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The point about the auto-transformer is it must not be directly on the supply L1-N-L2 at any point as any imbalance will cause a huge current as the transformer attempts to maintain it's winding ratio voltages.

When it is on the generator the source of power is only L1-L2 and the resulting transformer current is partly from that magnetising the core (and losses) but mostly it is from the load imbalance. The load in your house is not capable of such a huge current as your incoming supply, as fundamentally it is limited by the generator output.

However, as you are likely to have a transformer that is 50% or so of the generator output it really requires some protection in case of an overload that results in too much current on the transformer. Here a 3-pole MCB could be used to protect it and the generator, though as you have already realised they often just stall if seriously overloaded and don't deliver anywhere like the prospective fault current that a test would suggest (same as many UPS whose regulation suggests 2kA fault but in reality drop at 50A or so load).

So a 10kVA generator is about 42A at 240V. With a 240V-120V transformer for the centre tap at 5kVA is is the same on the 120V end (5000 / 120 = 41.67). Hence something like a 3-pole (linked operation) 45A MCB would provide reasonable protection for long-term thermal overloads, even if a short is likely to just stall the generator.

In terms of switching, in the UK the N-E link is on the supplier's side and it is prohibited to link N-E in the installation, so here we would have another N-E link in the generator and use a change-over switch that swaps neutrals as well as the line conductor(s). That way the E is always connected, but at any one time there is only on N-E link and it is always before the DB (panel).

But you are using USA style rules and there the N-E link is in the panel. In this case it really does not make sense to switch the N as you will have it linked in any case, and the less messing around with supply lines the better. That is why I later though you would be as well just switching the L1/L2 pair between the incoming supply and the 3-pole MCB, but leaving the N & E there all the time.
Ok, again thank you yet another diagram. What you said made much more sense with that.

1. On the neutral topic:

So with your latest diagram, when on generator power, it is fine that the neutral from the auto-transformer is connected to the supply's neutral?

2. On the generator output breaker topic:

Would it make sense to use a 3-pole RCBO (MCB+RCD) instead of just an MCB to make sure that any current imbalance in any non-RCD/GFI protected circuit in the DB panel will cut the generator output and protect the generator, people, and animals?

Also, what I don't understand is if there's already a built-in breaker in the generator that's fairly small, what is the reason of installing MCB's downstream if the generator breaker will be the very first one to trip in case of a fault anyway?

3. On the change-over switch topic:

So my current change-over switch is composed of 2 x 2-pole 200A MCB's (the line side of one connected to the supply and the line side of the other is connected to the generator output) and have that metal strip that only allows one of those breakers to be turned on. The load side of both is connected to the DB panel main circuit breaker.

With the 45A MCB/RCBO that we talked about for the generator output BEFORE the change-over switch, do I need to replace the 200A MCB on the change-over switch to something smaller?
 
1. On the neutral topic:

So with your latest diagram, when on generator power, it is fine that the neutral from the auto-transformer is connected to the supply's neutral?

Theoretically no issue. But I don't know if the USA code allows it to remain in place. It could be switched as for the L1/L2 but with the N-E link for all cases still at the panel side.

Probably @Megawatt will know what the USA would allow for generator change-over.
2. On the generator output breaker topic:

Would it make sense to use a 3-pole RCBO (MCB+RCD) instead of just an MCB to make sure that any current imbalance in any non-RCD/GFI protected circuit in the DB panel will cut the generator output and protect the generator, people, and animals?

A RCD before the N-E link won't protect anything really, other than a winding to chassis fault in the generator resulting in current outside of the senses set.
Also, what I don't understand is if there's already a built-in breaker in the generator that's fairly small, what is the reason of installing MCB's downstream if the generator breaker will be the very first one to trip in case of a fault anyway?
It is to protect the autotransformer against overload if a large imbalance or fault occurs in the home.

If you put any means of isolation in to the N it must also disconnect all L simultaneously for safety, so here it ensures that any 120V fault that might overload the transformer also isolates all lines so you don't get a floating N and over-voltage on one 120V set.
3. On the change-over switch topic:

So my current change-over switch is composed of 2 x 2-pole 200A MCB's (the line side of one connected to the supply and the line side of the other is connected to the generator output) and have that metal strip that only allows one of those breakers to be turned on. The load side of both is connected to the DB panel main circuit breaker.

With the 45A MCB/RCBO that we talked about for the generator output BEFORE the change-over switch, do I need to replace the 200A MCB on the change-over switch to something smaller?
No, keep the switch as it is. No point in modifying it as it is simpler to add a 3-pole one elsewhere.
 
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Theoretically no issue. But I don't know if the USA code allows it to remain in place. It could be switched as for the L1/L2 but with the N-E link for all cases still at the panel side.

Probably @Megawatt will know what the USA would allow for generator change-over.
@Megawatt , I'd really appreciate if you can chime in. I live in the Philippines but our electrical system follows USA standards so if I can wire up the neutral from both utility supply and generator that way, it would make the modifications I need to so much easier.

A RCD before the N-E link won't protect anything really, other than a winding to chassis fault in the generator resulting in current outside of the senses set.
And we don't necessarily need protection against winding to chassis fault, why? Does it not work like a normal RCD that is installed on the DB panel? It should still detect current imbalances on any of the circuits since it would technically be the protection device that is most upstream in the tree, no?

It is to protect the autotransformer against overload if a large imbalance or fault occurs in the home.

If you put any means of isolation in to the N it must also disconnect all L simultaneously for safety, so here it ensures that any 120V fault that might overload the transformer also isolates all lines so you don't get a floating N and over-voltage on one 120V set.
Ok, got it. That clears that up.

No, keep the switch as it is. No point in modifying it as it is simpler to add a 3-pole one elsewhere.
Ok. As for connecting the generator/transformer neutral to the neutral from the meter, what's the best way to tap the two neutrals together outside of the DB panel? I know I can always tap into the neutral bus bar in the DB panel but it would be harder route-wise.
 
Yes, that's it. I know we've been a bit around the houses - that's partly because we are used to different wiring standards and so we (or at least, I) have tended to work from what we'd do here and adapt.
Also, what I don't understand is if there's already a built-in breaker in the generator that's fairly small, what is the reason of installing MCB's downstream if the generator breaker will be the very first one to trip in case of a fault anyway?
It's only really for a genny winding-frame fault. If you add in the built in breaker to your diagram, you'll see that there's one side of the output that isn't really protected. You could circulate fault currents through the genny "neutral" which is really your L2, the auto-transformer, the neutral-earth connection, and back to the genny frame. The genny breaker may or may not trip - but even if it does, it can't stop the current flow in the genny neutral.
But now I think about it a bit more, and look at your last diagram, if properly sized the 3 pole breaker after the transformer should trip and that will stop the fault current.
With the 45A MCB/RCBO that we talked about for the generator output BEFORE the change-over switch, do I need to replace the 200A MCB on the change-over switch to something smaller?
I wouldn't bother - it just won't trip, and so you are using it just as a switch.
Hmm, I guess US standards are somewhat different to ours. I don't like that design as it's just too easy for someone to remove or bend the bit of metal that's "getting in the way" and close both switches. Also, there's no terminals for your neutral - in answer to you question of how to connect them.
The ideal would be to have a neutral block in your changeover switch housing - then you have 3 wires from meter to switch, 3 wires from transformer to switch, and 3 wires from switch to panel, and thus your wiring is made much simpler.
Over here I don't think that would be accepted as it's just too easy to connect both the genny and mains supply - which means you can back-feed the mains which over the years has led to a number of accidents due to circuits being live when they are supposed to be dead. We'd normally be using a switch where all poles are operated by one actuator - making it impossible to close one switch without the other having opened first. Rotary switches are common, but in single phase DBs, it's possible to replace the two pole isolator with a 4 pole device like this one (we use DIN rail mounted devices as standard over here) which is effectively two switches with a common operating handle, and one of the switches working "upside down" to the other.

As an aside, it's also important to run the 3 wires of each group in close proximity to each other - if you were to (for example) run the neutral wire a different route, then you'd create a single turn of a transformer, creating magnetic fields according to how much current there is in the neutral.

I've seen some interesting effects on old CRT screens when there's been magnetic fields like that ?On one occasion, I got called to a client because all the screens in one part of the office were "wobbling" at certain times of the day. I checked with the electricians, and they'd done some work in the shop below - but a neutral had come adrift in a ring final circuit*. So when the electric curtain fan heater was one, it's 13A was split both ways round the ring for the live, and only one way for the neutral - thus creating a large loop that effectively had something in the order of 6-7A flowing round it. This was enough to interfere with the magnetic scan of the CRT screens. As the screesn were using something near to our 50Hz mains, the result was a visible wobble rather than it going fuzzy as it would do if the wobble was at the full 50Hz.
The other occasion was when they were extending the offices. The steel frame was bolted, but also welded. The builders just clamped the welder earth to the steelwork, and went roound welding the joints. Thus the welding current similarly appeared in a loop. Because of the magnitude of the current, when they struck up and arc, the terminals the other side of the wall just went nuts - the display just shimmered well off both sides of the screen.

* Ring Final Circuit
Created during the second world war as a means of reducing the amount of copper needed, a quantity of sockets are wired in a ring that starts and ends at the distribution board. The sockets themselves are rated for 13A each, the cable we currently use is rated (depending on installation method) at a max of 27A, but it's protected by a 30A fuse or 32A MCB. The idea is that unless you apply a massive load right at one end of the ring, the currents gets split between the two routes between board and sockets, and it's not possible to overload the cable.
The downside is that without testing, it's quite possible for any of the individual wires to break, and no-one will ever know - unless it causes magnetic interference like I described above.
And the "which is better - ring or radial" (I believe US standards are for lots of radials) is one of those arguments that I think will go on for a very long time.
 
@Megawatt , I'd really appreciate if you can chime in. I live in the Philippines but our electrical system follows USA standards so if I can wire up the neutral from both utility supply and generator that way, it would make the modifications I need to so much easier.


And we don't necessarily need protection against winding to chassis fault, why? Does it not work like a normal RCD that is installed on the DB panel? It should still detect current imbalances on any of the circuits since it would technically be the protection device that is most upstream in the tree, no?


Ok, got it. That clears that up.


Ok. As for connecting the generator/transformer neutral to the neutral from the meter, what's the best way to tap the two neutrals together outside of the DB panel? I know I can always tap into the neutral bus bar in the DB panel but it would be harder route-wise.
I just trying to figure out what has previously been talked about. Here’s what I’m understanding is yes you tie the neutrals together from the panel to the generator and don’t break them through your transfer switch. As far as your breakers on your generator do you not have a 30 amp 4 wire plug or just a
 
I just trying to figure out what has previously been talked about. Here’s what I’m understanding is yes you tie the neutrals together from the panel to the generator and don’t break them through your transfer switch. As far as your breakers on your generator do you not have a 30 amp 4 wire plug or just a
Just a 3 wire plug. What are you referring as a auto transformer. I’m just trying to catch up
 
Ok. As for connecting the generator/transformer neutral to the neutral from the meter, what's the best way to tap the two neutrals together outside of the DB panel? I know I can always tap into the neutral bus bar in the DB panel but it would be harder route-wise.
As mentioned above, you really want the L1/L2/N cables to follow the same route. However, a detour of 0.5m or so is not a big deal.

Running the generator neutral to the panel along side the supply neutral is one option.

If the change-over switch lacks a neutral terminal then you need some means to connect there then be careful as a 200A join has to be very good or it will overheat. For joining to something like that I would be inclined to look at using a line-tap. This is the sort of thing I mean:

Basically the high-current wires goes straight through (minus a section of insulation that is carefully removed) and the tap wire is clamped next to it. Advantage is no additional joint(s) in the main high-current path.

Whatever you do, check if the cable is aluminium and if so you should be using something like 'noalox' on any joints to inhibit galvanic corrosion, especially against copper or brass.
 
And the "which is better - ring or radial" (I believe US standards are for lots of radials) is one of those arguments that I think will go on for a very long time.
As above, you will find threads running to hundreds of posts on the subject of ring vs radial.

TL;DR answer is "it depends". A couple of sockets somewhere, us a radial. Many sockets on a floor of a flat/house, use a ring.

Also you have to be aware that the UK has fused plugs. That has a huge impact on the trade-offs possible.
 
Just a 3 wire plug. What are you referring as a auto transformer. I’m just trying to catch up
Short version is they have a 240V generator but a split 120-0-120 home, so genny use causes one side to be over volts, other under-volts as not balanced load.

Suggestion is to use an auto-transformer to create a "virtual neutral" off the 240V genny and then supply the house, following that the issues of rules for generator-utility change over switching, protection of a partial load rated transformer, etc.

We should charge $500/day for this, and spend it on beer!
 
As mentioned above, you really want the L1/L2/N cables to follow the same route. However, a detour of 0.5m or so is not a big deal.

Running the generator neutral to the panel along side the supply neutral is one option.

If the change-over switch lacks a neutral terminal then you need some means to connect there then be careful as a 200A join has to be very good or it will overheat. For joining to something like that I would be inclined to look at using a line-tap. This is the sort of thing I mean:

Basically the high-current wires goes straight through (minus a section of insulation that is carefully removed) and the tap wire is clamped next to it. Advantage is no additional joint(s) in the main high-current path.

Whatever you do, check if the cable is aluminium and if so you should be using something like 'noalox' on any joints to inhibit galvanic corrosion, especially against copper or brass.
What about installing a Henley block as y’all call them. If you break the neutrals then it is consider an separately derived system which has another article with that so I would not break the neutrals
 
Hmm, with that switch the OP is using - doesn't the neutral have to go through the box along with the L1 and L2 ? And each set of 3 through the same hole ?
Thinking eddy currents here.
 
Ok, So I guess we're all on the same page regarding the final design. I checked locally and there's either a 40A or a 50A 3-pole MCB available for the genny output. Would it be better to go with the lower 40A for the 10.8kVA genny /5.4kVA transformer combo? And does it have to be a B curve or a C curve?

The only problem now is the concern about running L1/L2/N together from the gen to the changeover switch. The L1/L2 from the gen are already routed underground from the area around the B2 building to the changeover switch on the B1 building and adding another neutral wire would be hard. I thought abother neutral wire using a different route will be just fine. Can't I get away with that since the genny power is just "temporary" anyway?

@pc1966

When you said "running the generator neutral to the panel along side the supply neutral is one option.", does that mean I can directly go from the autotransformer to the panel neutral bus bar even if it's a different route from the genny L1/L2?

I'd definitely treat you guys for a beer for real if we were close to each other :) But seriously, thank you.
 
To save running an extra neutral through a different route, is there room to put the transformer near your distribution board ? If not, then you don't have much option really. It won't be carrying the full load current unless you manage to load only one of your 110V sides.
10.8kVA @ 220V is 49A. I'd personally go with the 40A breaker - you'll be hard pressed to trip it anyway unless you have electric showers and/or do a lot of cooking with electric. Normally I'd expect some amount of restraint when running off the genny. Go with B curve - the only difference is the magnetic trip setting (B curve is more sensitive), slow thermal trip is the same. I doubt the genny will be capable of tripping the magnetic trip on even the B curve.

As to the difficulty of running extra wires, now you know why people often recommend ducting so you can pull in the cable you need if requirements change.

BTW, for you changeover switch and distribution board - if they have ferrous metal (i.e. steel) housings. All three wires from the set (L1,N,L2) should enter through the same hole. So all 3 from your mains supply through one hole, the three from the genny/transformer through one hole, the three out to the DB through one hole, and the three into the db through one hole. If you use two holes, cut a very thin slot between the holes with a thin hacksaw blade.
The reason for this is that each individual wire carrying a current generates a magnetic field, and this will cause currents to flow in a sheet of metal if passed through a hole in it. But the fields from the three cables will total out to nothing - because the currents in the three wires must sum to zero (taking into account phase angle as well as magnitude). So if all the wires go through one hole, then the magnetic fields cancel and no eddy currents are created. If they go through different holes, then you get eddy currents which wastes power and heats up the box.
Cutting a slot where cables go through different holes breaks the circuit for eddy currents, so the whole thing acts like the wires go through one larger hole.
 
The only problem now is the concern about running L1/L2/N together from the gen to the changeover switch. The L1/L2 from the gen are already routed underground from the area around the B2 building to the changeover switch on the B1 building and adding another neutral wire would be hard. I thought abother neutral wire using a different route will be just fine. Can't I get away with that since the genny power is just "temporary" anyway?
You only need the L1/L2/N together from the autotransformer, so if the generator is already wired in you can leave it, just plan to route the transformer ones more or less together (often simpler just to use 4 core cable of adequate rating, earth is there as well).
 
To save running an extra neutral through a different route, is there room to put the transformer near your distribution board ? If not, then you don't have much option really. It won't be carrying the full load current unless you manage to load only one of your 110V sides.
It depends on the size of the autotransformer as I still don't have a clue how big a 5kVA variant is. The changeover switch and DB for B1 is actually just separated by a wall back-to-back so they are very near each other. If its size is reasonable enough, I probably can do something to make it fit in the vicinity of the changeover switch or the DB. But shouldn't it be easier if it's near the changeover switch side and not the DB? So the new MCB + autotransformer combo near the changeover switch would mean that the cables from the autotransformer to the MCB are shorter and together and the L1/L2 wires from the MCB to the changeover switch is also shorted. But the neutral from the MCB should be tapped to the supply neutral (as shown in @pc1966 's latest diagram).

10.8kVA @ 220V is 49A. I'd personally go with the 40A breaker - you'll be hard pressed to trip it anyway unless you have electric showers and/or do a lot of cooking with electric. Normally I'd expect some amount of restraint when running off the genny. Go with B curve - the only difference is the magnetic trip setting (B curve is more sensitive), slow thermal trip is the same. I doubt the genny will be capable of tripping the magnetic trip on even the B curve.
Ok, I'll settle with a 40A B-curve MCB. And yes, before we turn on the genny we manually turn off the breakers for shower heaters, airconditioners, etc. anyway.

As to the difficulty of running extra wires, now you know why people often recommend ducting so you can pull in the cable you need if requirements change.
Actually, the L1/L2 that currently runs from the generator to the changeover switch is underground but with a pvc pipe to protect them. Technically, they can be pulled but I'm not a 100% sure if it would be easy as it's a pretty tight fit and even if it's possible it would probably destroy the existing wires. A duct would've been easier, I agree.

BTW, for you changeover switch and distribution board - if they have ferrous metal (i.e. steel) housings. All three wires from the set (L1,N,L2) should enter through the same hole. So all 3 from your mains supply through one hole, the three from the genny/transformer through one hole, the three out to the DB through one hole, and the three into the db through one hole. If you use two holes, cut a very thin slot between the holes with a thin hacksaw blade.
The reason for this is that each individual wire carrying a current generates a magnetic field, and this will cause currents to flow in a sheet of metal if passed through a hole in it. But the fields from the three cables will total out to nothing - because the currents in the three wires must sum to zero (taking into account phase angle as well as magnitude). So if all the wires go through one hole, then the magnetic fields cancel and no eddy currents are created. If they go through different holes, then you get eddy currents which wastes power and heats up the box.
Cutting a slot where cables go through different holes breaks the circuit for eddy currents, so the whole thing acts like the wires go through one larger hole.
Ok, this makes sense. I'll have to check if they were done this way. As for the genny neutral-to-supply neutral bonding, with this recommendation where all wire sets go through one hole, where does the neutral-to-neutral bonding "ideally" take place?

You only need the L1/L2/N together from the autotransformer, so if the generator is already wired in you can leave it, just plan to route the transformer ones more or less together (often simpler just to use 4 core cable of adequate rating, earth is there as well).
I thought L1/L2 from either the generator or the autotransformer are one and the same? I mean, the autotransformer is in parallel with the L1/L2 output of the generator so I was assuming that L1/L2 can either be from the generator output itself (which is how it is wired from the generator to the changeover switch right now) OR get L1/L2 and N from the autotransformer output like what you're suggesting. Or are you referring to what Simon is suggesting above? Put the transformer near the transfer switch where the end of the L1/L2 generator output cables are, install the autotransformer there, and run another fresh set of L1/L2/N/E wires from the autotransformer to the changeover switch?
 
Ok, this makes sense. I'll have to check if they were done this way. As for the genny neutral-to-supply neutral bonding, with this recommendation where all wire sets go through one hole, where does the neutral-to-neutral bonding "ideally" take place?
Ideally I would expect the neutrals to be linked inside the enclosure for the changeover switch.
So three wires from mains, three wires from genny/transformer combo, three wires to DB.
I thought L1/L2 from either the generator or the autotransformer are one and the same? I mean, the autotransformer is in parallel with the L1/L2 output of the generator so I was assuming that L1/L2 can either be from the generator output itself (which is how it is wired from the generator to the changeover switch right now) OR get L1/L2 and N from the autotransformer output like what you're suggesting.
Yes, the L1 and L2 are the same wherever you put the joints. So work out what's going to be easiest to do.
One way is to take L1,L2 to the transformer, then L1,N,L2 from transformer to breaker.
Or you take L1,L2 to breaker, tee off the L1,L2 there and take a 3 core (+earth) cable to the transformer.
Or put a junction box in.
It really depends on what's going to give the "best" result in terms of neatness and ease of install.
 

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