# 100 amp subpanel from 100 amp main panel?



## Daniel Holzman (Mar 10, 2009)

To the best of my understanding, there is no code issue running a 100A subpanel off a 100A main panel, so long as the wire size is correct, and the installation is correct. For a subpanel, you need four wire service (two hots, a neutral, and an equipment ground). The neutral and the ground need to be separate at the subpanel. If you go with underground service to the subpanel, you need to use appropriate underground rated conduit, and appropriate wire (I believe THWN is acceptable in underground conduit). Of course you need to verify EVERYTHING with the local wiring inspector before you do any work, since local regulations may be different than NEC.


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## MTN REMODEL LLC (Sep 11, 2010)

Ditto as Dan....

I've had no issue running 200A sub from a 200A main several times.

The principal is not that uncommon when an addition is added at the location of the original main... often easier to install a new main and make the old main a sub.

Best


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## eholmes77 (Feb 12, 2014)

Thanks. I am probably going to have an electrician do the subpanel, and at the very least will hire one to inspect my work if I decide to do it myself.

Regarding the main panel, I understand upgrading it to 200 amps is not as simple as just swapping out the main breaker to one that is 200 amps and requires upgrading the whole panel and possibly the service. Is that correct, or is it sometimes a matter of just swapping out the main breaker? Of course, I would hire an electrician for anything dealing with the main breaker.


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## Oso954 (Jun 23, 2012)

While there might be an exception somewhere, it has never been just swapping the main breaker. The panel must be replaced. If the meter is on a seperate pan, the pan needs to be checked and possibly replaced. Assuming an overhead service, the wires up to the weatherhead need replacement. Increasing the wire size will almost always require that the riser pipe and weatherhead be replaced.

Whether the service drop from the pole to the weatherhead needs replacement depends on the POCO.


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## eholmes77 (Feb 12, 2014)

Thanks. Hence my desire to avoid replacing the main panel.


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## eholmes77 (Feb 12, 2014)

The two 100-amp panels will be 50 feet apart and I will bury the cable. What size cable or wire will I need? Is there an advantage or disadvantage to using cable versus four individual wires? Also, is aluminum acceptable? It is WAY cheaper.


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## Pharon (Apr 14, 2014)

Yes, you can certainly use aluminum for this feeder, but keep in mind that it's not apples to apples. A 100A copper feeder for this application is #3 AWG (plus a #8 equipment ground), and a 100A aluminum feeder is #1 AWG (plus a #6 equipment ground).


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## Arrow3030 (Apr 19, 2015)

Nothing wrong with 100A sub on a 100A service as others have said. The wire could be potentially as small as #4 copper. Most will recommend and feel better about at least #3 copper.


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## Pharon (Apr 14, 2014)

Arrow3030 said:


> The wire could be potentially as small as #4 copper.


Explain?

If you're referring to 310.15(B)(7), I thought the 83% rule only applied to service entrance conductors, not sub-panel feeders. No?


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## Daniel Holzman (Mar 10, 2009)

The advantage of individual conductors is that they pull far easier than cable through conduit, especially if you purchase stranded conductor wire, which pulls better than solid. Nothing wrong with aluminum, it is typically less costly than equivalent copper, but you need larger wire size, and a larger conduit, and you need to be careful to use aluminum compatible equipment.


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## Arrow3030 (Apr 19, 2015)

Yup. 310.15 (b)(7). 
Feeder conductors aren't required to have an allowable ampacity rating greater than their service entrance conductors.


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## WyrTwister (Jul 22, 2013)

You can rent a Ditch Witch and cut the ditch ( 24" or deeper for PVC conduit ) and lay the conduit in the ditch . Stub up with galvanized rigid steel conduit or schedule 80 PVC . Check your local codes .

Keep the inside of the conduit clean .

You can use direct burial cable , but if the animals chew into it , you are kind of in a bind .

Our local codes make it almost impractical to use Al wire . Most of our work , the specifications prohibit it . So , we do not use Al wire . Do as you think best .

God bless
Wyr


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## Pharon (Apr 14, 2014)

Arrow3030 said:


> Yup. 310.15 (b)(7).
> Feeder conductors aren't required to have an allowable ampacity rating greater than their service entrance conductors.


Ah, good to know. But just to be clear, in a scenario where you have a 200A service and a 100A sub panel, the 100A feeder needs to be sized at 100%, correct?


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## eholmes77 (Feb 12, 2014)

If I use cable rather than individual wires, what TYPE of cable should I use, assuming use PVC conduit rather than direct burial?


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## Pharon (Apr 14, 2014)

If you're going underground, the cable must be rated for a wet location, so UF should work. I don't believe you can use SER. I'm also not sure if either of those cables can be put in a raceway without derating, so you may have to size up.


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## WyrTwister (Jul 22, 2013)

I would not , under most circumstances pull cable through conduit . I would pull individual wires .

God bless
Wyr


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## poiihy (Aug 18, 2015)

Why do you need conduit if you use UF? Is it because it is not deep enough?


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## WyrTwister (Jul 22, 2013)

To protect the wire . If ground hogs chew into the direct burial cable , you and the gofer are going to have a bad day .

Also , in conduit , if a wire goes bad , you have a chance to re-use the old conduit with some new wire .

We use THHN wire that is also rated THWN .

100 amp circuit , I would run 1-1/4 " ( copper wire ) . Al wire , maybe 1-1/2 " ( which I would not use .

Check the price of 2" PVC . Depending on price , I might go 2" as " insurance " .

Keep the bends down to a minimum . Remember , wire pulling lube is tour friend . Especially for an amateur .

It has been so long since I used UF , I can not remember what the depth is suppose to be ? 24" , I think .

Best I remember , SDT cable is rated for direct burial . 

God bless
Wyr


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## Oso954 (Jun 23, 2012)

My reason for going conduit would be that it is a rural area and I do not want some burrowing rodent to be chewing on the Wires or cable. Personally, I would run it schedule 80 for the whole distance, it's only about 40 ft. I think the extra protection would be worth it. 

Remember that this is going to be the only feed to the main cabin, not a seldom used shed.

I would definitely run it as THWN, not UF.

A lot of beginners like cables because that is the majority of what they have seen or tried to work with. UF in the smaller sizes isn't that bad, but in larger sizes (eg 1 or 1/0 conductors) it is an absolute bear to work with.

Finally, if you do have a problem in conduit, you can often pull the conductors out and pull new conductors into it. With a direct buried cable, you just start digging.


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## WyrTwister (Jul 22, 2013)

I have rarely used schedule 80 . Best I remember , you use the same fittings ( couplings , etc. ) as schedule 40 . That means the schedule 80 is slightly smaller inside diameter . Keep that in mind when sizing wire and conduit .

God bless
Wyr


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## Arrow3030 (Apr 19, 2015)

Pharon said:


> Arrow3030 said:
> 
> 
> > Yup. 310.15 (b)(7).
> ...


In this case 310.15 (b)(7) can be used no matter the size of the service entrance because the feeder is more specifically a main power feeder for a dwelling unit.

I also recommend schedule 80 PVC and THWN-2 or the AL equivalent.


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