I have a 200 amp service panel, and I calculate I am using about 160 amps. (I used a formula I found online that takes the square footage of the house and then adds from there) The new 160 sqft shed is on the other side of the breaker box so the run will be about 80 - 90 feet. In the shed I want to run a mini split air/heat that requires a 240v 15 amp breaker. I would also like to have a 120v 15 amp breaker for lights and a 120v 20 amp breaker for plugs. Lights will be led fluorescents and the plugs will be light duty tools and battery chargers so not really very much draw at all. The shed will be insulated and sheetrocked. It is 10 feet at the peak of the roof. The mini-split is 27.5 seer and 550 Watts so should draw too much either.

I have space in my service panel to add a 240v breaker, two poles, 25 amps and 25 amps and I will run 2 hots, 1 neutral and one ground number 8 AWG THWN2 through 1" pvc at the service panel into the attic where I would switch to Carlon flexible Blue conduit, then pop out the other side of the house and go back to 1" pvc down the house and 18" into ground to the shed. I was thinking at that point I would run it to a 60 amp disconnect switch and then through the shed wall to a indoor subpanel maybe 100 amps with my three breakers. I read that the sub panel would need the ground and neutral unbonded and that I would need to use 2 8ft grounding rods 6 feet apart so will do that.

So does that sound reasonable?

If the new 240V 25 amp double pole breakers add up to 50 amps, that would technically give me 210 amps usage at the service panel. 160 + 50. Would that be okay on a 200 amp service? Or is it counted as only 25 amps in which case I would be fine.

Also, at the shed's sub panel, I would have those 3 breakers adding up to either 65 amps if the 240 15 amp gets added as 30, or else it would be 50 amps. Would the 240V double 25 amp breaker be big enough to support the 3 breakers in the shed? Is number 8 wire big enough or do I need to go to 6 awg?

The 60 amp disconnect has two line and two load connectors and a ground. However there is no place for the neutral. Would I just pass the neutral through the wall into the subpanel and not be involved at all with the disconnect wires? I'm thinking the disconnect just needs to break the hots and not the neutral, but wanted to make sure.

Thanks for any help. I'm relatively new to electrical wiring so want to be sure I'm doing it all correctly and up to NEC standards.

Thanks, Andy.

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    I'm going to respond in more detail later, but for now, I seriously doubt you are drawing 160 amps unless you have a huge house, not well insulated with electric furnace and electric everything else (water heater, clothes dryer, kitchen range, grow lamps, bit coin miners (being a bit factitious on the last 2)). If you were, your power bill would be astronomical. Take a look at your power bill and determine what you are REALLY using. – George Anderson Apr 19 at 1:17
  • If you make a sum of rating all your breakers , you may get much more then 200A. It is not the way how it calculates. – user263983 Apr 19 at 1:24
  • What's the minimum circuit amps rating on that minisplit? – ThreePhaseEel Apr 19 at 1:41
  • Stop. Do not buy wire. #8 copper is a waste of money, you can get 2-2-2-4 MH feeder or other 2-2-2-4 aluminum wire for less than that. And that's good for 90A. – Harper - Reinstate Monica Apr 19 at 23:41

First, a two pole 25A breaker would allow 25A on each 120v leg of your 240v circuit. All you would need is #10, and maximum allowed amperage for #10 is a 30A breaker. Your service is 200A of 240v, or you could think of it as 200A on each 120v half of the 240v service.

Load calcs are complicated, even if your existing load reaches 160A adding a 60A load/panel would not require a total 60A additional panel to the 160A due to various diversity allowances.

#10 copper is only good for 30A, the next size bigger wire is #8, THWN copper is good for 50A (with #10 ground), and would easily fit in 3/4" conduit. The loads you describe wouldn't require bigger than 30A, but I certainly wouldn't bother, things change. Drop a compressor into the plan and everything changes.

The panel in the shed must be rated for at least the rating of the breaker in the main panel feeding the sub-panel. Bigger rated sub-panel is fine, it will only be good for the breaker feeding it. Also almost all loads now require AFCI or GFCI protection, so full sized breakers must be provisioned, so a 12/24 space panel is now just a 12 space panel. Don't get one that small, things change.

Your shed requires a main disconnect, maybe that is what the 60A disconnect you referenced is for, but the easiest way to accomplish that is with a main breaker panel, with the main breaker 50A or above. A 100A 24 space panel is a good option. The panel needs to have a floating neutral, and a cabinet mounted ground bar. You may need to buy the ground bar separately.

Search "table 300.5" for depth requirements. The table shows cover, the minimum fill required above the conduit.

Edit: NEC 110.14 limits termination ampacity to 60°C if not marked, but allows 75° if marked, modern breakers are marked 75°C. Table 310.16 shows THWN-2 as 90°, but limited by 110.14 to the breaker rating of 75°. Table 310.16 #8 copper in the 75°C column shows 50A.enter image description here

  • #8 rate for 40A. – user263983 Apr 19 at 1:20
  • Great answer, so I won't bother to respond. The only thing is I'd go at least to 1" conduit...makes the pulls so much easier. Also, is 8ga thwn really rated for 50 amps? In conduit? In cable I thought it was only rated for 40? – George Anderson Apr 19 at 1:21
  • @user263983 I was assuming copper, at that length and that small I don't bother with aluminum. Will update. – NoSparksPlease Apr 19 at 1:27
  • I meant cooper too. – user263983 Apr 19 at 1:29
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    @GeorgeAnderson Yes, see this table Key is 75 C column - AIUI (and I'm not a pro, so I could be wrong) you can generally use the 75 C for panel-to-panel but not directly to receptacles. – manassehkatz-Moving 2 Codidact Apr 19 at 1:29

#8 copper is unnecessary

You do not need a wire size "bump" for such a short distance, given that it's a 240V line. For your draws and distance (I agree 25A will suffice), you can go 2 ways for less money:

  • #10 copper to be breakered at up to 30A.
  • #2 aluminum to be breakered at up to 90A.

These two options are the same price.

When you are up over about #8 wire, it makes more sense to go aluminum. There's never been anything with aluminum as large feeder - any negative press is about using it for small 15-20A branch circuits with receptacles improperly approved for aluminum wire, and nobody torquing screws to spec (a problem which continues to haunt all wires).

You cannot direct bury THHN wires. It sounds like you were talking about having conduit only for the stub-ups. Nope, can't do it, and even if you had buryable cable like 2-2-2-4 MH feeder, that would require 24" of cover.

If your wires are #6 or smaller, you must buy 3 colors of wire: green for ground, white or gray for neutral, and anything else for hots. No buying 1 spool and remarking.

See how #4 or #2 aluminum is looking better and better?

Use a 70A or larger breaker as that will have lugs large enough to take #2.

Do not waste money on an outside disconnect

It is simply not required. There needs to be a disconnect, but it can be inside the shed.

The cheapest way to get a disconnect, since you're getting a subpanel anyway, is to select a subpanel which has a main breaker. The main breaker is not used for its overcurrent function, only for its "big high-current switch" function.

The government doesn't care if you use a "main panel" for a subpanel, but you do have to separate neutrals and grounds, and might have to buy an accessory ground bar. ($7).

There are two banks of 120V in 240V.

Here's a primer on how US power actually works.

The upshot is that a 240V feed has two banks of 120V. 120V loads are connected only to one bank. 240V loads are connected across both, like 2 batteries nose to tail.

So a 120V load draws its stated amps, but from only one of the two banks A 240V load draws its stated amps from both banks.

Let's say you have a 25A/240V supply. That means you have 25A x 240V, or 25A x 120V twice. You plug in a 10A/240V load. The two banks are now flowing 10A and 10A. Now, you add a 15A/240V load to the #1 bank. Now, the two banks are flowing 25A and 10A.

As has been covered, your mini-split needs about 720 watts @ 240V == 3A @ 240v. So that alone will load your two "banks" to 3A + 3A.

Let's add 12A of Skil-Saw and 2A of lights. One onto each "bank". Now our two "banks" are loaded 15A and 5A counting the mini-split too. If we put both those on the same bank, it would be 17A and 3A.

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