What Wire Size for 100amp Subpanel

Question

I've seen a similar question in several other posts but the answers seem to include several caveats and differences of opinion. I'd really appreciate some thoughts on my situation and any recommendations you may have:

Situation & Question: I'm installing a subpanel in my yard. I've already run 1.5" electrical conduit (underground) before we installed pavers and am now preparing to pull the wires. My specific question is what gauge wire I should use.

Information:

• Subpanel: 100 amp Square D, 6 breaker/12 circuit: https://www.homedepot.com/p/Square-D-Homeline-100-Amp-6-Space-12-Circuit-Outdoor-Main-Lug-Load-Center-HOM612L100RBCP-HOM612L100RBCP/100143240
• Distance: ~80ft
• Location: California
• Loads on subpanel: 100amp breaker at main. Subpanel will power yard lights, bbq island, and workshop/shed (160sq ft with basic contractors table saw, drill press, etc.).

Current Logic: For a 100amp panel, it looks like this would typically require 3 AWG THWN wire (seems that SER isn't rated for use in conduit and I've seen 3AWG (all references to wire are copper) recommended on this forum elsewhere). With that said, the NEC 120% rule seems to apply here and considering I'm almost at 100ft, it looks like the wire needs to be rated for 120AMPs. Based on the ampacity charts I've found, I'll need 1AWG (rated for 130amps) wire for this.

Bottom line, cost is not too different so I really just want to do the best possible job on this project. I really appreciate your advice, guidance and thoughts!

Answer 1

Aluminum is our heavy feeder friend

#1 aluminum will be fine. Nothing wrong with aluminum at these large sizes, and they'll be landing on aluminum lugs anyway.

If you are looking for the magic that makes terminals not burn up, that's a torque wrench. Recent science shows it matters on ALL terminals.

HOMeline panels all have stamped aluminum bus bars.

Having 130 mph tires does not oblige you to drive 130 mph. That's a red-line, not a recommendation. Similarly, having a panel with a 100A bus rating does not oblige you to run 100A feeder. So you could use the more readily available #2 aluminum for 90A. That will more than satisfy most needs, 100 is just a number people pick out of thin air.

Large panels are good

100A is a lot more power than most people realize. Unless a Tesla is coming, you will max out the panel spaces long before you run out of amps. Since on total project cost the panel is a pittance, we recommend nice big panels.

We have a 24 space, 60A-fed panel in our shop, and it's 60% full.

You can't count on being able to use the "12 circuit" number, since increasingly, circuits require AFCI or GFCI protection, and those are not available in tandem/quad breakers.

The only gotcha is the Rule of Six: if there are more than 6 total breaker throws to turn it all off, then you need a main breaker in the panel. However if you want to avoid that, avoid tandems. Full-sized breakers can be handle-tied in groups of 3, making them count as 1 throw. (Only groups of 2 on HOMeline since they don't sell 3-phase panels).

125% derate for continuous loads and friends

The 120% rule does not work that way. You do a Load Calculation per NEC Article 220, and 125% derate is applied on a load by load basis - welders no, heaters yes, microwave no, EV charging darn right yes. Unless Article 220 gives you some other way to provision power, e.g. the formula for ranges is quite permissive.

Consider 6000 VA of heater and 2000 VA of table saw. The heater needs a 125% derate to 7500 VA, the 2000 VA table saw does not. Add them up and that is 9500 VA or 39.6 amps. A 40A feeder will suffice. There is no additional derate, we already applied it with the heater!

You also don't need a wire size bump for distance.. for a 240V circuit I don't even bother crunching the numbers for less than 150’.

Only one circuit to an outbuilding

There is one snag with your "subpanel outdoors serving shed" plan. A building can't have more than one circuit entering it.

If you want to have more than one circuit in the shed, mount the subpanel on the shed.

Answer 2

Get a bigger panel!

You need a local disconnect. The cheapest way to do that is with a main breaker. If you put a backfed main breaker in this panel then you are now down to 4 spaces. Staying with the same brand for a simple comparison. This panel is currently $48 at Home Depot. A 20 space panel with a main breaker - that's more than 3 times the size! - is $104. Plus that includes the 100A main breaker, 2 20A single breakers and a 30A double breaker. (You may not have a use for the 30A breaker - the usual use in a house is for a water heater or a clothes dryer.) Even if you never use more than 6 spaces it is a better deal.

No connection to Home Depot except as a satisfied customer - used that because OP's link is to HD. No connect to Square D - used that for an easy comparison. All major brands have similar "bundles".

Aluminum Wire

Don't use copper for panel to panel feed. Simply not needed to satisfy NEC, to fit in your conduit (1.5" is comfortably huge) or to satisfy local codes (the places I know of that restrict aluminum do so for branch circuits, not for large feeders).

The usual recommendation is 2 AWG Aluminum, which will handle up to 90A. Based on "6 space panel" and no mention of on-demand electric water heating (don't, just don't) or of EV charging, I can't imagine you really expect to use anything close to 100A peak/80A continuous. If you do, then jump up to 1 AWG Aluminum. If you don't 2 AWG Aluminum is usually priced right, but then you need a 90A breaker in the main panel at the end of the feed to the subpanel. (The breaker in the subpanel can be any size 90A or larger as it functions as a disconnect and not to protect the feed wires.)

And if you do decide to do EV charging from this subpanel, you probably have plenty of capacity for that anyway, as long as you don't go for 50A+.

As far as upsizing for distance, unless you expect to actually use that much power on a regular basis, there is no need to be concerned. The 125% or 80% (depending on perspective) derate is already built in elsewhere. Voltage drop? Not an issue. Max out a 2 AWG Aluminum run and you'll be under 2% voltage drop, and real-world usage probably way below that (unless you put in an on-demand electric water heater and EV charging).