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Voltage between phases (3Ph supply)

Voltage between phases (3Ph supply)

Voltage between phases (3Ph supply)

I'm sure the question I'm about to ask has a very simple answer, yet it's one that I'm not aware of!

When designing circuits from a 3-phase dis bd. why are single phase loads separated between rooms/areas? I understand there is a p.d. between the line conductors of 400V, but why must they be spatially separate?

For example, I have a TP+N dis. bd. where I am supplying a number of single phase ring circuits. I am aware that I shouldn't mix phases within rooms, but I am unsure as to why. Could someone please enlighten me?

Thanks in advance!

RE: Voltage between phases (3Ph supply)

It would, indeed. Some rooms require several circuits. What I would like to know is why I need to keep each phase separate within the rooms. Say I'm supplying a computer suite with 3No. ring circuits, they would all need to be from either L1, L2 or L3. Why can't they be mixed? For instance: one circuit on L1, another on L2 and the other on L3. Bearing in mind all circuits are independently earthed and contained within trunking of some description.

I have also noticed on some electrical system designs that phases are split over floors. For example, ground floor is supplied via L1, first floor is supplied via L2 and second floor is supplied via L3. I am guessing this is the same principle; keeping phases spatially apart.

I know this might seem like a stupid question, but it's not my primary field.


RE: Voltage between phases (3Ph supply)

Because while the mains are nicely coupled, the grounds might not be.  I've blown circuits not because the AC was wonky, but because the grounds weren't tightly coupled, resulting in massive voltage transients that fried ICs.

Moreover, from a pure user perspective, it makes more sense from a troubleshooting perspective that interconnected hardware be tied together, so that when a particular phase fails, that everything either stays up together or goes down together.  Having one equipment go down while others around stay up will lead to wasted effort trying to troubleshoot the equipment, when it's the power that's at fault.


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RE: Voltage between phases (3Ph supply)

I've never heard of it myself.  Look at homes.  The two legs are mixed all over the place.

Having multiple legs in the same area allows one to go down and you can still have some lights on to see with.

Mixing them would allow better balancing.  What if you have greatly different usage on different floors?  It might be impossible to balance the phases if only one phase is allowed per floor.

Sounds like a design by bean counters.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Voltage between phases (3Ph supply)

No requirement to separate phases into areas that I ever heard of...

In fact, when designing circuits for twin-type high bay (or any) HID fixtures, using alternate phases is sometimes needed to reduce or eliminate the stroboscopic effect.

Also, in hospitals, we often intentionally mix both phases and systems in rooms to add reliability --  a single-phase breaker trip won't leave a room dark or powerless, like Keith said.

Good on ya,

Goober Dave

RE: Voltage between phases (3Ph supply)

In the UK we do generally try to avoid mixing phases for socket outlets in an area purely from a safety perspective because with our 240V L-N supply there would be 415V between conductors and that creates an increased risk which can be reduced by avoiding mixing phases.

Perhaps the key factor is that UK safety legislation is founded on the ALARP principle - 'as low as reasonably practicable' - and as a designer it would be hard to defend mixing phases as being ALARP if someone was hurt or killed after managing to string themselves across two lines simultaneously. I don't know how someone would do that, but idiots are very creative. wink The prosecution might well argue that had they only had a line-neutral shock they might have lived. Avoiding mixing phases is not a requirement in BS 7671 as far as I know.

I agree that for lighting there is no reason to keep stay one phase, and possibly benefits to mixing phases.

If we learn from our mistakes I'm getting a great education!

RE: Voltage between phases (3Ph supply)

This is the first time that I have heard of this, but I was only first introduced to three phase panels and mixed circuits back in school doing stage lighting when I was about 14 years old.
Circuits are mixed to save wire as one neutral will serve three circuits if the wiring is laid out appropriately. Some codes allow a larger neutral to serve six circuits.
A phase for each floor, in North America?
The installed cost is greater. The losses are greater. If this is implimented in buildings where the single phase areas are larger than may be covered by 1/3 of a standard panel you will have underutilized panels adding to the cost. If you gang the phases together to make a single phase panel out of a three phase panel, the neutral ampacity may be insufficient.
If voltage drop on circuit conductors becomes an issue, the price may be further increased. When a conventional wiring scheme is used with shared neutrals, the voltage drop percentage may be calculated on a 208V base.
With a single phase feeding an area, the voltage drop must be calculated on a 120V base. This will often require larger cross section conductors.
Is this being designed by people with computer background?

"Why not the best?"
Jimmy Carter

RE: Voltage between phases (3Ph supply)

Scotty: I often use two outlets and a jiggered extension cord to dish up 240VAC in a house.  It's always nicer if I don't have to leave the room in search of "The Other Phase".  LOL

Keith Cress
kcress - http://www.flaminsystems.com

RE: Voltage between phases (3Ph supply)

Yes Keith, but you're 'special'. lol I wish I had 3-phase here too - would make running the machine shop a lot easier.

If we learn from our mistakes I'm getting a great education!

RE: Voltage between phases (3Ph supply)

Bill says: "Is this being designed by people with computer background?"

I don't think that I can give you a LPS for that. But am tempted to.

It was people with computer background that decided not to cool 1.2 MW DC motors (because the fans used energy and lowered efficiency). Result: five motors in the winder shop before I could convince them that cooling was necessary.

Gunnar Englund
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Voltage between phases (3Ph supply)

Like you Gunnar, too many years of being exposed to too much foolishness. Grin

"Why not the best?"
Jimmy Carter

RE: Voltage between phases (3Ph supply)

If you consider how it would be wired.
Cable comes from switchboard to first outlet box.
One phase & Neutral are used, the other 2 phases are joined and passed to the next box, and so on.
 To make the wiring safe you would have to supply it from a 3 pole breaker. So where's the added reliability gone trip one phase you loose 3 times as many outlets.
I can see it making some sense if you were to run a seperate cable for each phase but what contractor is going to follow that?
In NA we run 120/240 single phase to outlets in the kitchen to spread the load. I have yet to see 3 phase supply to a house here.

RE: Voltage between phases (3Ph supply)


North American wiring practices are way different to British practices. I've never seen anything close to what you describe in a UK residential application. Our final circuits all run as line, neutral, & earth. Radial circuits are possible, but ring circuits are far more common.

If we learn from our mistakes I'm getting a great education!

RE: Voltage between phases (3Ph supply)

Firstly, thanks for all of the replies.

I did forget to mention I am in the UK so this obviously will differ from other practices, such as those in America.

Scotty - as I assumed, the separation of the phases is purely for safety reasons. As you mentioned though, it would be quite difficult to create a short circuit or earth fault between separate L/N conductors in separate circuits. This is why I questioned the issue.

To me, logically, I would arrange the 3 phases as evenly as possible, even if it meant having, say, all 3 phases feeding separate circuits within one room. However, I am told this is not the standard UK practice due to safety reasons; potential of 400V passing electric shock. This just means that it makes it more difficult to balance the phases in some situations.

Also, for the Americans, by single phase circuits from a TP+N dis. bd. I mean via one line/live cable, one neutral and one cpc fed from an individual MCB/RCD, as standard SP+N circuits are wired here in the UK.

I guess I'll just go with it... Although I haven't seen this mentioned within the BS 7671 Wiring Regs.


RE: Voltage between phases (3Ph supply)

Keep socket outlets on different phases as far apart as possible - in the past I've used 2m as a minimum spacing so that someone couldn't 'accidentally' manage to stick their fingers in two sockets simultaneously and receive a 415V shock. Certainly don't put two adjacent socket outlets on different phases.

I'd also put a warning, perhaps on the door to the room, that a multiphase supply exists and 415V may be present between sockets outlets. In an industrial setting this condition would be reasonably expected and I wouldn't bother with the warning, but in a residential application it is unusual and so I'd draw attention to the condition. Cover yourself - the last thing you need is an ambulance-chaser on your case!

If we learn from our mistakes I'm getting a great education!

RE: Voltage between phases (3Ph supply)

There is also the isolation issue.
In the UK, domestically, three phase is rare. Someone inexperienced/not properly qualified/careless may assume from testing one part of a room that the whole room has been isolated...
(Personally, I'd love three phase at home. Wired in red, yellow, blue, not the new-fangled brown, black, grey as well. Why? Looks better!)

"I love deadlines. I love the whooshing noise they make as they go past." Douglas Adams

RE: Voltage between phases (3Ph supply)

Hi all, came across this forum as I'm a fixed wiring inspection and testing engineer in the UK. I've just finished a job inspecting a large care home consisting of five separate buildings/blocks where in most rooms throughout there were different phases supplying the socket outlets (two phases), lighting and heating/wall stats.
As a recommendation to the client I referred to regulation 514.10.1 paragraph 2 of BS7671 which states:

Where terminals or other fixed live parts between which a nominal voltage exceeding 230 volts exists are housed in separate enclosures or items of equipment which, although separated, can be reached simultaneously by a person, a notice should be secured in a position such that anyone, before gaining access to such live parts, is warned of the maximum voltage which exists between parts.

My interpretation of this is although this method of installation is unorthodox the regulations don't state that this approach cannot be used, but if it is appropriate labeling should be used.


RE: Voltage between phases (3Ph supply)

Can you reach between one socket and the next, or is there permanent equipment (e.g., a built-in heater) attached to each that you can reach?  If not, doesn't sound like it needs to be labeled.  In the States, sockets must be at least every 10', but my arms aren't that long...

Dan - Owner

RE: Voltage between phases (3Ph supply)


Thanks for the reference - I missed it when I looked a few weeks ago.

Oh, and welcome to the forum. Nice to see another Brit in here. Which part of the country are you in? I'm up the North East.

If we learn from our mistakes I'm getting a great education!

RE: Voltage between phases (3Ph supply)

Think about load balancing and all it implies.. very important

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