Three phase boards and switchgear used for 480V split phase. OK or not?

[Mark42]

Three phase boards and switchgear used for 480V split phase. OK or not?

Hi from a newbie.

I have already bought all the DBs and equipment for a major rewire and extension at my old farmhouse in Norfolk. I was expecting to have the existing ‘split-phase’ 240/480V supply (from antique overheads and a pole transformer) upgraded to ‘standard’ three phase. (There is quite new three phase HV at the pole – with one conductor not connected - which I paid for a few years back, but only the old tranny.)

Apparently it was a standard way of saving copper in rural areas after the war – having only two HV conductors, with the transformer delivering two LV phases 180 degrees opposed.

Despite my intial expectations the supplier will not share the costs of modernising this antique installation. So, being unwilling to shell out their quoted twenty-odd grand (!) I’ll live with the existing 2 x 20kVA supply.

My question is, is it OK to use standard switchgear and DBs, wiring only two of the phases, and leaving the third unconnected. I’m concerned that maybe standard modern equipment is not designed for 480V between phases.

There will be three sub-mains in different buildings. Since these will be used ‘one at a time’ I think it’s better practice to run both phases to all buildings, then arrange the final circuits to balance the loads across the two phases. I don’t want to run, for example, only phase A to the barn and Phase B to the house.

I have bought a shed-load of 25mm 4-core SWA for this purpose (wonderful EBay, Eh?). Clearly this cable’s fine for 480V between conductors, but what should be done when it gets to the downstream boards?

Elsewhere I have seen the cable terminated in a splitter box then run to two separate domestic consumer units. I think this is inelegant. I want to use my beautiful new Hager TPN boards, but are they certified for 480V?

And a further question: What are the ‘correct’ colours for marking up ‘split phase’ 480V? 17th edition is silent on this. Using two of the 315V colours feels wrong, and I believe potentially misleading. And what do you even call the bloody things? A/B? L1/L2? R/S?

Apologies for the long post. I didn’t have time to write a shorter one.

Cheers, Mark

 

[gaffer]

Hi Mark and welcome to the forum.

I would wire the sub mains and dis boards as a standard three phase install.But as you say only distribute the load equally across the two phases at the local dis board.
Youre Hager dis boards are rated for three phase(TPN,three phase and neutral).
But its very hard to give advice when not seeing the job,and I assume that youre not an electrician.
Youre best bet would be to ask around neighbours etc and get a well recommended local sparkie to come and at least give you some advice and guidance on site.
HTH,Gaffer.

 

[WayneL]

Hi,

Are you sure you're not getting mixed up - I always thought 'split-phase' was one incomming phase of 480v to a transformer.

Outgoing side of this transformer then center tapped to neutral/earth (similar to 110v site supply). - hence the name 'split-phase'.

This then gives two line conductors and a neutral - Line to Neutral or Earth voltage = 240v..... Line to Line voltage = 480v (180 degrees)


I might be wrong though

 

[Mark42]

Are you sure you're not getting mixed up - I always thought 'split-phase' was one incomming phase of 480v to a transformer.
Outgoing side of this transformer then center tapped to neutral/earth (similar to 110v site supply). - hence the name 'split-phase'.
This then gives two line conductors and a neutral - Line to Neutral or Earth voltage = 240v..... Line to Line voltage = 480v (180 degrees)
I might be wrong though

Hi Wayne

All concurs with my local reality, except your first paragraph, which I suspect describes some kind of LV isolation transformer arrangement, of which I know nothing. I don’t understand where 480V single phase could come from anyway.

In my case it’s two incoming HV phases to a pole transformer. The LV side is exactly as you say. The (earthed) neutral first appears at the centre tap on the pole trannie as you say (TN-C-S).

Metered this morning at the service head, I’m getting 239V from both lines to neutral/earth and 478V across the two phases. Allowing for the length of LV u/g cable from the pole to my house, that’s exactly as would be expected.

But none of this really matters anyway, except for academic interest.

I would wire the sub mains and dis boards as a standard three phase install.But as you say only distribute the load equally across the two phases at the local dis board.
Youre Hager dis boards are rated for three phase(TPN,three phase and neutral) …

Hi Gaffer

Thanks for your input. I agree that a ‘standard install’ would at first appear attractive. But on reflection I’ve changed my mind (and is one of the reasons I’m asking here).

This install is not ‘two phase’ as widely understood as using two of the usual three phases, 400/415V between phases. It is very different – split phase: 480V between phases. I believe it would not be good practice to introduce possible future confusion by causing the installation to appear ‘standard three phase’.

Due to cost, there will now probably never be three phase supplied to my house anyway. Therefore why ‘waste’ a core in the 4-core SWA sub-mains cabling? I’d prefer to use it as an additional PE conductor, rather than use only the armour, which I’ve never really liked, even with Piranha nuts.

So my question remains: Is it OK to have 480V present between phases in ‘standard’ modern three phase boards and switchgear?

Everything is marked up for 230/400 or 240/415. In some ways I’ve answered my own question by inspecting the supplier’s equipment this morning: the meter and timeswitch are all modern standard equipment, marked as above, but the actual voltage present exceeds the markings. Therefore if the supplier can do it, why can’t I?

I’m not suggesting that this is in any way dangerous – there are wide tolerances built into modern equipment – but is it strictly legal?

Of course nowhere will there be any actual 480V equipment (some legacy equipment of this type apparently exists on farms – has anyone ever seen any?). So the 480V would only ever be present within the DBs and switchgear. OK, and between separate T&E cables and final circuits within the premises, but never in the same enclosure.

This is by no means my main subject, so if I’m talking bol’x, anyone who deals with this stuff every day, do please say

Cheers, Mark

…But its very hard to give advice when not seeing the job, and I assume that youre not an electrician.

Youre best bet would be to ask around neighbours etc and get a well recommended local sparkie to come and at least give you some advice and guidance on site.
HTH,Gaffer.

No, I’m not an electrician, but I’m not an idiot either, and like to understand things. Local advice here comes in the ‘it’s always been done that way’ flavour, often without full understanding, which is often less than impressive … : )

 

[WayneL]

Hi Mark,

Did a bit more reading up and found this - quite interesting, I thought, kind of confirms a bit from each of our posts

A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the former Edison direct current distribution system. Like that system, it has the advantage of saving the weight of conductors for the installation. Since there are two live conductors in the system, it is sometimes incorrectly referred to as "two phase".

A transformer connected to a 3-wire distribution system has a single phase input (primary) winding. The output (secondary) winding is centre-tapped with a conductor called the neutral on the centre tap, which is normally connected to earth ground. Split phase is most common in countries with a standard phase-neutral voltage of 120V (or thereabouts). In this case, the transformer is rated 120 volts on either side of the centre tap, giving 240 volts between the two ungrounded terminals. It is also feasible to have a 240V/480V split phase system and this is sometimes used in the UK for running large farms or groups of a few houses off a single transformer where only two of the 3 phase cores of the 11KV systems are available (it is a lot cheaper to run two wires than three). However with such systems power is usually split out to single ended single phase before entering the houses.

The neutral conductor ensures that the voltages on the two legs do not get (far) out of balance. Lighting and small appliances operating at 120 V can be directly supplied, by connection between a live wire and the neutral. By distributing the load between the two live conductors, the system provides better voltage regulation and allows for the use of smaller conductors than a single-ended single phase system would allow. Large applicances, such as cooking equipment, space heating, water pumps, clothes dryers, and air conditioners are connected across the two live conductors and operate at 240 V, requiring less current and smaller conductors than they would need at 120 V. No individual conductor will be at more than 120 V potential with respect to ground (earth) possibly reducing the insulation requirements compared to a 240-volt single-ended system.

The practice originated with the DC distribution system developed by Thomas Edison. By dividing a lighting load into two equal groups of lamps connected in series, the total supply voltage can be doubled and the size (and cost) of conductors can be cut in half. Since the load will vary as lamps are switched on and off, just connecting the lamps in series would result in excessive voltage variations and brightness variations. By connecting the two lamp groups to a neutral, intermediate in potential between the two live legs, any imbalance of the load will be supplied by a current in the neutral, giving substantially constant voltage across both groups. The total mass of conductors required to supply a given load is reduced, compared to a system operating only at the utilization voltage.

It looks like my version works on a single phase, 240v supply - split down to two 120v lines, whereas the 480v that you're talking about requires two of the three phases to give that voltage

You learn something new every day

PS You certainly don't sound like an idiot - My lecturers in college didn't even know what 'split phase' or 'single-phase, three wire' was (Ihad to tell them), so you're doing well.

 

[davelerave]

i 'd guess the 230/460v supply is split phase- otherwise the voltages would be 240/415

for the standard 55-0-55 (here in the ROI) i'd be using 2 browns and a cpc

with the dual voltages the center tap will be a mid-point conductor or PEN i think-i'd have to refresh my memory on that i'm afraid