Neutral as a Current Carrying Conductor

[denders]

Hello folks,

I'm asking whether the neutral conductors between the transformer and service entrance switchboard are current carrying conductors for deciding whether the conductors should be derated.

1. The transformer secondary is 480VAC Wye.
2. There are multiple sets of secondary conductors. 4 Wire, 3 phases plus neutral.
3. The switchboard is the service entrance. The neutral is grounded in the switchboard.
4. There are multiple 480VAC 3 phase loads (motors, etc.). Not to muddy the waters, but some motors are on VFDs, nothing larger than 30 HP.
5. Also various 480/240/120 transformers for other loads.
6. There are also single phase 277VAC loads, Lighting (LED) and Heat tracing.
7. The various loads will probably change over the life of this facility.

If you don't mind, I'd like a bit more of an answer than Yes or No.

Thanks
Dave

 

[Jraef]

How about this:
Yes, because a Neutral is ALWAYS considered a Current Carrying Conductor. There really isn't any more than that needed.

Article 100 of the NEC, Definitions:

Neutral Conductor. The conductor connected to the neutral
point of a system that is intended to carry current under
normal conditions.

 

[infinity]

Hello folks,

I'm asking whether the neutral conductors between the transformer and service entrance switchboard are current carrying conductors for deciding whether the conductors should be derated.

1. The transformer secondary is 480VAC Wye.
2. There are multiple sets of secondary conductors. 4 Wire, 3 phases plus neutral.
3. The switchboard is the service entrance. The neutral is grounded in the switchboard.
4. There are multiple 480VAC 3 phase loads (motors, etc.). Not to muddy the waters, but some motors are on VFDs, nothing larger than 30 HP.
5. Also various 480/240/120 transformers for other loads.
6. There are also single phase 277VAC loads, Lighting (LED) and Heat tracing.
7. The various loads will probably change over the life of this facility.

If you don't mind, I'd like a bit more of an answer than Yes or No.

Thanks
Dave

In order for the neutral to be considered a CCC in the installation you've described it would need to have a non-linear load of more than 50%. I don't see anything that would suggest that it does.

 

[kwired]

Most cases the service neutral is not a current carrying conductor for the purpose of ampacity adjustments as covered in 310.15(B)(3). It will carry any unbalanced line current if there is line to neutral loads though.

 

[denders]

Thank you.

I guess I'm trying to get it in my head why there isn't current in the neutral conductors from switchboard to transformer because of those 277VAC single phase loads. There aren't any intervening transformers, etc. just various circuit breakers at the appropriate points. (switchboard main, etc.) I'm assuming the single phase loads will be balanced across the 3 phases.

 

[Dennis Alwon]

Does the neutral carry current--- Yes however in terms of adjustments because of heat it will not add any more current to the load then the actual conductors.

In a single phase load the Neutral = L1- L2 which is the unbalanced load. If L1 carries 12 amps and L2 carries 10 amps then the neutral will only carry 2 amps.

 

[Dennis Alwon]

I know this isn't a correct analysis but it is the best I know to get the idea across.

Again single phase- because it is easier- Say you have a 2 hots and a neutral with #12 awg. The max #12 can carry is 20 amps. With 2 conductors the max is 40 amps-- I see it as 40 amps of heat. Now the neutral will carry the unbalanced load as I stated above

20 amps on L1 and 20 amps on L2 = 0 amps on the neutral. Ant combination will not give more than the 40 amps of heat thus the neutral is not a current carrying conductor even though it may carry current.

 

[kwired]

Thank you.

I guess I'm trying to get it in my head why there isn't current in the neutral conductors from switchboard to transformer because of those 277VAC single phase loads. There aren't any intervening transformers, etc. just various circuit breakers at the appropriate points. (switchboard main, etc.) I'm assuming the single phase loads will be balanced across the 3 phases.

There is current in the neutral, but when it comes to how much heat it will contribute to the raceway or cable, it doesn't add any more heat when it is only carrying unbalanced current.

Take a single phase 120/240 system with 10 amps each ungrounded conductor - you have two conductors carrying 10 amps, the neutral is carrying zero. 10+10+0=20 amps of current altogether between the conductors to cause heating effects in the cable or raceway. Now unbalance it to 10 on one line and 5 on the other, leaves the unbalance of 5 amps on the neutral, but there is still 10+5+5=20 amps of current altogether to cause heating effects in the cable or raceway.

Now consider that on a three wire multiwire circuit (two lines and neutral) from a wye system, because of the phase angle involved there the neutral doesn't handle unbalance current the same way, but does cancel out if you are carrying unbalance of all three phases.

 

[Smart $]

Thank you.

I guess I'm trying to get it in my head why there isn't current in the neutral conductors from switchboard to transformer because of those 277VAC single phase loads. There aren't any intervening transformers, etc. just various circuit breakers at the appropriate points. (switchboard main, etc.) I'm assuming the single phase loads will be balanced across the 3 phases.

When the single-phase line-to-neutral load is balanced among the line conductors, they share the current passing through each other via the circuit neutral point, but no current travels on the neutral conductor between the circuit neutral point and the source neutral point.

When the load is unbalanced, what current can be shared among line conductors is, and the balance is carried from the circuit neutral point back to the source neutral point. The neutral conductor must carry this current to maintain the voltage where the loads connect between line and neutral.

Three phase balancing is similar to single phase balancing but is a little more difficult to imagine because the phase timing is 120° instead of 180°... but the timing is required to share current between three lines rather than two. In both cases, each cycle is 360°.

Welcome to the forum. :thumbsup:

 

[Carultch]

How about this:
Yes, because a Neutral is ALWAYS considered a Current Carrying Conductor. There really isn't any more than that needed.

Article 100 of the NEC, Definitions:

It is not as cut and dry as that.

In this context, "current carrying conductor" doesn't just mean a conductor that carries current. It means a conductor that carries enough current to generate heat you have to account for, when calculating conductor ampacity. In most cases, where all a neutral does is carry the unbalanced load that otherwise would be carried exclusively by the ungrounded conductors, or where a neutral never carries a single full ampere of current per the operation nature of the load and only is present for voltage sensing purposes, it is unnecessary to include it in the ampacity calculation.

To summarize the times when you do have to count it:
1. When it is a mandatory part of the return path of the circuit current, such that it is impossible to balance the currents amoung only the ungrounded conductors. Examples: phase-to-neutral circuit, 2-phases & neutral pulled from a 3-phase wye system.
2. When non-linear loads are significant. Particularly 3rd harmonics, because they accumulate on the neutral, rather than add up to zero.

 

[kwired]

It is not as cut and dry as that.

In this context, "current carrying conductor" doesn't just mean a conductor that carries current. It means a conductor that carries enough current to generate heat you have to account for, when calculating conductor ampacity. In most cases, where all a neutral does is carry the unbalanced load that otherwise would be carried exclusively by the ungrounded conductors, or where a neutral never carries a single full ampere of current per the operation nature of the load and only is present for voltage sensing purposes, it is unnecessary to include it in the ampacity calculation.

To summarize the times when you do have to count it:
1. When it is a mandatory part of the return path of the circuit current, such that it is impossible to balance the currents amoung only the ungrounded conductors. Examples: phase-to-neutral circuit, 2-phases & neutral pulled from a 3-phase wye system.
2. When non-linear loads are significant. Particularly 3rd harmonics, because they accumulate on the neutral, rather than add up to zero.

Actually OP was not clear on what was being asked, the neutral is a current carrying conductor unless you have no line to neutral loads, but only for the application of 310.15(B)(3) is it considered non current carrying if it only carries unbalanced current of the circuit. Choice of words used in that section make it somewhat confusing, as the conductor still carries current though it is considered non current carrying in this particular context.

 

[Smart $]

Actually OP was not clear on what was being asked...

Clear enough for me.

...
I'm asking whether the neutral conductors between the transformer and service entrance switchboard are current carrying conductors for deciding whether the conductors should be derated.
...

 

[kwired]

Clear enough for me.

I went back and read OP again, for some reason I missed him asking about "for deration purposes" which led me to thinking he wasn't so clear on what he was asking about.

Sorry about that.

 

[Jraef]

I went back and read OP again, for some reason I missed him asking about "for deration purposes" which led me to thinking he wasn't so clear on what he was asking about.

Sorry about that.

Yeah, I missed that too. I saw CCC and stopped at that.
My bad.

 

[denders]

Thank you all for the comments and information. I have worked a long time in an industrial segment where I have never done a large transformer w/ a wye secondary, always delta, and I'm wanting to make sure I look at everything and don't 'shoot from the hip'.

Thank you again.

 

[don_resqcapt19]

...
Take a single phase 120/240 system with 10 amps each ungrounded conductor - you have two conductors carrying 10 amps, the neutral is carrying zero. 10+10+0=20 amps of current altogether between the conductors to cause heating effects in the cable or raceway. Now unbalance it to 10 on one line and 5 on the other, leaves the unbalance of 5 amps on the neutral, but there is still 10+5+5=20 amps of current altogether to cause heating effects in the cable or raceway.
...

But since the heat is really an I²R issue there is only 3/4s of the heat in the second example.

 

[kwired]

But since the heat is really an I²R issue there is only 3/4s of the heat in the second example.

I didn't write the NEC rules we have to follow on this

 

[Smart $]

But since the heat is really an I²R issue there is only 3/4s of the heat in the second example.

Unbalance (a, b, n) to 10+0+10=20A and you have the same amount of heat as 10+10+0=20A

 

[kwired]

Unbalance (a, b, n) to 10+0+10=20A and you have the same amount of heat as 10+10+0=20A

Yup.

 

[don_resqcapt19]

Unbalance (a, b, n) to 10+0+10=20A and you have the same amount of heat as 10+10+0=20A

But the example I was talking about is below. There is less heat in the second example.

you have two conductors carrying 10 amps, the neutral is carrying zero. 10+10+0=20 amps of current altogether between the conductors to cause heating effects in the cable or raceway. Now unbalance it to 10 on one line and 5 on the other, leaves the unbalance of 5 amps on the neutral, but there is still 10+5+5=20 amps of current altogether to cause heating effects in the cable or raceway.