Running from a new 200amp main interior in the attached garage, out a 150 amp breaker into 4/0 cable (4/0-4/0-4/0-2/0 assumed). The cable will travel about 30 feet inside through raceways and EMT (2in), then travel outside another 50 feet straight run also in EMT (2in). The EMT will get within 6 inches of the earth at the closest point (about 2 feet of travel), never terminating or touching/buried the earth.

Finally the cable will enter an exterior JBox where it will splice down to 1/0 Cu before re-entering the basement (also in EMT). Why the 1/0 splice you ask? I want to future-proof the main run with 4/0 Al so I could go up to 200amp in the future, but I can't use 4/0 into the existing 100A subpanel (the old main). So the 1/0 Cu is the perfect ampacity intersection between a 150A feeder breaker that can take 4/0 and the 100A subpanel. So even though it will initially be fused to 150A feeding a 100A panel, I want to size to a day when I replace the 150 feeder and 100a subpanel with 200 all the way (and replace the 1/0 Cu with 2/0 Cu).

Problems: NEC 338.12(B)(2): USE cable cannot be use for interior, and must terminate in outdoors enclosure. -- So this type is a no-go for the interior garage run.

SE and SER: (Can't run in the EMT at 90C from what I understand.)

Confusing. Brian

  • 1
    Is the cable going to be inside conduit the entire way, or will it be direct burial or direct attach to walls in some places? Nov 12, 2018 at 9:44
  • Inside conduit or raceway the whole way
    – bfoddy
    Nov 12, 2018 at 13:12
  • This is a well written question. But are you installing a 200A enclosed breaker or a 200A MCB panel in the garage? It appears you are trying to install a 150A feeder to a 100A subpanel and later upgrade the subpanel to a 200A, but you can't install a 150A main breaker to a 100A rated subpanel. If what I asked is true you need to upgrade your subpanel to 200A before you start to consider your feeder size, or make your new main breaker 100A. Nov 12, 2018 at 15:21
  • Installing a 200 amp main panel on the garage. Then coming off it will be a 150amp breaker(qo2150) that will feed the 100 amp sub panel. My understandment was I had to protect the wire to match the source breaker, and all my wire would be 150+ amp. Why would I need to protect the 100amp breaker?
    – bfoddy
    Nov 12, 2018 at 18:31

1 Answer 1


You don't want cable at all here, but individual wires instead

Pulling cable down a conduit is a waste of time/effort (as it's a pain in the rear to pull by all accounts I have heard) and fill space (as the cable's treated as round instead of oval/flat for conduit fill purposes).

Instead, what you want to be using are individual conductors -- the 200A-sized run can be made using 3 4/0 Al XHHW-2 wires for the phases and neutral -- this consumes much less fill than a 4/0-4/0-4/0-2/0 SER cable (529mm2 for the individual wires vs 1135mm2 for the cable) and is cheaper than the cable too, as you can use the EMT as an equipment grounding conductor here. Given that you can remark wires 4AWG and larger freely to neutrals with a wrap of white electrical tape at the terminals as per NEC 200.6(B), a single 250' spool of 4/0 Al XHHW-2 will get the job done here.

As to that Al to Cu splice...

The Al to Cu splice, by the way, should be made up using 2-port, Al9Cu rated mechanical splice connectors ("Polaris", "Unitap", or equivalent). These provide a reliable and rugged connection when torqued to specification with a torque wrench, which is a requirement in the NEC nowadays (as per 110.14(D)) in addition to being good installation practice anyway, and are simple to install (compared to split bolts). You'll also want to use an inch-pound torque wrench on the breaker and panel lugs as well.

  • Excellent detailed answer, just what I was needing.
    – bfoddy
    Nov 13, 2018 at 6:01

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