1. Electrical Phase Question

Here's something I came across years ago that has always bothered me. It was in an apartment built in the 60s or 70s, so none of the outlets I'm discussing has a ground plug.

There were two 110 circuits in the kitchen area, opposite phase from one another. They used one common wire for the return circuit to the breaker box, so there were three wires running to the kitchen area.

Here's my question. There were two outlets above the counters in each unit. Each outlet was split so that the top half was one phase and the bottom half the other. I don't see what they were trying to accomplish by doing that. Why not just have one outlet be on one phase and the other outlet on the other one?

I'd come across that once before in a building built about the same time, but it was only in an outlet next to a window. There I thought it was possibly to give some option for having 220 at that outlet for an air conditioner, although without being a dedicated circuit I don't see how that would work.

Anyway, what's the theory behind doing this?

2. Re: Electrical Phase Question

Most likely this was only single phase. Multi-phase is typically reserved for commercial and industrial services.

What I think you are trying to describe would more correctly be called a multi-wire branch circuit feeding the duplexes.

This may have been done to have 2 circuits on the duplex to avoid tripping the breakers if two appliances were used in one receptacle. Same concept as A-B-A-B arrangements, just a different method.

3. Re: Electrical Phase Question

Originally Posted by Jim Port
What I think you are trying to describe would more correctly be called a multi-wire branch circuit feeding the duplexes.

This may have been done to have 2 circuits on the duplex to avoid tripping the breakers if two appliances were used in one receptacle. Same concept as A-B-A-B arrangements, just a different method.
In Canada up until a couple of years ago, all kitchen counter outlets are on 15 A split duplex circuits like what the poster is describing. Because we used 15 amp breakers, you had to supply top and bottom with a different circuit, and no two adjacent outlets were allowed to be on the same circuit.

The result was two double 15 A breakers and two runs of 14/3 for as few as two receptacles.
New rules. Now with the GFCI requirement, (in Canada only required 1.5 m adjacent to the sink, BTW) we are allowed to go 20 amp, no split. Adjacent outlets are allowed if they're 20 amp. Max 2 duplex outlets per circuit. Repeat: That is Canadian rules.

4. Re: Electrical Phase Question

Originally Posted by Jim Port
Most likely this was only single phase. Multi-phase is typically reserved for commercial and industrial services.

What I think you are trying to describe would more correctly be called a multi-wire branch circuit feeding the duplexes.

This may have been done to have 2 circuits on the duplex to avoid tripping the breakers if two appliances were used in one receptacle. Same concept as A-B-A-B arrangements, just a different method.
I'm sorry, but my electrical terminology isn't great! What I mean by opposite phase is if you connected the two positives together (e.g. you didn't break the tab off the outlet to separate the top and bottom halves), you'd have a massive short because one circuit is off on leg of the panel and the other off the other leg. If they were off the same leg but still with different breakers you wouldn't notice any difference in operation unless you turned off one of the two breakers, in which case everything on that circuit shut off would be fed through that outlet.

If you took a 220 volt dryer/stove outlet and connected the wires, you'd have 220, but not with enough amperage to run such devices.

5. Re: Electrical Phase Question

Originally Posted by John Kogel
In Canada up until a couple of years ago, all kitchen counter outlets are on 15 A split duplex circuits like what the poster is describing. Because we used 15 amp breakers, you had to supply top and bottom with a different circuit, and no two adjacent outlets were allowed to be on the same circuit.

The result was two double 15 A breakers and two runs of 14/3 for as few as two receptacles.
Yes, that sounds similar, but your statement "no two adjacent outlets were allowed to be on the same circuit" I don't think is met because there were only two outlets in the kitchen (ignoring the refrigerator), and only two circuits. So the top and bottom of each of the two outlets would have been the same two circuits on each of the two outlets.

Also, the outlets were daisy chained, such that if you disconnected the wires from the first outlet and reset the breakers, the second outlet would be completely non-functional on both circuits. So you're really only dealing with one run of 3 wires for the two circuits--they shared the common line. (I don't remember the gauge of the wires or the amperage of the breakers).

It's almost like they were trying to guess what outlets people would plug things into.

6. Re: Electrical Phase Question

I have seen this done many times and though I strongly disagree with every aspect of this set up, as long as the two circuits are on the same phase with separate neutrals it has been allowed. If you see this with opposite phases and a shared neutral it was never allowed because a standard duplex receptacle device is only rated for 15 or 20-amps at 120-volts not 30 to 40-amps at 240-volt. I have had people argue that point and say it is ok if you use a double pole breaker but It always falls back on the device rating. Show me where it's rated for that and I will shut up is what I tell them!

Even if you are on separate phases with separate neutrals you are still exceeding the devices rating because you are still connecting it to 2-circuits with the potential for twice the load in amperage to run through the device which is rated for 15 or 20-amps! If someone plugs a microwave rated at 1500 -2000-watts in the top and a toaster oven rated the same in the bottom you now have 25-34-amps running through a device rated for 15-20-amps

Last edited by Lou Romano; 06-07-2010 at 02:04 PM.

7. Re: Electrical Phase Question

Originally Posted by Lou Romano
I have seen this done many times and as long as the two circuits are on the same phase with separate neutrals it is allowed. If you see this with opposite phases and a shared neutral it was never allowed because a standard duplex receptacle device is only rated for 120-volts not 240-volt. I have had people argue that point and say it is ok if you use a double pole breaker but It always falls back on the device rating.
Lou-
It is perfectly legal to use a multiwire branch cicuit on a duplex receptacle.
The NEC article 210.4 allows this and requires the multiwire circuit to be simultaneously disconnected ( multipole circuit breaker)

In the 2005 NEC - article 210.4(B) was written for devices or equipment
on the same yoke.

This section was rewritten in 2008 to include ALL multiwire circuits not just those on devices.

In regard to the duplex receptacle ratings-
A duplex receptacle rating of 120 volt is for EACH receptacle of the duplex receptacle. A duplex receptacle has 2 receptacles ( top and bottom are each a receptacle)

8. Re: Electrical Phase Question

If the tab is broken out on the device, it's two separate devices entirely. Unless someone sticks it with an opened up paper clip . . .. ;-) (Seriously, that's part of what bothers me about this--someone could pull out the receptacle without turning off any power and they'd have a real issue if they touched it a certain way, etc.)

As to the common neutral wire, that clearly wouldn't work if the circuits were on the same phase, right?

9. Re: Electrical Phase Question

I will agree to disagree! I don't interpret the code you quoted as having any meaning in this situation.

ken horak;133290]Lou- It is perfectly legal to use a multiwire branch cicuit on a duplex receptacle. The NEC article 210.4 allows this and requires the multiwire circuit to be simultaneously disconnected ( multipole circuit breaker)

How does this apply to connecting 240-volts to a receptacle rated at 120-volts? This is only talking about MWBC's not device ratings?

In the 2005 NEC - article 210.4(B) was written for devices or equipment on the same yoke.This section was rewritten in 2008 to include ALL multiwire circuits not just those on devices.

Again I ask you the same question? How does this apply to using a device beyond its rating? That is what I am talking about. And where does it say that 210.4(B) was
written for devices or equipment on the same yoke

In regard to the duplex receptacle ratings- A duplex receptacle rating of 120 volt is for EACH receptacle of the duplex receptacle. A duplex receptacle has 2 receptacles ( top and bottom are each a receptacle)

OK, so you have split the device and made it two devices each rated for 120-volts! Show me where on the device it says you can connect 240-volts and it is rated for that! Granted you can split it and use it as two receptacles but it is still one device and the rating for the device is 120-volts and 15 or 20-amps.

You can do it and it may even be accepted by some, but it is flat out wrong!

Show me where it says the device is rated and I will STFU! This is a rating issue.

Last edited by Lou Romano; 06-07-2010 at 02:44 PM.

10. Re: Electrical Phase Question

Now some one is trying to say that if you split out a 15 A-120 V duplex receptacle you got two 7.5 A-60 V single receptacles.Becouse if is lake that I need to restart my electrician classes again.

11. Re: Electrical Phase Question

Lou, that would not be using the device at 240 volts. You have 2 120 volt receptacles on the same mounting strap. You would not get 240 volts out of a receptacle wired that way. Also cords and receptacles for 240 volts are a different NEMA configuration and would not fit into a standard duplex.

12. Re: Electrical Phase Question

Originally Posted by ken horak
Lou-
It is perfectly legal to use a multiwire branch circuit on a duplex receptacle.
The NEC article 210.4 allows this and requires the multiwire circuit to be simultaneously disconnected ( multipole circuit breaker)
Lou,

I believe that a better code reference for this would be 210.7(B), shown below:
From the 2008 NEC. (underlining and bold are mine)
- 210.7 Branch-Circuit Requirements for Device Connections and Locations.
- - (A) Receptacle Outlet Location. Receptacle outlets shall be located in branch circuits in accordance with Part III of Article 210.
- - (B) Multiple Branch Circuits. Where two or more branch circuits supply devices or equipment on the same yoke, a means to simultaneously disconnect the ungrounded conductors supplying those devices shall be provided at the point at which the branch circuits originate.

As others have stated, a multiwire branch circuit does not apply 240 volts across a receptacle rated for 125 volts. The multiwire branch circuit simply provided two 120 volt circuits to the duplex receptacle device, and the receptacles share the neutral, but there is only 125 volts across EACH (or 'either') receptacle outlet.

Just in case one is thinking that the above code section for "Multiple Branch Circuits" does not apply to "multiwire branch circuits" ....
- 210.4 Multiwire Branch Circuits.
- - (A) General. Branch circuits recognized by this article shall be permitted as multiwire circuits. A multiwire circuit shall be permitted to be considered as multiple circuits. All conductors of a multiwire branch circuit shall originate from the same panelboard or similar distribution equipment.
- - - FPN: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic currents on the neutral conductor.
- - (B) Disconnecting Means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.
- - (C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads.
- - - Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment.
- - - Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.
- - - - FPN: See 300.13(B) for continuity of grounded conductor on multiwire circuits.
- - (D) Grouping. The ungrounded and grounded conductors of each multiwire branch circuit shall be grouped by wire ties or similar means in at least one location within the panelboard or other point of origination.
- - - Exception: The requirement for grouping shall not apply if the circuit enters from a cable or raceway unique to the circuit that makes the grouping obvious.

13. Re: Electrical Phase Question

I really don't see the 220 rating argument as being legitimate when it comes to the capacity of the outlet. There's no more power running through the thing than if it were two separate circuits that were both the same phase (and thus only 110 volt), and since they're independent, there's no more strain on the device either way. But I can see it doesn't seem as safe for someone working on the outlet. And BTW, although I used the term breakers, I'm pretty sure this place had fuses, so there's no way they were interconnected.

But rather than code, I'd like to get more on theory. I just don't get what they were after with this design. I get that by having two circuits with opposite phase they saved running one additional wire a total of maybe 30 feet. Maybe the conduit was also smaller and less expensive. What I don't get is why each outlet had to be split, as opposed to one outlet being on one of the two circuits, and the other outlet the other. What did they gain or save by doing that?

14. Re: Electrical Phase Question

Originally Posted by Lou Romano
You can do it and it may even be accepted by some, but it is flat out wrong!

Show me where it says the device is rated and I will STFU! This is a rating issue.
Maybe, just maybe, if you are in code enforcement, it'd be great to have your facts straight.

To put the matter to bed why don't you contact a manufacturer and ask them straight out if it is OK? Just make sure you talk to a product engineer and not a catalog jockey.

Hard to believe the Canadians REQUIRED this type of set up for years if it was the issue you say it is.

I just love it when guys with your attitude of "show me" come around and aren't willing to do their own leg work.

While you're at it, why not ask for the torque specs for the receptacle screws. There's something legitimate you can grouse about. Gives you the excuse to check all the calibrated wrists out there.

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