Sorry, I have read the threads and I know this has been beat into the ground but... please give me your opinion on how to feed my subpanel.
I have a 200 amp service on the main house and need to go 100' through the attic and down the wall, then through the footing and underground another 50' to the 1500' shop for a total run of 150'. My electrician friend wants me to run a 2-2-2-4 romex through the attic to a junction box at the plate, then #2 THWN in conduit to the shop. I believe that this would need to be protected by a 90 amp breaker at the service panel.
First critical question: Was this recommendation based on use of copper wire, or aluminum? This makes a HUGE difference.
Notably, AWG 2 aluminum is limited to 90A by code; and Type NM-B, a.k.a. "Romex", is further limited to 75A. But either way, over a 150-foot run that would produce excessive voltage drop at anything over 50-75A (depending on which "threshold", 2% or 3%, you consider acceptable for a feeder).
Now, if your "electrician friend" was assuming copper all the way, that could potentially provide up to 115A to the sub-panel, depending on the specific cable types used (and the temperature ratings of their insulation/jackets). Voltage drop would be "OK" (but only "OK"), at about 2.4% @ 100A.
The romex is very pricey, and I think I would rather just run 1 1/2" conduit the whole way and pull 3 #1 THWN and a ground. I can buy a 500' roll of #1 for less that the 100' of romex. Then I believe that I could use a 100 amp breaker at the service panel.
Again, copper or aluminum? With aluminum THHN/THWN, AWG 1 would be code-compliant at 100A; but it would still produce more voltage drop (by a little or a lot, again depending on the "acceptable threshold" chosen) than you really want over that length run. And remember, THHN/THWN would need to be in conduit the ENTIRE way from one panel to the other, including the runs inside the house. That could be a major PITA.
Is this the correct wire size to stay within 3 percent voltage drop over this distance?
As noted above, no. But at least in the last case (AL AWG 1 THHN the whole way), if you consider 3% acceptable, you'll likely also consider 3.167% to be "close enough".
Use 100 amp aluminum ser to go from panel to outside ,then switch over to pipe and #4 thhn to garage. You could then use a 100 amp breaker but legally I think you have to use 75 amp breaker.
As others have pointed out, this is Just Plain Wrong.
I like the Idea of conduit I am a conduit fan, I prefer conduit,
The minimum you will need is A #3 copper THWN
Make that "DEAD minimum"; IMCO, it would be an edge case, at best.
#2 and #1 may be readily available at the box stores. I prefer copper when it is in the budget.
As do I, in general; but it is indeed considerably more damaging to the checkbook. AWG 2 copper would also keep voltage drop well below 3%; but to keep it below 2% would require AWG 1 copper -- and that starts to get pricey indeed (~$2/ft., x3).
Lets discuss Aluminum as it is more economical
#1 Aluminum XHHW (the insulation I prefer on aluminum). You will need (3 ) #1 XHHW and (1)#6 aluminum ground
But that would bring voltage drop back up over 3% (@ 100A load).
#1 Aluminum will be fine for voltage drop based on 240 volt. 90 amp load at 3 percent
Yes, if we limit things to 90A, we squeak by at just under 3% V.D..
Voltage drop is subjective. Code recommends 3 percent branch and a 2 percent on feeders. Combined for a 5 percent drop total. It is recommendation only not required.
I would definitely NOT call it "subjective". It is what it is, and can be calculated precisely; that is completely objective. Those limits being in the form of a "recommendation" as opposed to a hard-and-fast requirement puts it at least somewhat into the realm of a value judgement; and thus, it can be subject to some degree of "adjustment" in the field as the situation warrants. But the recommendation itself
IS based on good, solid OBJECTIVE reasons. And for these same reasons, you really don't want to significantly exceed the recommendation; and staying well under it is better yet.
I would install a second conduit to your out building for systems such Data/voice/security/cable etc.
Good idea.
Use 1-1-1-3 aluminum SER for the inside run, not Romex (NM-b).
I agree with the SER recommendation. But as noted above, the "1-1-1-3 aluminum" part would be a bit "iffy" in terms of voltage drop. In practical terms, it would probably OK in the overall scheme of things, presuming he doesn't habitually load down that feeder cable for all it is worth; but it would still be better to go with a larger cable. It would be a bit pricey, but there
IS this:
http://www.elliottelectric.com/Products/Detail.aspx?v=COP&c=SER241000
or even this:
http://www.elliottelectric.com/Products/Detail.aspx?v=COP&c=SER13WG500
either of which would kill several birds with one stone (including eliminating the need for conduit inside the house, which will help offset some of the cost).
Thanks for the replies. Going around the house all underground would add another 80' to the run. Going through the attic is a straight shot.
If you say so; and lacking any sort of drawing/diagram, I have no choice but to take your word for this. But it does seem a bit odd that it would still be "shorter" to snake up/down/sideways through the house interior for ~100 feet, before even hitting the trench. Where within the house is the main service panel located? Typically, these are put into basement or garage areas. If so in your case, it stands to reason that there MUST be some shorter/straighter route to an appropriate outside location than going all the way up to the attic and back down again. FWIW, and all that.
As far as the other recomendations about smaller wire, I would like to have a full 100 amps at the shop. I may add on to the building later and dont want to need to upgrade.
In which case, size EVERYTHING generously from the get go. Don't even attempt to "squeak by" on such things as voltage drop, etc.; you'll be far happier in end, long after the pain of writing the check is but a dim foggy memory.
No I am not kidding.
