270 feet subpanel feed

Question

I need to connect a 100Amp subpanel at a guest house that I am building in the back of my property. My main panel is 400Amps. I need 120/240V at the subpanel to run a couple of mini split ACs that are on 240V plus your usual lighting and kitchen stuff (cooking and hot water is gas). The distance from the main panel to the subpanel is 260f. I plan to run the wires in conduit, some underground but majority of conduit is along a block fence. I live in the Phoenix, AZ area.

I had planned to use 1/0 aluminum or 3AWG copper wire but it looks like this will not be sufficient to allow for a 3% maximum voltage drop and I should use 3/0 aluminum or 1/0 copper which the online calculator recommends. NEC seems to suggest no more than 5% voltage drop. For 5% allowable voltage drop 1/0 aluminum looks sufficient. Should I use 1/0 aluminum, or use 2/0 or even 3/0 for the guest house? Thank you for your feedback.

Does this answer your question? What wire gauge do I need for a 100 Amp subpanel at the end of a 60' wire run? — JPhi1618, Jan 20 '20 at 19:59
I picked a popular "wire size" question, but we need to find a good canonical answer to link to when this comes up. There are many wire size questions to choose from. — JPhi1618, Jan 20 '20 at 20:00
How many square feet is the guest house, and how many amps do the minisplits there pull? — ThreePhaseEel, Jan 21 '20 at 00:35
@JPhi1618 "Canonical" posts is something we're looking at adding on Codidact Unfortunately, we are still quite a ways away from production software. The problem at SE (as Harper & I know from trying) is that if you create a Canonical post as a Question it falls by the wayside within a day or so because it isn't "active". — manassehkatz-Moving 2 Codidact, Jan 21 '20 at 05:06

Ed Beal · Answer 1 · 2020-01-20T20:12:22.677

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3% and 5% are there in the form of Fine print notes. These are not enforceable. If your max demand (power used at 1 time) is 100 amps 1/0 will be fine. We would really need to do a load calculation to provide a real value but 270’ 1/0 aluminum is only 4.68% since you have gas cooking and water heater I would guess your demand will be below 60 amps and that would even allow for 2awg at 3.96% so there is no need to go larger than 1/0. : added Aluminum wire is a must for cost here copper would just be nuts for this.

edited Jan 20 '20 at 20:12

answered Jan 20 '20 at 20:06

Ed Beal

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Definitely Al for the feeder, no where else.+ – JACK Jan 20 '20 at 21:22
Thanks I do use AL for these loads but with high AMP industrial copper lasts longer .+ – Ed Beal Jan 21 '20 at 04:37

Harper - Reinstate Monica · Answer 2 · 2020-01-21T08:23:41.393

Yeah, as Ed Beal discusses, the 3% isn't serious, but they like to encourage you to "bump" to sell fatter wire.

When you calculate voltage drop, you should really use the practical load you expect to have realistically. I have a comparable cottage with no major electric appliances, and we're fat and happy with 30A@120V service. So I'd expect 10A @ 240V to be a normal load for the rest of the house. The two mini-splits, you have to guess at duty cycle, let's see, Phoenix, so 100% then :)

I jest, but that's exactly what I'd do. Full nameplate of both mini-splits, plus 10A, and that's the amperage figure to put into the Voltage Drop Calculator.

Heck, you definitely should never enter more than 80% of breaker trip. Because you're required to derate (up-rate) your service by 125% of whatever your expected load would be; if someone did really want 100A, they'd need a 125A breaker and wires.

Because it's breakered at 100A, the minimum size is #1 aluminum. I don't know the size of the two mini-splits, but if they're 20A, you may not need a bump at all. Just the same, I would consider a bump to 1/0. That'll give you some expansion room for the unexpected, like the Chevy Bolt.

I would not even consider copper for this, unless your last name is Kennecott. Aluminum is actually a superlative conductor, lighter and far cheaper for the same ampacity (although more bulky). In the 70s they tried it on small branch circuits and the copper lugs didn't play well with it, but it's always worked fantastic for large feeder like this. And the panel lugs will be aluminum, which is the universal donor.

Don't forget to make the cottage panel nice and huge. At build time, extra breaker spaces are dirt cheap; running out of spaces is an expensive and difficult problem. We recommend 40 space panels without hesitation. It's totally OK to use a 225A sub-panel fed by a 100A breaker; the "main breaker" is only there to be a disconnect switch. Beware "12 space/24 circuits; the larger "circuits" number is no practical use, because most breakers these days need to be AFCI or GFCI, and those take a whole space.

Guest house is about 1500sf. I do not expect ever getting close to 100 Amps but wanted to be on the safe side in case I do install an additional charger for an electric car. The mini-splits require 1x 20A and 1 x 25A breaker. Dryer is going to be gas as well. And I may even throw a few solar panels on the roof (standing seam metal roof, easy to install solar panels on) at some point. Bought the 1/0 and 2 AWG and will start the installation process on the weekend. And I will look for a large panel. Thank you all! — JBE, Jan 21 '20 at 22:57
@JBE -- the breaker sizing for the mini-splits isn't relevant here, what we need to know is how many amps they're actually rated to draw (Minimum Circuit Ampacity, not Maximum Overcurrent Protection, in other words) — ThreePhaseEel, Jan 23 '20 at 04:36

270 feet subpanel feed

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