Sub panel feeders

[Dmrod@NASD]

It has been awhile since I had to do any conductor size calculations, derating, voltage drop and such. So to simplify; my code book knowledge is way behind the times of today's NEC coded book. That said; we are constructing a storage shed and looking at installing a 100 AMP sub panel, single phase, 240volt; to supply power to the shed. My first review was leading to #4 or #2 copper. However, looking at some addition info, due to distance, suggests possibly 1/0 copper. The distance from the main panel to the sub is approx 140'. The conductor (THHN or similar) will be in sch40 conduit(pvc) buried 18". can someone quickly state what the minimum conductor size and grounding wire size would be for this distance?

 

[retirede]

NEC minimum would be #3CU with #6 EGC.

But it sounds like you are concerned with voltage drop since you mention length. That’s a design consideration. You need to know the actual load and maximum allowable voltage drop in order to calculate.

 

[Dmrod@NASD]

NEC minimum would be #3CU with #6 EGC.

But it sounds like you are concerned with voltage drop since you mention length. That’s a design consideration. You need to know the actual load and maximum allowable voltage drop in order to calculate.

 

[Dmrod@NASD]

Thanks for the response.
Much appreciated.
Looking at cost of materials....Is use of Al wire acceptable in this case? rule of thumb use to be to up the wire size two points when going from copper to aluminum.
I would need to review the equipment specifications to look up allowable voltage drop.
And refer to the code book to refresh my memory on the calculation.

 

[electrofelon]

Thanks for the response.
Much appreciated.
Looking at cost of materials....Is use of Al wire acceptable in this case? rule of thumb use to be to up the wire size two points when going from copper to aluminum.
I would need to review the equipment specifications to look up allowable voltage drop.
And refer to the code book to refresh my memory on the calculation.

I would never use copper in that situation. I would probably buy AL "Mobile home feeder" which has all four conductors Plexed together.
Of course you can also run single conductors, usually around here single aluminum comes in XHHW.Generallyy I would use number two, a 90A breaker, and inch and a quarter PVC. Sounds like you might want to go up a couple sizes.

 

[infinity]

The distance from the main panel to the sub is approx 140'. The conductor (THHN or similar) will be in sch40 conduit(pvc) buried 18". can someone quickly state what the minimum conductor size and grounding wire size would be for this distance?

Without VD compensation as retirede stated #3 Cu would give you the minimum of 100 amps, with a #8 EGC. #4 is too small. #1 Al would also give the minimum of 100 amps. You can try a VD calculator to see how much you need to increase the conductor size for VD compensation. As stated you would need to know the total current.

 

[LarryFine]

rule of thumb use to be to up the wire size two points when going from copper to aluminum.

Go by actual published ampacities, not by rules of thumb.

 

[wwhitney]

Go by actual published ampacities, not by rules of thumb.

But it is a very good approximation to what the tables say for AWG Cu sizes. Where you call 250 and 300 kcmil the next two sizes after 4/0 AWG.

Up through 4/0 AWG, the 60C and 75C Cu ampacities of Cu are the same or 5A less than the respective Al ampacities for two sizes larger. For 90C ampacities the same is true except 2/0 Cu is 195A and 4/0 Al is 205A

Cheers, Wayne

 

[electrofelon]

But it is a very good approximation to what the tables say for AWG Cu sizes. Where you call 250 and 300 kcmil the next two sizes after 4/0 AWG.

Up through 4/0 AWG, the 60C and 75C Cu ampacities of Cu are the same or 5A less than the respective Al ampacities for two sizes larger. For 90C ampacities the same is true except 2/0 Cu is 195A and 4/0 Al is 205A

Cheers, Wayne

Yes they are very close. Probably the most likely case of the 5 amp difference being an issue is when using the next size up rule. For example 2/0 copper is 175 which is a standard size, 4/0 aluminum is 180 which could then next size up to 200.

 

[kwired]

And not much has changed code wise in many years for the tasks inquired about.

 

[Dmrod@NASD]

I want to thank everyone for chiming in on this topic. I have had the time to sit and review the project and pull the info together for calculating the VD (to be no greater than 3%); and then sizing the conductors accordingly based on the length of run and the equipment that will be placed in the shed...it looks like #2 Cu is required.