on the "ring"; square boxes are link chambers; most cable sizes are in sqinches but some are metric eg 185mm^2



If you look at Crosby st for example, the sub has two pillars (8w) with 7 used on each.

 

[Julie.]

Interesting to read the USA details, though I'm not sure I grasp all of the details.

Looking at references to "Taps Not Over 3 m (10 ft) Long" and similar it looks a bit like our regulations for conductors not in themselves protected at source for over-current.

The closest I can think of in my own limited experience are the likes of a bus-bar chamber where you can have multiple 'taps' taken off to fused-switches that in turn feed some device or DB elsewhere. The UK regulations allow this form of protection at the far end of the reduced-section conductor provided the length of the vulnerable conductor is less than 3m and there are additional measures to avoid damage / short circuit (such as enclosed in the bus bar chamber).

So if you have, for example, an 800A supply you can run off 25mm (approx #3 AWG) conductors to a 100A fused-switch or similar that protects it from down-stream faults (such as the outgoing cable or DB end short, or even overload from too much DB load where the sum of breakers exceeds the supply capacity).

Yeah, I don't really understand what is meant by "tap"; given the description I took it to mean supply to customers off the main supply from a transformer.

Beyond that the only similar thing is as you say where you tap off a busbar for smaller loads where the conductors are undersized for the upstream overcurrent device. But as far as I know there isn't any limit to numbers, just distance and additional measures.

The vaguely similar thing is where you reduce the conductor size off say a radial or rfc to a fixed load and/or fused load where the overload protection is at the remote end (fault protection still provided by the upstream "oversized" device)

The reality is non of these appear to line up with the implied description of a "tap".

 

[JWallstrop]

I should clarify as it really is a two part question.

Part I - Tapping a secondary tap conductor off a transformer
This first setup shown below is not Kosher in the US if the transformer is customer owned.




Figure 3. Multiple taps from a single transformer secondary must originate from the transformer.
Customer owned transformer, this is not allowed. We don't allow a tap conductor to be tapped. Oh, wait, unless....

if it's a utility owned transformer like in the second and third picture it's Kosher to tap a tap conductor coming off the transformer for up to 6 disconnects (setup B in the third picture is the same as the setup in first picture except the transformer is utility owned).






Utility owned transformer, now we allow up to six disconnects tapping the tap conductor.


Curious as to whether the UK has the same constraints and distinction (which I found somewhat arbitrary and stems from the fact that utilities in the US are governed by a different code than non-utilities).




Part II

We do allow tapping the transformer directly (of course!). Those taps have to follow the "secondary tap rules" listed below, and I am curious what the UK equivalent is there. I can't seem to access the BST standards or the IEC standards without paying through the nose and since this is just curiosity, I don't think I want to do that, but perhaps a kind soul can enlighten me:

​

(C) Transformer Secondary Conductors​

A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). Section 240.4(B) shall not be permitted for transformer secondary conductors.
Informational Note: For overcurrent protection requirements for transformers, see 450.3.

(1) Protection by Primary Overcurrent Device​

Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

(2) Transformer Secondary Conductors Not Over 3 m (10 ft) Long​

If the length of secondary conductor does not exceed 3 m (10 ft) and complies with all of the following:

1. The ampacity of the secondary conductors is
   1. Not less than the combined calculated loads on the circuits supplied by the secondary conductors, and
   2. Not less than the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the termination of the secondary conductors.
   3. Exception: Where listed equipment, such as a surge protective device(s) [SPD(s)], is provided with specific instructions on minimum conductor sizing, the ampacity of the tap conductors supplying that equipment shall be permitted to be determined based on the manufacturer's instructions.
2. The secondary conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.
3. The secondary conductors are enclosed in a raceway, which shall extend from the transformer to the enclosure of an enclosed switchboard, switchgear, a panelboard, or control devices or to the back of an open switchboard.
4. For field installations where the secondary conductors leave the enclosure or vault in which the supply connection is made, the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary to secondary transformer voltage ratio, shall not exceed 10 times the ampacity of the secondary conductor.

Informational Note: For overcurrent protection requirements for panelboards, see 408.36.

(3) Industrial Installation Secondary Conductors Not Over 7.5 m (25 ft) Long​

For the supply of switchgear or switchboards in industrial installations only, where the length of the secondary conductors does not exceed 7.5 m (25 ft) and complies with all of the following:

1. Conditions of maintenance and supervision ensure that only qualified persons service the systems.
2. The ampacity of the secondary conductors is not less than the secondary current rating of the transformer, and the sum of the ratings of the overcurrent devices does not exceed the ampacity of the secondary conductors.
3. All overcurrent devices are grouped.
4. The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

(4) Outside Secondary Conductors​

Where the conductors are located outside of a building or structure, except at the point of load termination, and comply with all of the following conditions:

1. The conductors are protected from physical damage in an approved manner.
2. The conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors. This single overcurrent device shall be permitted to supply any number of additional overcurrent devices on its load side.
3. The overcurrent device for the conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto.
4. The disconnecting means for the conductors is installed at a readily accessible location complying with one of the following:
   1. Outside of a building or structure
   2. Inside, nearest the point of entrance of the conductors
   3. Where installed in accordance with 230.6, nearest the point of entrance of the conductors

(5) Secondary Conductors From a Feeder Tapped Transformer​

Transformer secondary conductors installed in accordance with 240.21(B)(3) shall be permitted to have overcurrent protection as specified in that section.
(6) Secondary Conductors Not Over 7.5 m (25 ft) Long

Where the length of secondary conductor does not exceed 7.5 m (25 ft) and complies with all of the following:

1. The secondary conductors shall have an ampacity that is not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device protecting the primary of the transformer.
2. The secondary conductors terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity that is permitted by 310.14.
3. The secondary conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means.

Feeder and Secondary Tap Rules

 

[Julie.]

The method of connection on the secondary of a transformer is irrelevant in the uk, hence why we can't understand the "tap" nomenclature.

From a protection point of view the protection on the primary of a transformer must protect through beyond any protection on the secondary.

Normally this could just be a set of fuses at MV (or proper protection relays etc) , any fault on the secondary side must be sufficient to operate the primary fuses/relay , this naturally limits how long the secondary conductors can be before fuses or other devices are fitted.

This applies for both utility owned or privately owned transformers, typically for a utility owned transformer the primary protection would therefore cover through to the pillar fuses as per the photo I posted yesterday.

Privately open pillars are no longer used and the transformer usually feeds into an enclosed busbar system with fuses or mccb.

Each utility has their own guidelines, basically a 500kVA sub would follow a standard arrangement which complies with the above philosophy, their design may indicate that for MV fault levels of say 100MVA and above, the fuseswitch shall be x, transformer y, maximum length to pillar z metres etc.

Privately each sub is designed specifically for the application (although we all tend to re-use common and proven previous designs)

The situation is very unlike the manner used in the US.

 

[pc1966]

if it's a utility owned transformer like in the second and third picture it's Kosher to tap a tap conductor coming off the transformer for up to 6 disconnects (setup B in the third picture is the same as the setup in first picture except the transformer is utility owned).

View attachment 93777

I don't really know anything about the HV/MV supply side of things, but here in the UK the first example "code violation" I presume is due to the lack of any means of isolation. That would also go against the UK regulations.

However, there is no limit to the number of "disconnects" in our case as the DNO (supply) is already fused against overload, so providing your intended load is not above the supply then it is up to you how it is distributed.

However, in the UK case once you are inside a LV installation the normal BS 7671 wiring regulations apply and every sub-main supplied from the incoming supply has to meet a 5s max disconnection time on a fault. Typically you are not allowed to assume the DNO fuse will do that, and it is possible it won't due to the supply fault loop impedance being too high, so our systems would normally have fused-switches feeding the sub-mains (or sometimes MCCB).

The DNO fuse might be OK, and you can get agreement from the DNO to do so, but it is normally easier to do it yourself by designing in the over-current protection!

As I mentioned above, we do have a "3m rule" for conductors that have reduced in size and so are no longer protected at the supply end against over current, but are protected at the load end and have enhanced protection against damage/faults.

 

[Julie.]

The point is really, the two systems are quite different, we don't "tap" off in the same way, ordinarily the utility supplies a single connection point for each customer that is fused.

There is a limit of 3m cable length from this point before which a fuse/mccb must be fitted by the customer, however the customer may split this as many ways as they want, one for the main house, one for a ev charge point, one for a garage, one for a solar panel set up etc (although tbh most properties just have one consumer unit for the whole property).

There are occasions where the utility supplies to one "customer" who actually operates as a small utility within the building(s) , but again the same arrangement as above is undertaken, we have special boards designed to distribute to sub-customers in this sort of arrangement, but it is no more than a formal implementation of the above (each sub-customer in this case having a metered fused supply).

Electricians in the uk generally do not make the connections to utility transformers, or their secondary cabling , all this side of thing is done by the utility.

 

[pc1966]

Here is an example of the distribution board that you might find in a UK block of flats to split up something like a 200A or more 3P supply:

15 Way Ryefield Board taking 60A Fuse Units, 15 Way TPN

Sparks Electrical Wholesalers sells a wide range of electrical items including 15 Way Ryefield Board taking 60A Fuse Units, 15 Way TPN Fuseboard.

www.sparksdirect.co.uk

The billing meters would usually be in the flat, and the flat's "consumer unit" (CU = panel = distribution board) would have a double-pole isolation switch feeding the RCD/MCB for the final circuits.

Most UK flats would only have a 60A 230V single-supply, as typically the max demand would be something like a 40A electric shower. However, medium size houses might be on 100A single phase, or occasionally 3-phase for either large properties or where there is something like a rapid EV charger that demands it.

In fact for domestic use it is practically unheard of to have something using 3-phase (other than fast EV chargers, or maybe coming soon big air/ground source heat pumps). In a large domestic property with a 3P supply you would probable have several single-phase sub-boards in different locations, each distributing one phase to a floor or two.

 

[JWallstrop]

Ok so in the UK the utility provides a utility owned overcurrent protection device on the secondary and then of course you can connect that to as many panels as needed.

But I’m thinking of larger commercial and industrial sites where you receive service at higher voltages and the customer supplies a transformer or transformers to step down. For example a university campus or an industrial site.

For arguments sake say utility provides medium voltage protected with a fuse but then you have an industrial customer owned step down xformer that steps down to 3 ph 240/416V (say ~900 kva xformer) and two adjacent buildings and you wanted to feed 450 kva to each. Could you feed the two different load centers from that single xformer, one load center for each of the two buildings and what are the associated UK “tap” rules given that the xformer is customer owned?

Specifically, if UK rules require you to first have a single tap connected to a single load center off the customer xformer you’d need a 3 ph ~1250A master breaker or fusible disconnect first to the first load center and THEN connect two load centers, one for each building from there.

Which would often be much more expensive than a pair of feeds directly off the xformer each going to a load center at each building, with each load center having a ~625A master breaker (since the cost per amp skyrockets above ~400A breakers, plus there are more enclosures in the former case vs the latter we have the large difference in economics).

 

[Julie.]

Ok so in the UK the utility provides a utility owned overcurrent protection device on the secondary and then of course you can connect that to as many panels as needed.

But I’m thinking of larger commercial and industrial sites where you receive service at higher voltages and the customer supplies a transformer or transformers to step down. For example a university campus or an industrial site.

For arguments sake say utility provides medium voltage protected with a fuse but then you have an industrial customer owned step down xformer that steps down to 3 ph 240/416V (say ~900 kva xformer) and two adjacent buildings and you wanted to feed 450 kva to each. Could you feed the two different load centers from that single xformer, one load center for each of the two buildings and what are the associated UK “tap” rules given that the xformer is customer owned?

Specifically, if UK rules require you to first have a single tap connected to a single load center off the customer xformer you’d need a 3 ph ~1250A master breaker or fusible disconnect first to the first load center and THEN connect two load centers, one for each building from there.

Which would often be much more expensive than a pair of feeds directly off the xformer each going to a load center at each building, with each load center having a ~625A master breaker (since the cost per amp skyrockets above ~400A breakers, plus there are more enclosures in the former case vs the latter we have the large difference in economics).

Depends on the arrangements, but there are a number of ways.

For simple applications you could bring the secondary onto the LV busbar, which would then have multiple feeders out, each rated for their corresponding loads , could be fuse switches, mccb, acb or whatever.

If you are dealing with critical applications you would often bring the secondary through an Acb to busbar, then as above, usually though in this case you would have a second transformer feeding into the other end of the busbar via an Acb, and often another acb midway along the busbar.

This latter case you would feed say 4 different buildings off the lh bus and 4 off the rh bus. In the event of a transformer failure, you open the transformer feed acb and close the centre bus acb putting all the loads on the one transformer (may require load shedding if the transformer isn't capable of feeding the full load).

We would pretty much never have a long feed from the transformer to the disconnector - you would split in the substation to then go to all buildings, or you would build the sub in the one building and feed it and the others from there.

Very minimal unprotected length of secondary cables.

 

[pc1966]

This latter case you would feed say 4 different buildings off the lh bus and 4 off the rh bus. In the event of a transformer failure, you open the transformer feed acb and close the centre bus acb putting all the loads on the one transformer (may require load shedding if the transformer isn't capable of feeding the full load).

Do they do something similar with the MV/HV side? I.e. have separate feeds to each transformer, but fed via switches and a crossover-switch so you can have one HV feed used for both transformers in the event some part of the HV grid is out?

 

[Julie.]

Do they do something similar with the MV/HV side? I.e. have separate feeds to each transformer, but fed via switches and a crossover-switch so you can have one HV feed used for both transformers in the event some part of the HV grid is out?

Yes, sort of.

For major sites, the intake is usually from two different MV/HV independent routes.

So even if one grid connection goes there is still supply via the other route.

In this case the intakes are usually spread over the (large) site, and there are interconnections acting like the one board.

On larger sites there may be multiple intakes to the same MV switchboard set up similar to my earlier post, with interconnections as above.

I have worked on many sites that take in at 33kV and above and have an 11kV distribution set up as complex as many utilities (city based), which often includes 11kV motors, generators and so on.

Unfortunately they vary in quality from modern and well maintained, to something scaringly different!!!

 

[JWallstrop]

Depends on the arrangements, but there are a number of ways.

For simple applications you could bring the secondary onto the LV busbar, which would then have multiple feeders out, each rated for their corresponding loads , could be fuse switches, mccb, acb or whatever.
....
We would pretty much never have a long feed from the transformer to the disconnector - you would split in the substation to then go to all buildings, or you would build the sub in the one building and feed it and the others from there.

Very minimal unprotected length of secondary cables.

Ahh see, this is what I am interested in. What are the BST rules on the length of the unprotected length of secondary cable?

 

[Julie.]

Ahh see, this is what I am interested in. What are the BST rules on the length of the unprotected length of secondary cable?

None.

I don't think you quite understand, our rules are not like your rules, (but a bit different,) they follow a quite different philosophy.

You have it spelt out "x length in these circumstances" etc etc.

Our rules are "distribution must disconnect within 5 seconds " etc etc.

Therefore there isn't actually any unprotected cable (although I and others use the term), it really means there isn't "our" protection on the cable.

In the case of a transformer the cable from the secondary to the switchgear bus, and the secondary itself is protected by the primary side protection.

How long therefore - well it must disconnect in 5s in the event of a fault.

So ulike your codes, the length isn't specified, it is determined by the characteristics of the installation.

(Utilities don't necessarily have this rule, although they usually follow the 5s just the same now. They used to have whatever was the limiting time, so if the transformer can withstand a direct short for 15s, but adiabatic calculations on the secondary cable indicate 14s, but then again adiabatic on the primary cable is 10s, then the design limit would be less than 10s)

The only time length is used is where it is actually arbitrary because the installer doesn't have the information.

So for example, a utility will provide a supply to a customer, it will have a fuse (100A, 80A, 60A or rarely 40A) - which size you probably don't know.

The installer then knows that it must go through their own protective device within 3m - could be a simple fuse switch, or could be the actual distribution panel (known as the consumer unit - cu here) with fuses/mcb/rcbo...

This philosophy follows right through our regulations, final circuits don't have a maximum length defined , that comes out of calculations, it must disconnect within x seconds (depends on the type of installation/earthing arrangements), and have less than y voltage drop... etc.

There are guide books that rationalise the above into very conservative guidelines, so on a typical installation, on a 6A type B mcb using 1.5mm^2 cable "z" metres of cable is the maximum length to stay within volt drop and disconnect times etc.

These are not part of the regulations, just shortcuts reducing the need for the calculations.

 

[Julie.]

Just to add - if it's a customer owned transformer, then the utility will provide the information of the MV side, so something like "11kV 107MVA , supplied via a fuse switch having 63A fuses type 12TDLEJ63 or equivalent"

Then you have all the information to design the installation yourself, you just select an appropriate transformer - giving you the % voltage impedance, an appropriate set of cables and switchboard etc and of course the maximum cable length drops out via your calculations, - not suitable? Well start again, relocate transformer or switchboard or change cable size etc.

 

[pc1966]

As @Julie. says our regulations are much more calculation-based in terms of what is allowed. In terms of the LV side then most circuits need overload protection (regulation 433.2), unless fixed loads where that is not likely and some other details (regulation 433.3), but practically all circuits need fault protection (regulation 434).

The closest we have to your rules is the 3m limit on conductors not adequately protected in themselves (but protected down-stream) which typically is taps of a bus-bar chamber and the case mentioned above where the DNO supply is not known to protect the first stage of the installation.

Here is the main bit for 433 on overload:


And here is the main bit for 434 fault protection:




Apologies for the warped photos of the regs book!