Counting neutrals as current carrying conductors

[PbMXer_218]

Good afternoon! 2017 NFPA 70, 310.15 (5) A neutral conductor that carries only the unbalanced load from conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a). For NFPA 70 2020 THE REFERENCE IS 310.15(C)(E). Both basically say the same thing.

With that said, my question is when does this rule apply? For example, if we have a multi-branch circuit like an oven that requires a neutral, then the neutral is not required to be counted?

Also, if you have a 120V circuit that is drawing 6 amps, the neutral would also carry 6 amps. This would be a balanced circuit. To be quick and to the point, when does 310.15(5) apply?

When you read further, it talks about multi wire b ranch circuits and basically sys it must be counted. I am trying to understand when this rule can be applied. And if a dedicated circuit is carrying the same load from phase conductor and neutral conductor why are we required t count the neutral as current carrying. This rule is deceiving to me. I do not understand when and how to apply it properly.

TIA

 

[Dennis Alwon]

In most cases a neutral for a multiwire branch circuit will not be a current carrying conductor. Here is something written by our member Infinity. It should help.


Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC: by Infinity
3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*
Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception,
*if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.

 

[Dennis Alwon]

Remember on a multiwire branch circuit it does not have to have the same current on both circuits to be a current carrying conductor. There are exception for non-linear loads mentioned above.

 

[infinity]

With that said, my question is when does this rule apply? For example, if we have a multi-branch circuit like an oven that requires a neutral, then the neutral is not required to be counted?

It depends on the system that is supplying the power. Take a look at the different systems in Dennis' post. A 1Ø circuit from a 3Ø Wye system would have a neutral that is counted as a CCC.

 

[PbMXer_218]

Thank you for the excellent information and clarification.

 

[winnie]

An important point to remember: we are deciding if the neutral 'counts' as a current carrying conductor for the purpose of derating calculations. IMHO a better way to think of it is 'given this set of N conductors, do we have N or N-1 units of heat generation in the worst case'.

Consider a single phase 20A MWBC. You have 3 conductors, two hot one neutral. In general _all_ will be carrying some current. But consider the worst case for heat generation, the two hot conductors loaded to 20A. In this case the system is balanced and the neutral doesn't carry any current. Thus 2 wires worth of heating. Now consider the maximum unbalanced case. One hot loaded to 20A, the other hot with no load. Now you have 20A on one hot, and 20A on the neutral. Again: 2 'wires worth of heating'.

In any other circumstance (say 20A on hot 1, 10A on hot 2, and 10A on the neutral) you have less heat produced, even though all conductors are carrying some current.

So for purpose of derating we treat this set of 3 conductors as only producing 2 conductors worth of heating.

When you analyze the other situations where the neutral doesn't 'count as a current carrying conductor' you find a situation where with N conductors the worst case heating is N-1 wires worth.

-Jon

 

[roger]

An important point to remember: we are deciding if the neutral 'counts' as a current carrying conductor for the purpose of derating calculations. IMHO a better way to think of it is 'given this set of N conductors, do we have N or N-1 units of heat generation in the worst case'.

Consider a single phase 20A MWBC. You have 3 conductors, two hot one neutral. In general _all_ will be carrying some current. But consider the worst case for heat generation, the two hot conductors loaded to 20A. In this case the system is balanced and the neutral doesn't carry any current. Thus 2 wires worth of heating. Now consider the maximum unbalanced case. One hot loaded to 20A, the other hot with no load. Now you have 20A on one hot, and 20A on the neutral. Again: 2 'wires worth of heating'.

In any other circumstance (say 20A on hot 1, 10A on hot 2, and 10A on the neutral) you have less heat produced, even though all conductors are carrying some current.

So for purpose of derating we treat this set of 3 conductors as only producing 2 conductors worth of heating.

When you analyze the other situations where the neutral doesn't 'count as a current carrying conductor' you find a situation where with N conductors the worst case heating is N-1 wires worth.

-Jon

To show a visual of what Jon is saying.

 

[Strathead]

Regarding the non linear load part of this, I am going to say it. As an installer, don't worry about it. If you follow the other rules by Dennis, do what the engineer specifies. If he tells you to run 250 copper for a 200 amp load, that is what you do, regardless of whether that is for voltage drop or heat dissipation.

 

[PbMXer_218]

To show a visual of what Jon is saying.

View attachment 2566835

This is an interesting illustration. I appreciate the further clarification. Though, I find it odd that a neutral of a MWBC that is carrying the 10 amps of unbalanced load is not required to be counted. It still generates some heat, does it not? So my thought process for a 2 wire circuit, 1 phase conductor, 1 ungrounded conductor, both conductors are carrying the same load. Is the difference in heat generation so much greater in a 2 wire circuit that much more than a MWBC only carrying the unbalanced load? The science behind this rule is what I am questioning. When you have a MWBC you have differences in positive and negative peaks of the phase conductors and harmonics. But a 2 wire circuit you have the same current and the same peaks.

 

[Strathead]

This is an interesting illustration. I appreciate the further clarification. Though, I find it odd that a neutral of a MWBC that is carrying the 10 amps of unbalanced load is not required to be counted. It still generates some heat, does it not? So my thought process for a 2 wire circuit, 1 phase conductor, 1 ungrounded conductor, both conductors are carrying the same load. Is the difference in heat generation so much greater in a 2 wire circuit that much more than a MWBC only carrying the unbalanced load? The science behind this rule is what I am questioning. When you have a MWBC you have differences in positive and negative peaks of the phase conductors and harmonics. But a 2 wire circuit you have the same current and the same peaks.

Because it is the unbalanced load. Simplify it. The one conductor carrying 20 amp is generating it full allowance 100% of heat, each of the wires carrying 10 amps are only generating 50% of their heat. So the total heat allowed is 3 wires=300%, the total generated is 300%.

 

[winnie]

The heat produced goes as the square of the current flow.

In the specific example of a 20A single phase MWBC, made with 12ga wire (0.00162 ohm/ft) consider 3 states:

2 hots running 20A, neutral 0A -> 2 * 20^2 * 0.00162 = 1.3 watts per foot of heating
1 hot running 20A, 1 hot running 0A, 1 neutral running 20A -> 2 * 20^2 * 0.00162 = 1.3 watts per foot of heating
1 hot running 20A, 1 hot running 10A, 1 neutral running 10A -> (20^2 + 2 * 10^2) * 0.00162 = 0.97 watts per foot of heating

The worst case heating happens when the current is most concentrated, on two wires. When the current is more distributed (flowing to some extent on all 3 wires) then heating is reduced.

-Jon

 

[winnie]

Because it is the unbalanced load. Simplify it. The one conductor carrying 20 amp is generating it full allowance 100% of heat, each of the wires carrying 10 amps are only generating 50% of their heat. So the total heat allowed is 3 wires=300%, the total generated is 300%.

Heating on the wires goes as the square of the current. So if we define a wire carrying 20A as generating its 100% allowance, then when the wire carries 10A it generates 25% of its allowance.

-Jon

 

[wwhitney]

When you analyze the other situations where the neutral doesn't 'count as a current carrying conductor' you find a situation where with N conductors the worst case heating is N-1 wires worth.

Nice write up. A simple way to express the rules on whether to count the neutral as a CCC, then is this: suppose all the other circuit conductors are fully loaded, linearly. Is the neutral current zero? If so, don't count it. Is the neutral current more than zero? If so, count it.

Cheers, Wayne

 

[LarryFine]

Though, I find it odd that a neutral of a MWBC that is carrying the 10 amps of unbalanced load is not required to be counted. It still generates some heat, does it not?

It does, but any current being carried by the neutral is current not being carried by the lesser-loaded line conductor, so the total current on the three conductors is never greater than what can be carried by two.

 

[PbMXer_218]

Very nice. I have a better understanding of why this method is used. I very much appreciate all who have shared their knowledge. As a newly licensed EC, August of 2022, 29 years in the trade now. I am always eager to learn an understand more of the codes we adhere to and especially why or the science behind why. I have learned throughout my career there is far more to passing a test in this trade. Knowing and understanding the code and how to apply it are extremely important and even after nearly 30 years of trade experience I am still learning everyday. Thank you all!