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Adding a 100amp subpanel, total run is about 70 ft - and 1 1/4 Sch40 PVC is already in the ground. There are a fair amount of turns, it looks like:

Main Panel -> out rear hole through wall -> Pull box -> 90 -> 90 -> Pull box -> 45 -> 45 -> 45 -> 90 -> Sub Panel.

Trying to decide what cable to run for feeders. Initially was thinking 3 @ #3 Copper THHN and #6 Bare Ground as it would be thinner and easier to pull - especially due to the (45,45,45,90) run under concrete.

Then I read that XHHW is more flexible despite being slightly thicker (can't tell if this is true or good marketing) and of course #2 AWG Aluminum is an option - which I dismissed as being too hard to pull in 1 1/4" conduit with those bends.

Any practical advise for how hard it would be to get #2 Aluminum through those bends and/or if I should consider XHHW?

("Pull box" is a LB Conduit Body)

[Edited] Edited original post - I meant to say #3 Cu vs #4. My calc for #3 was assuming a QO 125Amp Sub panel (75C rating) and then I adjusted for ambient (Using 310.15(B)(2)(a)) by 1.11 (all underground and our average air is 58F) and ended up with 111 amp at 75C.

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    You know #4 copper is 85A wire, and #2 aluminum is 90A wire. You need to use the 75C column of 310.15(B)(16) because consumer tier panels are only rated 75C. Typical "100A subpanel" doesn't need anywhere near 100A so 85A is fine. May 12, 2022 at 6:21

3 Answers 3

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#2 AL/#4 CU isn't allowed to be protected at 100A in this application. It would be allowed for overhead, full load of a residential service, or for terminations (UL) Listed for 90°C. Your panel at best will be listed for 75°C so for a full 100A feeder you would need #1 AL/#3 CU for circuit conductors and #6 Al/#8 CU for ground.

You likely don't really need a full 100A, for pricing you could consider #2 AL "mobile home feeder" or other aluminum options and protect it with a 90A breaker in the service panel. You don't have to feed a 100A panel with a full 100A, the (100A) rating of a panel is the maximum allowed protection for the busing in the panel, so a 100A or more rated panel is the proper size for 90A feeder.

Aluminum does bend easier than copper, next time you at a hardware store go to the bulk wire section and grab a piece and see. I don't know what is available in your market, but not all brands of wire have the exact same surface and coating, which will effect pulling ease.

Any wire size discussed would legally fit, and the length of pulls looks like a maximum of 50 feet, so if you manage your wire so that it doesn't tangle when entering the conduit it shouldn't be a difficult pull. Pulling lube would make it slippery and easier to slide through conduit, but makes midpoint pull harder to handle, easier to pick up dirt, and tends to make a mess in the panels, so I would use sparingly only on the pulling heads, and wipe it off as soon as visible.

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  • #3 cu thwn may be a unicorn if shopping a box stores, but my experience is electrical distributors typically have it in stock and is priced competitive. For instance Platt.com shows 250,000 feet in stock at their stores, and is priced 20% less than #2. May 13, 2022 at 0:48
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Your wires are undersized for Full 100 Amps unless you use #2 copper, and #2 copper will cost a LOT. (#3 will actually work, but good luck finding that in stock, and it will still cost hugely)

You can do 90A on 2-2-2-4 aluminum "Mobile home feeder" which pulls just fine in 1-1/4" conduit and is very affordable. You can't put a 100 amp breaker on the input of that, though.

Price it out, calculate your actual loads, and ask yourself if the extra 10A is really worth the expense. The panel on the far end can be 100, or even 200 amps (200 amps usually comes with more breaker spaces, which are good to have) but the feed to the wire needs to be limited to 90A unless it's #2 #3 copper.

If you can find the unicorns (odd wire sizes not commonly stocked,) 1-1-1-3 Aluminum is rated for 100A and just fits in 1-1/4" schedule 40 (but not schedule 80) - the trick with 2-2-2-4 is that it's a dead common bulk commodity, which is not the case with three #1's and a #3.

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  • You are correct. Double-checked my calcs and meant to say #3 copper. Calc for #3 was assuming a QO 125Amp Sub panel (75C rating) and then I adjusted for ambient by 1.11 (underground and average air is 58F) and ended up with 111 amp at 75C. Curious why #2 copper required - do I need to derate for length? I don't think I need to derrate for bundling. I can start with a 90a breaker as it just for two new bedrooms plus a washer/drier - so my adjusted load is 7500 watts (32amp). Good to know that 2-2-2-4 pulls OK - that is certainly a more attractive option than #2 copper $-wise. May 12, 2022 at 14:08
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    #3 copper would also be legal for 100A at 75C rating, it's just "also a unicorn" and it's large copper and absurdly expensive. Edited.
    – Ecnerwal
    May 12, 2022 at 14:12
  • www.wireandcableyourway.com says they have #3 at $2.05/ft which is only 5c more than Platt. Which is still 2.5x the cost of 2-2-2-4 Al... May 12, 2022 at 14:20
  • From looking at wire OD dimensions online - I see #3 copper is 0.344" and #6 bare is 0.184" (1.216" total bundle) vs. 2-2-2-4 mobile feeder is 0.944" total bundle. ID of 1 1/4" Sch40 is 1.36". Surprisingly (to me) - this suggests the mobile feeder will be easier to pull than #3 copper. May 12, 2022 at 14:41
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I am assuming it is in the US. Your installation should not pass inspection. Reason per the NEC (National Electrical Code) limits the total number of bends in one continuous run to 360 degrees or four 90 degree bends. You have 360 the limit plus an additional 45. The code specifically states, “There shall not be more than the equivalent of four quarter bends (360 degrees total) between pull points, for example, boxes.” FYI, not a recommendation they sell a pulling compound available at the box stores that allows you to lubricate the wires making them easier to pull. I highly recommend before you go to all the work check with your local inspector to be sure it will be approved.

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    The 360 limit is "between pull points" right? My max bend between pull points is 225. Yeah - I'm planning on using pulling lube. May 12, 2022 at 5:50
  • That is correct. Placing another pull point will solve the problem or if you are lucky you already have one. I am assuming 225 is a typo for 360.
    – Gil
    May 12, 2022 at 5:56
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    FWIW - 225 isn't a typo. In my specific case, I have a max bend of (45+45+45+90) = 225 between the sub panel and the LB Conduit Body pull point. Which doesn't exceed the max NEC bend limit of 360 between pull points. May 12, 2022 at 6:02
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    Yes, as OP described it I get two pulling segments of 180 and 225. That's well within limits. May 12, 2022 at 6:20

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