Sub Panel wiring Q

[Jamisjockey]

What wiring do I need to run a 100 amp sub panel?  I'm going to have an electrician do the connections, but it's retarded of me to pay for them to supply and run the wiring.  It'll get run through the attic from the South side of the house to the attached garage. 

[zxcvbob]

Off the top of my head, you need 4 gauge copper with 75°C or better insulation (like THWN).  The actual ampacity is only 85A, but there's a note in the NEC to specifically allow it.  You can also use 2 gauge aluminum with the right connectors and black goop.

You might wanna verify this before you buy a bunch of expensive wire or cable.

You said 100A, but is that the panel rating or the actual size circuit you want?  (you can use a 100A panel for 60A or 70A service and run smaller wires)

[cassandra and sara's daddy]

That how I remember it too but it's been a couple years so I'd look it up in case they changed requirements. And don't buy it 5 feet too short because you forgot to allow for a run. It's a special feeling


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[zxcvbob]

There appears to be much disagreement online among people who should know, whether you need #3 or #4 copper.  (assumes  75°C)  The relevant section in the 2011 code is 310.15(B) Table 7, and they are arguing about how to interpret it.

Note that you cannot get by with 90°C smaller wires, because the lugs in the panel are only rated for 75°.

[cassandra and sara's daddy]

When in doubt use the heavier rated product but paying for that kinda wire is no joke.
I've never been quite sure what to allow for drop in long runs too so I error on the side of caution

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[zxcvbob]

http://www.homedepot.com/p/Southwire-1-1-1-3-Aluminum-SER-Wire-By-the-Foot-13103790/205001799

[Jamisjockey]

http://www.homedepot.com/p/Southwire-1-1-1-3-Aluminum-SER-Wire-By-the-Foot-13103790/205001799

Roger that thanks. 

When in doubt use the heavier rated product but paying for that kinda wire is no joke.
I've never been quite sure what to allow for drop in long runs too so I error on the side of caution

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Yeah I figured expensive was in the cards but the project is a big part of why we bought this house.  The 20x40 garage and 12x40 office to be 

[K Frame]

Electrified midget walking!

[Hawkmoon]

Mike Holt's forum is where the pros hang out:

http://forums.mikeholt.com/

[zxcvbob]

Mike Holt's is where I looked, and they couldn't agree whether table 7 applied to a subpanel or not.

[Hawkmoon]

Mike Holt's is where I looked, and they couldn't agree whether table 7 applied to a subpanel or not.

Heh, heh --

The folks who hang out on Mike Holt's forum (particularly the electrical areas) are either licensed electricians or building (electrical) inspectors -- or both. If they can't agree on what the code says, it's unlikely you'll get agreement anywhere. In that case, it would be prudent to go with the more conservative (larger wire) approach.

[cassandra and sara's daddy]

So here's a question then. Do the screws that mount the switch plate count as "conductors in the box"


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[RocketMan]

If you're going the permit route, call your local inspector.  Most of them are happy to answer questions before you start the project.  And they can tell you about the little nooks and crannies of the local requirements, too.  That saved my bacon here recently when I installed my new 200 amp panel in the basement.  Found out I had to have AFCI breakers instead of standard units for circuits serving living areas.

[zahc]

If you're going the permit route, call your local inspector.  Most of them are happy to answer questions before you start the project.  And they can tell you about the little nooks and crannies of the local requirements, too.  That saved my bacon here recently when I installed my new 200 amp panel in the basement.  Found out I had to have AFCI breakers instead of standard units for circuits serving living areas.

Ignorant question: how does one locate his local inspector? Yellow Pages?

[RocketMan]

Ignorant question: how does one locate his local inspector? Yellow Pages?

They are usually with the permitting agency, at the city/town or the county level.  Depends on the jurisdiction.  Mine was at the county permit office.  Looked them up at the county website, their names and phone numbers were there.  Plus some information on requirements, but actually talking with the inspectors was the most helpful.

[cassandra and sara's daddy]

Sadly in some areas they are barred from advising you. "Liability issues"


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[zxcvbob]

So here's a question then. Do the screws that mount the switch plate count as "conductors in the box"

Not sure if serious.  

No.  But they are conductive (unless you use nylon screws) and should be grounded.  Nylons screws are a great way to deal with an inspector who's being a hardass about about grounding the screws when you have a nonmetallic box and a blank cover.

[cassandra and sara's daddy]

I had a nimrod inspector insist those metallic screws entered the box and counted as conductors. Was quite a lil argument for a while took 2 days to get him straightened out. Once upon a time the electrical inspector had to be an electrician usually retired. Not so anymore, they are cross trained. So we get plumbers doing electrical electricians doing plumbing etc. the results are as you might expect . Well usually they are, there was "the incident" where jack helped an electrical inspector out a window, an open one. Albeit on the second floor but the dumpster was right below it the fall was only 4 or so feet, unfortunately


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[Hawkmoon]

So here's a question then. Do the screws that mount the switch plate count as "conductors in the box"

Never heard of that as an interpretation. "Conductors" in code-speak is "wires" for the rest of us.

[zxcvbob]

Next time, remember the nylon screws.  You can change them out after he leaves.   

[Nick1911]

Only think I'll add is make sure you have a separate ground and neutral wire, and separate bus's in the sub-panel.

Inspectors usually look for that.  The only place the ground and neutral can be bonded is in the building's main panel board.

For a 100A subpanel, I recall running 2-2-2-4 aluminum SER.  That was for a short run though, you may need to upsise if your run is long enough to cause more then the allowed voltage drop at full ampacity.

[cassandra and sara's daddy]

They test the ground rods here now too. We drive two full length anymore just for one service panel. Never used too but it's less hassle than a rejection


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[cassandra and sara's daddy]

Never heard of that as an interpretation. "Conductors" in code-speak is "wires" for the rest of us.

I think it was a personality conflict and against all,odds the kid won. And that was not the one with the window


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[Nick1911]

They test the ground rods here now too. We drive two full length anymore just for one service panel. Never used too but it's less hassle than a rejection

And on that note, around here, they require that each ground rod be run with it's own copper wire to the main panel.  Size depends on ampacity.  I think when I did my 200 amp panel, the city required 2x ground rods, each with #4 AWG copper wire to the ground bus.

[cassandra and sara's daddy]

And don't cut one rod in 1/2 and drive em both in to try and fool em. Or so I hear


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