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I would like to install a new 100 amp sub-panel, from my 200 amp main, to my garage (75 feet away) to supply a hot tub and electric car fast charger. At this time I am limited on conductors because I ran 1 1/4" underground PVC conduit, which only allows three #1 conductors (no room for ground).

I am trying to size the largest conductor for the 100 amp panel and conduit.

In my research, I found that #2 THHN, @ 75° is rated at 115 amps, and you can use this ampacity if you use a breaker rated at 75° for the conductor. If I use a 75°, 100 amp, two pole breaker at the main and the sub-panel, would this meet the code?

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    Required reading on EV charging. You might be surprised!
    – FreeMan
    Commented Jun 14 at 12:09
  • Are you sure you have to have 100 amps for this? Your hot tub probably needs 30A, and if you can tolerate a 20A 240V fast charge that's well within the limits of 4 gauge aluminum feeder.
    – Machavity
    Commented Jun 14 at 13:16
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    0. Since you're in PVC, to be clear, you must have a ground wire. New three-wire feeds have been illegal for decades. 1. Don't forget, when doing your conduit-fill calculations, that the ground wire does not need to be as large as the current-carrying conductors. For #2 copper / 1/0 Al (or smaller) the ground can be #8 Copper or #6 Al. 2. Unless you really need to provide a 100A feeder, #2 aluminum (90A) is generally the way to go - it is particularly inexpensive thanks to economies of scale.
    – nobody
    Commented Jun 14 at 13:24
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    You just need to protect it with a 90A breaker on the feed end, the destination sub-panel can be anything rated for 90A or more - a 200A panel with lots of spaces is perfectly fine. 3. Don't forget to do a load calculation to verify your service can actually feed the new loads you want. The load calculation does not affect the sub-panel size (it would always be perfectly legal to set up a 200A feeder if you had the conduit space), but you must plan to not overload your service.
    – nobody
    Commented Jun 14 at 13:25

2 Answers 2

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There are several different things that go into determining the feed size for a subpanel:

  • Conduit - That sets a maximum size on the wire. 1-1/4" PVC 80 (you didn't specify 40 or 80, but likely at least some parts are 80 for damage protection) can easily handle 3 x 2 AWG aluminum + 1 x 6 AWG aluminum, based on this Conduit Fill Calculator. That's enough for 90A.

  • Wire Size - Aluminum is physically larger than copper but generally much less expensive. Using that 1-1/4" conduit, you could get more power by switching to copper - 3 AWG 100A or 2 AWG 115A. But you likely don't have that much power to spare.

  • Load Calculation - You actually need two load calculations. The first is to determine how much power your home currently requires. With 200A service (which is typical), you might have a Load Calculation from anywhere well under 100A (especially if you have gas for cooking, water heating and space heating) to well over 100A (all electric everything, large HVAC, etc.). So you need to figure out what you currently have, subtract that from 200A and see what's left.

The second Load Calculation is to see how much you need. You probably don't actually need a lot. A hot tub (heater, pump, etc.) might use 30A @ 240V, possibly a little more. EV charging is extremely flexible. You can provision most EVSE for anywhere from 20A @ 240V (minimal, but actually sufficient for typical driving) to 60A @ 240V, and sometimes even more. 30A @ 240V provides enough to add around 20 miles per hour of range according to Tesla, though your mileage may vary.

That means 50A or 60A may be enough capacity to do everything you want to do. I would work in a little extra capacity - 20A @ 120V x 2 (but wired so really 20A @ 240V) will provide enough extra for you to have a couple of receptacle/lighting circuits for tools or whatever.

Note that Load Calculation has a very specific meaning in the NEC. It is not (except with purely fixed loads that can all be on at the same time) adding up power requirements of the devices. It is not adding up breaker handles (that is usually too high). It is not looking at your electric bill (for a bunch of reasons). It is more complex, especially for a whole house, but it is required for major changes like this.

  • Panel Size - A panel bus is rated for a particular capacity. That may actually be much higher than the included main breaker if it is a "main" panel. Obviously if you want to have close to 100A of loads and a 100A feed you need at least a 100A panel. But you can use a 200A panel with a 100A feed. You can even use a 200A panel with a 50A feed!

But wait, what about that "main" breaker? In a subpanel then "main" breaker is purely the required disconnect. (Required when a subpanel is in a different building from the main panel.) You can have a 200A "main" panel with a 50A feed and 90A-rated wire. The key is: disconnect size >= feed breaker, wire size >= feed breaker.


So now that you have determined how much wire you can fit (easily enough for 90A aluminum), how much power you can spare (based on Load Calculation of utility service), how much power you need (must be <= power you can spare), you can figure out what minimum wire size you need. 2 AWG aluminum is perfectly fine for anything up to and including 90A and is often the best price, or very little more than some smaller sizes. It is actually better to oversize a bit, within reason, as then you can upsize later by just changing the breaker - e.g., if you make changes in your home that result in more power available for the subpanel.

Typical aluminum sizes:

  • 6 AWG = 50A
  • 4 AWG = 65A
  • 2 AWG = 90A

If you see something that says 2 AWG is good for a full 100A, that's when the entire service is 100A, which does not apply here.

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    Additionally, the price difference between AL and CU might make it cheaper to rip out the small conduit and replace it with larger metal conduit (using the conduit as the ground), and run the larger AL wire through it, if more amps are really needed.
    – FreeMan
    Commented Jun 14 at 17:38
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    Pish, tosh, @FreeMan - piddling fiscal concerns are not mentioned in the question, so Silver is the wire of choice, as it is slightly more conductive than copper. ;^)
    – Ecnerwal
    Commented Jun 14 at 18:25
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    @ecnerwal good point, pick up an extra 10 amps! Commented Jun 14 at 20:48
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Just the way feeder is priced, #2 aluminum lives at a cost and availability (and capacity) sweet spot at 90 amps.

#1 aluminum is a bit less common and is 100 amps. Or 1/0 aluminum is 120A. All these will fit in 1-1/4" pipe, although not as comfortably as 5 amp lower copper would. Of course copper also costs a lot more money, which is why it is not widely used for feeder. If there's any safety gain, I don't see it - the lugs you'll be landing on are made of aluminum, and the critical thing there is screw torque, not metal.

I would use copper for ground, because then you can use a bare wire, which helps with conduit fill especially for #1/0 AL where you're kissing right up against the 40% fill limit. #8 for up to 100A feeder or #6 for 101-200A feeder.

However, you will still be constrained by the service load calculation on the entire house. The easiest load calc to work with is 220.82, nicely rendered in worksheet form by Sacramento here. Line 2 only pertains to kitchen general-use circuits, and lines 1-3 sweep up all lighting and portable plug-in loads. Line 3 includes washer and 120V dryers. Fridge isn't mentioned because it's a trivial load.

That will define how much power you can throw to the subpanel, and thus informs wire size. HOWEVER... there are two cheats. These come in the form of Load Management systems, which work exceptionally well for EVs - here's a page on that (thanks Freeman). And for the hot tub, you can use a "dumb load shed" device (also detailed there) - however you may need to have a fairly long data cable in that conduit (data cable in conduit is allowed when it's entirely between AC mains equipment and inside the mains voltage compartments and conduits).

Depending on your your Load Calculation works out, the EVEMS or dumb load shed system can be either at the main panel or the subpanel. You have to think about worst-case loads in any situation and size wires for that.

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