Multiwire Branch Circuit Definition ?

[Joethemechanic]

Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

So would that make a circuit consisting of 3 phase conductors and a neutral that is supplied by a transformer connection like below NOT a MWBC?

 

[synchro]

Yes, it's not a MWBC because the L-N voltages are not equal. The grounded conductor is not really a neutral with respect to the A, B, and C 3-phase line conductors. It is only a neutral in circuits using, just A, C, and N (i.e., single phase aka split-phase circuits).

 

[retirede]

Since this portion shown in the drawing has no OCPD, it’s not a branch circuit of any kind.

 

[wwhitney]

Two questions:

1) Why does the definition have that last phrase "and that is connected to the neutral or grounded conductor of the system"? How can you have a grounded branch circuit conductor that isn't "connected to the neutral or grounded conductor of the system"?

2) The circuit shown in the OP isn't an MWBC, but it contains an MWBC as a subset (A-N-C). What rules in the NEC would treat the circuit differently if the whole thing were an MWBC?

For example, 210.4(B) requires simultaneous disconnecting of just A and C, rather than A, B, and C. But that is moot, I believe, as 240.15(B) requires a 3 pole breaker.

Cheers, Wayne

 

[jaggedben]

I'm curious what equipment would actually use all three hots plus the neutral of a high-leg delta.

I didn't look but it's possible that this definition hasn't been modified since the definition of a neutral point was added, evidently in the 2008 NEC. I'm surprised the neutral definition is that new.

 

[Joethemechanic]

Lets say just for example I run all three phases and the grounded conductor from the breaker panel to a pole in the yard. A-N supplies a receptacle, C-N supplies another receptacle, B-N supplies some flood lights on top of the pole. According to the definition it is not a MWBC. What would be the requirement for breaker handle ties?

And now that I think about it, if all three phases were loaded equally at lets say 10 amps, what would the current on the grounded conductor be?

 

[wwhitney]

Lets say just for example I run all three phases and the grounded conductor from the breaker panel to a pole in the yard. A-N supplies a receptacle, C-N supplies another receptacle, B-N supplies some flood lights on top of the pole. According to the definition it is not a MWBC. What would be the requirement for breaker handle ties?

Supplying loads B-N is discouraged. 240.15(B) requires a 3 pole breaker, so the handle tie question is moot.

And now that I think about it, if all three phases were loaded equally at lets say 10 amps, what would the current on the grounded conductor be?

10A in your example, assuming identical loads A-N and C-N (no phase difference). Then looking at just the paired loads A-N and C-N, they produce currents of (10, 0, 10) on (A, N, C). Looking just at the 10A load B-N, it produces currents of (10, 10) on (B, N). In general you'd have to add the currents on the neutral vectorially, but as one of the neutral currents is 0, the sum is just 10A.

Cheers, Wayne

 

[Joethemechanic]

Supplying loads B-N is discouraged. 240.15(B) requires a 3 pole breaker, so the handle tie question is moot.

10A in your example, assuming identical loads A-N and C-N (no phase difference). Then looking at just the paired loads A-N and C-N, they produce currents of (10, 0, 10) on (A, N, C). Looking just at the 10A load B-N, it produces currents of (10, 10) on (B, N). In general you'd have to add the currents on the neutral vectorially, but as one of the neutral currents is 0, the sum is just 10A.

Cheers, Wayne

Yeah, 10 on the grounded conductor with all 3 phases loaded at 10 amps makes sense. Now what if we load A-N at 10, and B-N at 10 and C-N is at ZERO. Really with only those two loaded we kind of have a 2 phase 3 wire circuit. A-N and B-N are 90 degrees apart.

Trying to remember my 2 phase calculations. The number 1.41 keeps coming to mind as a multiplier

 

[wwhitney]

Yeah, 10 on the grounded conductor with all 3 phases loaded at 10 amps makes sense. Now what if we load A-N at 10, and B-N at 10 and C-N is at ZERO. Really with only those two loaded we kind of have a 2 phase 3 wire circuit. A-N and B-N are 90 degrees apart.

Trying to remember my 2 phase calculations. The number 1.41 keeps coming to mind as a multiplier

Yes, assuming the current-voltage phase shift on the A-N load and the (discouraged) B-N load are the same, the currents are 90 degrees apart. So then vectorially 10A + 10A = 14.1A, the vector addition is an isosceles right triangle.

Cheers, Wayne

 

[Joethemechanic]

Why are B-N loads discouraged? Because B-N puts additional loading on the transformer supplying A-C?

I used to see open delta B-N / A-C being used to power old 220 volt 2 phase equipment in Philly all the time. I'm not saying it was right, but I saw a lot of machines run for years like that with no apparent ill effects.

 

[jap]

Why are B-N loads discouraged? Because B-N puts additional loading on the transformer supplying A-C?

I used to see open delta B-N / A-C being used to power old 220 volt 2 phase equipment in Philly all the time. I'm not saying it was right, but I saw a lot of machines run for years like that with no apparent ill effects.

Your example was to feed the "flood lights" on top of the pole with the wild leg and a neutral.

Although most ballasted fixtures are rated 120-277v we try our best to keep any 1ph load that may require a neutral off of the wild leg for those who may not know any better.

Most see a phase and a neutral on a 240v system and assume its 120v circuit.

In your example someone installing 120v fixtures who assumed the voltage at the top of the pole was 120 and turned them on would be climbing back up the ladder to change out the brand new fixtures as soon as they got down from installing them.

JAP>

 

[jaggedben]

.... 240.15(B) requires a 3 pole breaker, so the handle tie question is moot.

...

Um, no it doesn't. He has three line-to-neutral loads, but it's not offically an MWBC. 240.15 doesn't cover that situation.

Seems to me it's a loophole.

Also interesting that a 3-wire branch circuit from a corner grounded delta meets the definition of MWBC even though it serves only line-line loads. 210.4(C) Exception 2 requires a common trip breaker for that. But I don't see how our OP's proposal gets covered.

 

[wwhitney]

Um, no it doesn't. He has three line-to-neutral loads, but it's not offically an MWBC. 240.15 doesn't cover that situation.

Actually, the part about supply loads A-N, B-N, and C-N came after my comment, but isn't relevant to this question.

I'm considering all 4 wires (A,B,C,N) to be one circuit. 240.15(B) requires a circuit breaker to open all ungrounded conductors unless one of the subsections applies. (1) doesn't apply as it's not an MWBC. (2) doesn't apply as it's not a single phase circuit. (3) doesn't apply as the B voltage to ground exceeds 120V. And (4) doesn't apply as it's not DC.

So with a circuit breaker, it would need to be 3 pole as long as you consider all 4 wires to be one circuit. [Vs, say, the case of just supplying two different two wires loads, like a 120V load A-N and a 240V load B-C, which isn't really in the spirit of the OP. That would just be two circuits.]

As to why you might want to run a single circuit as in the OP, the example that comes to my mind for a 240V center tapped delta is that you need to supply some 3 phase 240V equipment, as well as a 120V receptacle for servicing the equipment, and the load calc works out to supply everything from a single 20A 3 pole breaker.

Cheers, Wayne

 

[jaggedben]

The first part doesn't say 'simultaneously'. But I'll grant that the 'unless otherwise permitted' doesn't make a whole lot of sense if that isn't what they meant.

 

[jap]

Then, on top of that, if we consider the pole out in the yard a "structure",,,,


JAP>

 

[wwhitney]

The first part doesn't say 'simultaneously'.

No, it says "both manually and automatically." I.e. the breaker trips (automatic), all ungrounded conductors must be opened. You manually move the handle, all ungrounded conductors must be opened.

Cheers, Wayne

 

[jaggedben]

No, it says "both manually and automatically." I.e. the breaker trips (automatic), all ungrounded conductors must be opened. You manually move the handle, all ungrounded conductors must be opened.

Cheers, Wayne

And you manually move three separate handles at separate times and you still open all ungrounded conductors.

Reading it again, I agree with you. I just dont understand why they don't use 'simultaneously' as in other parts of the code. It would be clearer.