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I'm installing a new 100A subpanel, about 10' away from the main panel (so no concerns about voltage drop). I believe all wire and terminals on the main + subpanels are rated for 75° or higher, so AFAICT 3-3-3-5 copper SER would be the easiest cable to use here. However, due to local availability issues with small amounts of this cable, I'm wondering if there are other options besides copper SER.

Can I use 3x #3 AWG copper THHN and 1x #5 AWG copper THHN, run in 1-1/4" metal flex conduit, to feed the subpanel?

Per this conduit fill table, it seems that 1-1/4" flex conduit can take up to 5x #3 AWG conductors. But I'm not sure if there are other issues with using flex conduit or THHN wires for a feeder. I'm also not sure if derating would apply here since it's just 1 feeder circuit.

I'm also open to other cheaper or easier (or otherwise relevant) suggestions.

I'm in a NEC 2020 state.

p.s. I'm planning to run a load calculation before doing this, and will use a subpanel with lots of spaces but control circuit size via the breaker on the main panel. So need to spend time debating this part here :-)

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First let me cover two common errors.

A panel's bus rating (such as 100A) is merely a redline "never-exceed" maximum - like the 112 MPH rated tires on your car. You don't have to feed it 100A, all that's required is bus rating >= wire >= breaker. We advise PLENTY of breaker spaces.

The key to safety with large lugs is using a torque wrench to torque to spec. Without it, copper won't buy any safety. With it, aluminum is your friend. Most panel lugs are made of aluminum, and aluminum heavy feeders are proven reliable. This has an effect on the marketplace, which is the root of your problem.

As far as stiffness, people think the larger AWG of aluminum would make it stiffer, but copper is a very dense metal - almost 4x the density of aluminum and it pays for that in stiffness. So really, for same ampacity cables it's a wash.

There's no such thing as #5 wire, except for that bundled into pre-made cables. #3 is scarce enough.

At 10 feet we don't really care about the price. But since you clearly prefer cable, consider 2-2-2-4 aluminum SER if 90A is enough (I say that because it's a commodity, and certainly the most popular large cable anywhere near 100A), otherwise for full and proper 100A try 1-1-1-whatever aluminum, not as much a commodity but still popular.

2-2-2-4 is a commodity because it's allowed in 100A whole service to a dwelling, due to diversity of loads.

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  • Thank you, this is helpful -- I didn't think this was big enough for aluminum to make sense yet but it sounds like it is. I'll look into 1-1-1-3 if I can find it or 2-2-2-4 otherwise. FWIW I know I don't have to get 100A cable but the first panel is out of space so might as well make the subpanel as big as possible for future loads.
    – peter
    Jun 18 at 23:03
  • Just for my knowledge though, would the THHN + metal flex conduit solution have worked? (Eg if I replaced #5 with #4 copper)
    – peter
    Jun 18 at 23:03
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    3-3-3-4 in 1-1/4" FMC does work, assuming you can find the wires (in THHN or XHHW insulation types.) The main issue with FMC is that you do actually need a ground wire in it, rather than being able to not have a ground wire if you use EMT, RMC, or IMC. 2-2-2-4 also fits, and may be more easily found.
    – Ecnerwal
    Jun 18 at 23:20
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    @peter if the wire is #1Al or #3Cu or smaller, the ground only needs to be #8Cu or #6Al or smaller. Cables come with +2 ground so it can be used for very long runs, where the large wire is for voltage drop. Having the ground +2 from conductors allows it to be used for things like RV stands, often hundreds of feet away. (RV stand is 50A so #8Cu/#10 ground). With voltage drop, conductors and grounds must be adjusted in proportion.
    – Harper - Reinstate Monica
    Jun 19 at 2:59

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