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I am installing a 60 AMP subpanel in a detached pole garage 150 feet from the house. Garage subpanel will be supplied from an existing 100 AMP subpanel in the basement of the house (running from the 200 AMP main is not an option). What size of wire and type of wire is required? Can it be direct burial with just short runs of conduits as it exits/enters the structures? Is a NEMA 1 load center suitable for mounting on the inside surface of the pole garage?

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    Do the other answers on this topic not answer your question? For example, diy.stackexchange.com/questions/41235/… – Hari Ganti May 9 '18 at 16:47
  • Are you planning for a minimum spec garage (1 SABC + 1 lighting circuit), or for the full 60A load, or something in between? Is underground the only option, or is an overhead run possible? How many square feet is the garage? – ThreePhaseEel May 9 '18 at 22:50
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    Garage is 24x36. Was thinking I would need 3-20A circuits and 1-15A lighting circuit. Would it be OK by code to use a 100A subpanel in the garage, fed from 100A subpanel in the basement? I would only need maybe 40 amps at a time, but having a larger subpanel would be more convenient than trying to squeeze 4 circuits into a 60A subpanel. I could still use a 60A breaker in the basement panel, couldn't I? Either way, looks like #2 Al feeder is needed (URD in conduit underground, then switching to SER when it enters the unfinished basement area).... – Chris May 11 '18 at 13:09
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Ditch the URD

Underground Residential Distribution cables are really uni-taskers: all they're good for is buried residential service from the utility. Most of the commonly available ones lack a ground wire (hot/hot/neutral), and they're often made from AA-1350/EC aluminum, which is picky about proper termination.

Instead, especially since you're considering using conduit anyway, I'd use individual XHHW-2 AA-8000 alloy conductors in a generously sized schedule 80 PVC conduit -- 2" would not be at all out of place for this run. A box can be used to make the transition to SER cable once you get into the basement, or you can simply keep going with the conduit run. Make sure to leave enough access points (pull elbows, conduit bodies, boxes) to make the run practical, and use expansion fittings in aboveground PVC runs subject to temperature changes!

As to conductor sizing, 2AWG aluminum should be fine for your purposes -- voltage drop will still be acceptable even at the full 60A you propose for this feeder. Given the amount of it you'll need (over 450') and that it's large enough for phase taping to be permissible for wire identification, it may be cheapest simply to buy a full 500' spool of the stuff instead of buying it by the foot. You'll also want some 6AWG bare copper for the grounding electrode conductor at the garage, and 10AWG bare copper for the equipment grounding conductor in the feeder run.

Keep 'em separated!

Speaking of grounding, you'll need to be careful on this point. Past practice with outdoor feeders treated them like services, bonded via the neutral, but this is no longer allowed -- instead, your feeder will be a full 4-wire feeder (hot/hot/neutral/ground) with neutrals and grounds separated onto their own bars and the loadcenter's bonding screw/strap removed.

Furthermore, you'll need to provide a grounding electrode at the garage and tie that to the grounding system: a concrete-encased (Ufer) electrode is ideal, or if your garage is steel-framed, you can tie into that. If neither of those are practical, a pair of ground rods driven at least 6' apart will do the trick.

Go big, or go home!

There is no shame in getting more loadcenter than one strictly needs -- it is often cheaper now to buy a somewhat larger loadcenter up front than to replace an undersized loadcenter later. Given that, the fact you'll need a main breaker loadcenter to provide the structure with a local disconnecting means (it's OK if the subpanel main is oversize as all it needs to be is a disconnecting means), and that a NEMA 1 cabinet will suffice for going inside the garage (all the space needs is four walls with doors that close and a roof that won't leak), I'd recommend a Siemens P3030B1100CU for this application as a minimum specification -- it provides plenty of spaces for an outbuilding application (30 spaces for a 100A panel is quite a bit), and ships with all the terminal bars needed (including separate ground bars). Of course, you are more than welcome to drop a few extra bucks on a larger panel (but you'll have to go up to 200A bussing to do so, most likely) -- a 42 space/200A loadcenter would not be at all out of place here.

TORQUE ALL CONNECTIONS TO SPEC

Last but not least, get an inch-pound torque wrench and torque all breaker and panelboard lugs you are connecting to up to manufacturer specified torque when you're doing this -- it's a new Code requirement for 2017 (see 110.14(D)), and just a darn good idea from a reliability standpoint, unless you want to pull a Greg Biffle with your garage's electrical system, that is.

  • Thanks for your suggestion on the underground run. I'll probably stick with SER in the basement. There will be a section that will run aboveground below a deck before it enters the basement. Should the transition from individual conductors in conduit to SER be done inside the basement, or under the deck? – Chris May 16 '18 at 16:33
  • Does the 60 Amp breaker in the basement for the garage panel need to be a GFI breaker? – Chris May 16 '18 at 16:38
  • If I do run URD unground, how is it picky regarding termination? – Chris May 16 '18 at 16:59
  • @Chris -- re: the feeder breaker: it can be a GFCI if you want every garage outlet to be GFCI protected, or you can provide GFCI protection to the individual branch circuits instead. As to the URD? It's picky because it's made from AA-1350/EC aluminum, the same stuff that gave aluminum wiring its bad rap back in the 60s and 70s, as opposed to the aluminum alloy (AA-8000 series) conductors required for modern aluminum building wiring. – ThreePhaseEel May 16 '18 at 23:02
  • @Chris -- also, why are you wanting to run URD within conduit? Can you not find XHHW-2 singles at the electrical supply houses (note: NOT big-box stores) in your area? – ThreePhaseEel May 16 '18 at 23:03

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