Sizing of Neutral

[Clb123]

We are getting ready to start a job and the Utility Transformer will be supplying the service with a 480/277 3phase 4wire system. This project doesn't have any 277volt loads. The feeder size is (4) sets of #350's for 1600amp. What is the minimum size I could use for the neutral conductor?


Also this job has an existing emergency generator that is to be rerouted to feed this new service. It does not have a neutral conductor in its raceway that I have to intercept and reroute to the new transfer switch. If there are no 277volt loads is there a need to install a neutral conductor in this existing raceway?

 

[texie]

We are getting ready to start a job and the Utility Transformer will be supplying the service with a 480/277 3phase 4wire system. This project doesn't have any 277volt loads. The feeder size is (4) sets of #350's for 1600amp. What is the minimum size I could use for the neutral conductor?


Also this job has an existing emergency generator that is to be rerouted to feed this new service. It does not have a neutral conductor in its raceway that I have to intercept and reroute to the new transfer switch. If there are no 277volt loads is there a need to install a neutral conductor in this existing raceway?

I think I have more questions here than answers. It would seem you are referring to service conductors not feeders. That would require sizing of the minimum neutral to 250.66. But your 4 sets if 350's will only give you 1240 amps.
For the generator more info it needed as this will depend on where the generator neutral is bonded.

 

[raider1]

250.24(C)(2) is the code section that specifies the size of the required grounded conductor that is brought to the service equipment when the service is made up of parallel conductors.

Chris

 

[kwired]

We are getting ready to start a job and the Utility Transformer will be supplying the service with a 480/277 3phase 4wire system. This project doesn't have any 277volt loads. The feeder size is (4) sets of #350's for 1600amp. What is the minimum size I could use for the neutral conductor?


Also this job has an existing emergency generator that is to be rerouted to feed this new service. It does not have a neutral conductor in its raceway that I have to intercept and reroute to the new transfer switch. If there are no 277volt loads is there a need to install a neutral conductor in this existing raceway?

The grounded conductor can not be any smaller than calculated neutral load or values in 250.66 which ever is larger.

texie did bring up a potential problem though - 4 sets of 350 only gives you 1240 amps of conductor. This is not acceptable if supplying a single 1600 amp service disconnect, but can supply 2-6 disconnects (plus allowable additional disconnects like maybe a fire pump) if the calculated load is 1240 or less.

 

[Clb123]

Type

Type

Sorry the service is 1200 amp not 1600amp

 

[Smart $]

The grounded conductor can not be any smaller than calculated neutral load or values in 250.66...

...or parallel 1/0 AWG where in two or more raceways...

...which ever is larger.

Refer to 250.24(C)(2), as previously noted by Chris.

 

[texie]

Sorry the service is 1200 amp not 1600amp

Then your neutral for each conduit would be sized per 250.24(C)(2) and 250.66. In your case 250.66 shows a #2 CU, but since this does not meet the 1/0 minimum, you must use a 1/0 in each conduit.
Regarding the genset, you must have a neutral fault return path. Exactly how this is set up will depend on the design being a non separately derived or separately derived system. This is an import aspect to be safe and NEC compliant.

 

[augie47]

Is there an equipment grounding conductor in the generator feed ?

 

[kwired]

...or parallel 1/0 AWG where in two or more raceways...

Refer to 250.24(C)(2), as previously noted by Chris.

Correct, not that it can't happen, but often when paralleling ungrounded conductors you have enough capacity the neutral is going to be 1/0 or larger but one must not forget that in general you can not parallel conductors smaller than 1/0.

Equipment grounding conductors however could be smaller than 1/0 in a similar parallel application, and in general would not need to be larger than 1/0 until the overcurrent device is over 600 amps.

 

[Clb123]

Is there an equipment grounding conductor in the generator feed ?

We are not sure at this point we haven't visited the job site as of yet and we are going off a general note on the drawings to extend the existing generator feed to the new transfer switch

 

[ecuinc]

Neutral Sizing

Neutral Sizing

I also have a question on sizing a neutral conductor. I have a small pump control panel that is 240v single phase 40 amps. In this panel it will have two pilot lights, one starter and a level control relay. I plan to fed the panel with 2 - #6 THHN and 1-#10 THHN ground. The conductor was sized for voltage drop. I would like to add a #10 THHN neutral and use one leg for my 120 control power. My control load will be less than 3 amps. Can anyone tell me where in the NEC code I can find this.

 

[augie47]

Your basic Code guide would be 220.61 which basically states your neutral will be size to supply the calculated load. With feeders you also need to keep 215.2 in mind for a minimum.
This situation may be a bit tricky however since you have increased your wire size due to voltage drop, 250.122 requires you to increase your equipment ground proportionally and 215.2 would require the neutral to be sized accordingly.
Without calculating, I would think your eq,ground and neutral in this case might need to be a #8.

 

[kwired]

For feeders the grounded conductor can not be any smaller than what is required for an equipment grounding conductor.See 215.2(A)(2).

For services it can not be any smaller than the required grounding electrode conductor. See 250.24(C)

I thought we were limited to the EGC size being minimum for branch circuits as well but currently am not finding it.

 

[ecuinc]

I will check that out.

Thank you for your help

 

[ecuinc]

Table 250.122 shows EGC for a 60amp to be #10. So wouldn't a #10 ground and #10 neutral be good for a 40amp feeder?

 

[augie47]

Bit of a twist in when 250.122 comes into play.
No question, if you install a 60 amp circuit and use a #10 equipment ground you have met Table 250.122
However......
250.122(B) states IF you increase the ungrounded conductors you must also increase the eq ground proportionally so when you increased your 40 amp circuit to a #6 for voltage drop reasons that comes into play.
A #8 would be "standard" for most 40 amp circuits. When you upsized to a #6 that was a 16510 to 26240 kcmil increase or 1.59% increase. The 250.122 equip. ground for a #8 is normally a #10. It's 10380 are would need to be increased 1.59% to 16504 kcmil or a #8.

 

[ecuinc]

Bit of a twist in when 250.122 comes into play.
No question, if you install a 60 amp circuit and use a #10 equipment ground you have met Table 250.122
However......
250.122(B) states IF you increase the ungrounded conductors you must also increase the eq ground proportionally so when you increased your 40 amp circuit to a #6 for voltage drop reasons that comes into play.
A #8 would be "standard" for most 40 amp circuits. When you upsized to a #6 that was a 16510 to 26240 kcmil increase or 1.59% increase. The 250.122 equip. ground for a #8 is normally a #10. It's 10380 are would need to be increased 1.59% to 16504 kcmil or a #8.

Ok I got it.

Thank you Augie47

 

[augie47]

of course...depending on your scruples....
you can also say "I ran a 60 amp circuit out there in case it was ever needed" and never mention voltage drop and use your #10

 

[kwired]

of course...depending on your scruples....
you can also say "I ran a 60 amp circuit out there in case it was ever needed" and never mention voltage drop and use your #10

That is one flaw I have always hated about that rule. How come a 10 AWG protecting a 40 amp circuit isn't large enough but put same conductors on a 60 amp breaker and it now is:?

I understand the intent of the rule, but it has some holes in it IMO.