Need help with neutral sizing

[crtemp]

I am running 100 amps to a shop (approx 250 feet away) from a 200 amp panel. I am going to run the feeders in 1/0 aluminum and a #6 aluminum ground with it. My question is how do I go about sizing the neutral? The wiring is going to be for future use in the shop so there is really no way to know what any of the loads will be. Should I just run it in 1/0 also or is there a way of calculating a neutral size just based off the 100 amp service?
Thanks

 

[Dennis Alwon]

You can calculate the neutral for the feeder if you want by using art 220.61 or just run a full sized neutral since you don't know the loads. The neutral must be at least as large as the egc if you do decide to calculate the load.

 

[petersonra]

You can calculate the neutral for the feeder if you want by using art 220.61 or just run a full sized neutral since you don't know the loads. The neutral must be at least as large as the egc if you do decide to calculate the load.

It is unlikely that you will ever need a full sized neutral, but since you don't have any way to know, and the additional cost is not that much overall, I would run something at least well above the bare minimum.

A lot really depends on who you are running it for, what they anticipate doing with the space, and how cost sensitive they are.

Since you don't have any information on what loads will be connected just how did you come up with 100A in the first place?

 

[Ponchik]

Assuming the OP installation is for a dwelling with 120/240V system, The feeder is basically a multiwire circuit.

So, Lets assume future loads will be 120V only and the total connected load will be 20KW, and further assume that all of the loads will be ON at the same time. The maximum neutral current will be around 43Amps.

Am I not seeing this correctly??

 

[petersonra]

Assuming the OP installation is for a dwelling with 120/240V system, The feeder is basically a multiwire circuit.

So, Lets assume future loads will be 120V only and the total connected load will be 20KW, and further assume that all of the loads will be ON at the same time. The maximum neutral current will be around 43Amps.

Am I not seeing this correctly??

If all the loads are 120V and they are all on at the same time and they are balanced by line, than the neutral current would be zero.

The thing is that with 120V circuits there is a potential to have just the loads on one line on at a particular instance. in that case you would have 100A neutral current.

 

[crtemp]

There will be at least one 240 volt welder circuit and various other 120 volt circuits. I guess I will just pull a 1/0 neutral with it. Thanks for all the replys!

 

[suemarkp]

Any 240V only appliance would allow you to reduce the neutral. But I woould not since your feeder is so long. The voltage drop of a 1/0 neutral (and the ungrounded conductor) at that distance will kind of suck if you end up with a decent amount of unbalanced load. Making the neutral smaller will just make it worse.