Correct wire for 100A subpanel feed?
davidmeiland
| Posted in Energy, Heating & Insulation on
Upcoming job requires a 100A subpanel in the detached garage, about 40′ from the house. I am assuming some large romex in the crawl space under the house, then a junction to an appropriate direct burial wire. One option that I know of is what’s called 2-2-4 around here (aluminum, individual conductors). My electrician mumbled something about another option (copper, I think) that would also work. What’s used where you’re at?
Replies
........1 1/4 conduit and #3Cu THHN
........1 1/4 conduit and #3Cu THHN
You win!
2-2-4 Al is OK for a 100A service, or for the main feeder to a dwelling unit, under 310-15(b)(6). But it can't be used for a generic 100A feeder.
........1 1/4 conduit and #3Cu THHN 2 inch or bigger.
2" is a bit large .... nothing wrong with that , but here its 1 1/4 , resi contractors are not going to put in something they don't need .....the pricing on pipe and fittings goes into the stratosphere as soon as you say 2" to the counterman
That is of course for pipe that has metal in it........LOL"
Barry: what is meant by a generic 100A feeder? Is that what I have?
All: there are very few occasions where I see buried conduit here. Lots of direct burial wire instead. 2-2-4 has to be in a conduit riser starting 18 or 24" below grade, I forget which. So, the riser has a service ell on the top popping into the building.
The main panel in the house is about 30 feet from where the subfeed will enter/exit, so I figured it would be easier to junction and convert to a NM cable, rather than running conduit to the main panel to continue the 2-2-4. In any case I can't get conduit into the main panel without opening a wall, so the plan was to fish the NM.
There will be a breaker in the main panel.
A "feeder" is any length of wire between the service entrance and the final overcurrent protection device. The line from your main panel to the garage subpanel is a feeder.
Normally, feeder ampacity is computed the same way as branch circuit wiring, according to Table 310-16 in the NEC. For 90 degree C insulation, that's #3 Cu and #2 Al. You can use THHN in conduit. Or you could direct-bury wires with the suitable direct-bury rating.
There's another section of the NEC (Table 310-15(b)(6)) that allows a bit smaller wire for Service Entrance cables, and for the case where an entire dwelling unit is fed by a feeder between a Service Disconnect located by the meter, and the main panel in the Dwelling Unit. You can use #2 Al in this case.
A detached garage doesn't meet the NEC definition of "dwelling unit", so you can't use Table 310-15(b)(6).
Hmmm.... now I'm confused. The buried 2-2-4 is the typical install around here from a main panel to a shop or garage. Based on what you are saying it's not NEC approved. Also, your term 'final overcurrent protection device", in this case I assume that would be the 100A breaker in the main panel, because I assume there WILL be a breaker in the main panel and a simple shutoff in the subpanel rather than a breaker there.
This may be too much to explain to a carpenter. I have an electrical inspection on my other job on Monday and I'm going to ask the inspector what he will approve.
If it's OK with the local inspector, then it's OK. The NEC is just a recommendation from the NFPA on what they consider safe electrical practices. It has no force of law by itself. Local jurisdictions make their own exceptions and extensions all the time.
.......woohoo, I accept cash or checks.............hahahah"
2-2-4 all the way from box to box.
Use the direct bury. But run in 2" conduit.
Add a #4 copper for ground from box to box.
Add a new ground for the new box anyway. Complete ground just like the main box.
Of course the #4 copper is "do as I say, and not as I do". LOL I simply ran mine with 2-2-4 direct bury, no number 4 copper ground from box to box. Then put a complete ground in at the sub-box. Since it is a separate building.
Of course, I also ran the sub box off of a 60 amp 220v breaker, instead of feeding directly to the power lugs in the main box. This gives me a main shut off somewhere away from the sub-panel. And I like the idea of that extra breaker protection anyway.
If you are going to feed directly from the power lugs, put that #4 copper ground in there from box to box...
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>>Of course, I also ran the sub box off of a 60 amp 220v breaker, instead of feeding directly to the power lugs in the main box.Most electricians in my area I know say you shouldn't double tap off the main lugs (and if you do, they should be fused within a couple of feet, not just at the remote site)
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Your point is ?I didn't tap off the main lugs. As I said.... I put a 60 amp 220 breaker in the main box and tapped off of that, instead of off the main lugs.
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AHJ told me #2 copper (#1 al) for 100A subpanel (in the attic). On their advice, went with #6 cu to 60A subpanel.