Armed Polite Society
Main Forums => The Roundtable => Topic started by: Jamisjockey on September 29, 2014, 03:41:40 PM
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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.
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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)
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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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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°.
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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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http://www.homedepot.com/p/Southwire-1-1-1-3-Aluminum-SER-Wire-By-the-Foot-13103790/205001799
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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 [popcorn]
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Electrified midget walking!
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Mike Holt's forum is where the pros hang out:
http://forums.mikeholt.com/
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Mike Holt's is where I looked, and they couldn't agree whether table 7 applied to a subpanel or not.
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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.
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So here's a question then. Do the screws that mount the switch plate count as "conductors in the box"
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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.
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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?
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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.
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Sadly in some areas they are barred from advising you. "Liability issues"
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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.
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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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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.
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Next time, remember the nylon screws. You can change them out after he leaves. ;)
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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.
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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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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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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.
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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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Was warned about that too. Apparently they have some conductivity meter around here which can tell if the ground rods are full size.
Not sure I buy it, with everything being all connected when the inspector arrives. Regardless, proper grounding is a good idea. Ground rods are cheap and easy to install. [With a clever water drill and a fitting size air hammer anvil, carefully TIG'd together from an old bit of air chisel and a 13mm socket from a garage sale. =) ]
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I use a hammer drill but we have rock layers in some areas it's hard
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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.
2-2-2-4 AL SER is perfectly fine for... [ahem] "120/240-volt 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that supply the total load to a dwelling unit and installed in raceway or cable with or without an equipment grounding conductor. The grounded conductor shall be permitted to be not more than 2 AWG sizes smaller than the ungrounded conductors..."
The point of contention is the "total load to a dwelling unit" part. 1-1-1-3 AL can handle 100A without any qualifiers. He should be able to get away with 2-2-2-4, but that's at the discretion of the inspector.
(I used 4-4-4 aluminum "Periwinkle" triplex cable, overhead to my detached garage for a 60A feeder, then transitioned to two #6 copper and one #8 from the weather-head to the subpanel. That was code at the time but would be illegal today.)
If you really want to see something scary, look up the cost of 3-3-3-5 copper cable. :O
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2-2-2-4 AL SER is perfectly fine for... [ahem] "120/240-volt 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that supply the total load to a dwelling unit and installed in raceway or cable with or without an equipment grounding conductor. The grounded conductor shall be permitted to be not more than 2 AWG sizes smaller than the ungrounded conductors..."
The point of contention is the "total load to a dwelling unit" part. 1-1-1-3 AL can handle 100A without any qualifiers. He should be able to get away with 2-2-2-4, but that's at the discretion of the inspector.
That's good to know. I'll have to review that bit of the NEC. Thank you for pointing it out to me.
I use a hammer drill but we have rock layers in some areas it's hard
Hammerdrill is a good idea. I probably would have used my rotary hammer, if I had a convenient way to attach the rod to the SDS drive output of the tool.
It's weird. Sometimes I do things "the hard way" because it's easier for me with my tools and skills then the suggested route.
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They make a "bit" for driving rods it's a hardened cup that twists it a lil and hammers it. It was 150 bucks.
And be careful a guy locally drove a ground rod in and nicked a gas line . It filled the basement and killed him the next day when he ignited it with a sparking tool. The word was that the gas leaking so slow and filtering through the soil filtered out most of the added scent. The utility guy marked the line as coming off the other corner of the house. It's a shame the guy was working in the basement and coulda caught the marking error. The line was visible in basement. I think he just assumed the markings were correct. As a guy who hit an underground service with a digging bar I know longer assume that
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Hammerdrill is a good idea. I probably would have used my rotary hammer, if I had a convenient way to attach the rod to the SDS drive output of the tool.
It's weird. Sometimes I do things "the hard way" because it's easier for me with my tools and skills then the suggested route.
a "T-post driver" works pretty well, as long as you don't hit a layer of solid rock. Even if you do, it works as well as anything else. When the rod get far enough in you can't use the driver, switch to a small sledge hammer.
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Sadly in some areas they are barred from advising you. "Liability issues"
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Very true. Here they will advise you on whether what you intend to do will meet code and pass inspection. That much is very helpful.
They can give general, high level advice. They are barred from telling you how to do the nitty-gritty of the actual work. And rightly so, I think.
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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
Yep. Most builders around here do the same. (Or did, when there was any building going on.)
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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.
That's not what the NEC calls for. The two ground rods (where two are used) are required to be bonded. Using two "home runs" to the same bus accomplishes that, but it's no better than jumping directly from one to the other, and just wastes wire.
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http://www.homedepot.com/p/Southwire-1-1-1-3-Aluminum-SER-Wire-By-the-Foot-13103790/205001799
Gee, thanks for that link. [/sarc]
Now Facebook is trying to sell me spools of wire!