Derate neutral

[karn]

I've been told to pull #10 neutrals on 20a and 30a branch circuits that were up sized to #8/#6 due to voltage drop (although some probably did not have to be up sized because they really did not go that far, the print just called for it to be up sized, also the print really just specifies the wire size for the breaker if that helps any)

 

[JFletcher]

Are they 240V split phase or 208V 3 phase circuits? If the neutral is a CCC of a single phase/single leg system, I believe it has to be sized the same as the ungrounded conductor. Dont have a code ref; Im sure someone will shortly.

 

[karn]

Are they 240V split phase or 208V 3 phase circuits? If the neutral is a CCC of a single phase/single leg system, I believe it has to be sized the same as the ungrounded conductor. Dont have a code ref; Im sure someone will shortly.

Its 208v 3phase system, feeding mostly 120v duplex receptacles, air curtains, one feeds a 208/120v deli hot case but I'm sure the neutral in the hot case is just for the lights and maybe a recep

 

[ActionDave]

Are they 240V split phase or 208V 3 phase circuits? If the neutral is a CCC of a single phase/single leg system, I believe it has to be sized the same as the ungrounded conductor. Dont have a code ref; Im sure someone will shortly.

Here's something to look at,

210.19 Conductors — Minimum Ampacity
and Size
(A) Branch Circuits Not More Than 600 Volts.
(1) General. Branch-circuit conductors shall have an am-
pacity not less than the maximum load to be served. Where a
branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable am-
pacity not less than the noncontinuous load plus 125 percent
of the continuous load.

240.23 Change in Size of Grounded Conductor
Where a change occurs in the size of the ungrounded con-
ductor, a similar change shall be permitted to be made in the
size of the grounded conductor.

 

[Smart $]

I've been told to pull #10 neutrals on 20a and 30a branch circuits that were up sized to #8/#6 due to voltage drop (although some probably did not have to be up sized because they really did not go that far, the print just called for it to be up sized, also the print really just specifies the wire size for the breaker if that helps any)

There is no NEC requirement to upsize any conductor for voltage drop for all but a couple special purpose branch circuit wiring. Additionally, there is no requirement to upsize all conductors of a circuit equally when upsized for voltage drop. However, 250.122(B) requires upsizing the EGC proportionately to the upsize of the ungrounded conductors.

 

[kwired]

How do you size the ungrounded conductor for voltage drop if you didn't have a size for the neutral established before hand?

Actual voltage drop when you have just one phase to neutral in operation will be dependent on both conductor sizes and length. If both are same length and size then half the drop occurs in each conductor. If they are not same size or length, you need to calculate each segment of the circuit with different characteristic separately.

 

[Smart $]

How do you size the ungrounded conductor for voltage drop if you didn't have a size for the neutral established before hand?

Actual voltage drop when you have just one phase to neutral in operation will be dependent on both conductor sizes and length. If both are same length and size then half the drop occurs in each conductor. If they are not same size or length, you need to calculate each segment of the circuit with different characteristic separately.

Your last paragraph answers the question of the first.

 

[kwired]

Your last paragraph answers the question of the first.

Well yes, my point was if you are increasing conductor size because of voltage drop you typically increase all conductors of the circuit not just one of them.

If drop is severe enough that a 20 amp circuit is being increased to #8, then I doubt you would want a 12 AWG neutral, if anything increasing both grounded and ungrounded to 10 AWG is a better trade off VD wise then having 8 on one conductor and 12 for the other - didn't run any calculations, just a WAG.

 

[Smart $]

Well yes, my point was if you are increasing conductor size because of voltage drop you typically increase all conductors of the circuit not just one of them.

If drop is severe enough that a 20 amp circuit is being increased to #8, then I doubt you would want a 12 AWG neutral, if anything increasing both grounded and ungrounded to 10 AWG is a better trade off VD wise then having 8 on one conductor and 12 for the other - didn't run any calculations, just a WAG.

I'm in agreement. But apparently not everyone is on the same page.

However, in a situation with a multiwire branch circuit where the likelihood of an unbalanced load is minimal, running a Code-minimum size neutral might save that extra penny someone is so desperate to see. :blink:

 

[kwired]

I'm in agreement. But apparently not everyone is on the same page.

However, in a situation with a multiwire branch circuit where the likelihood of an unbalanced load is minimal, running a Code-minimum size neutral might save that extra penny someone is so desperate to see. :blink:

If desparate enough they just take a chance on letting the neutral float

 

[rhovee]

I have been in the same boat you are. We did a study with computers and power supplies and studied the current going back on the neutral and it could exceed 20 amps because they were non linear loads with lots of capacitance in the circuit. They would have us pull double the conductor size for neutrals on a transformer also. This was at Microsoft.

 

[kwired]

I have been in the same boat you are. We did a study with computers and power supplies and studied the current going back on the neutral and it could exceed 20 amps because they were non linear loads with lots of capacitance in the circuit. They would have us pull double the conductor size for neutrals on a transformer also. This was at Microsoft.

Capacitance wasn't the main factor it was harmonic currents. And you needed to be on a neutral conductor that was common to more then one ungrounded conductor. A two wire circuit would have the same current on both lines, a multiwire circuit would have additive effects of harmonic currents on the neutral -and it is worst with neutral conductors of wye sources.