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I'm wiring a shed to use as an insulated tech workshop. A 20A circuit for 10 outlets, 15A for lights, and a 20A for a mini-split A/C. My main breaker appears to be 150A.

Should I just match the 150A on the subpanel, or is that overkill?

Is my house panel even big enough to run a sub-panel?

How should I go about choosing the correct/best size for my sub-panel?

I live in Georgia (USA). The building is 100 feet from the house. I'm currently planning on using rigid metal conduit and digging the trench by hand.

Here's a pic of my house panel.

Picture of my House Panel

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    How many square feet is your house, and how many square feet is the proposed shed? Also, how many kW is your stove, and can you post a photo of your air conditioner's nameplate? Last but not least, what do the four unlabeled 20A breakers in your panel feed? Jan 10 '21 at 19:25
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  1. Your main panel has several spaces open! Hurray! That means you have no problem adding a double-breaker for the subpanel. It also means that if you start adding more circuits inside the house then you are not in immediate panic mode. If you do start adding more circuits - e.g., a kitchen remodel - then you should consider a subpanel rather than going to double-stuff/tandem breakers. But no rush on that - even with the planned subpanel added you will still have 4 spaces open.

  2. Conduit is great because you can change the wires later if needed. Rigid metal is even better because it takes care of the ground - one less wire to run.

  3. The subpanel can be any size you want. But bigger is better, because you may decide later that you need a bunch of additional circuits for "reasons". The subpanel can be a "main panel" that comes bundled with a big main breaker (e.g., 100A or even larger), because that main breaker is used purely as a disconnect switch and does not have to match the power supplied to the panel. Your main panel is 30 spaces - a 24 or 30 space, or larger, subpanel would be fine.

  4. The double-breaker in the main panel feeding the subpanel must match the wire. You could probably get by with even a 40A breaker & 8 AWG wire, but 60A & 6 AWG would be more typical and allow you to add a few more circuits without upgrading breaker & wire. The key is to make sure your conduit can handle larger wire in case you ever need it. 3/4" conduit would do fine for 6 AWG, but if you go up to 1" then you're good for up to 2 AWG and > 100A.

  5. You will need ground rods at the subpanel.

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Lots and lots and lots of spaces.

Electricity works for us. It does our bidding. Running out of spaces is a huge, project-killing problem. And spaces are dirt cheap when you buy it (rather more expensive after the fact)... so we earnestly advise gross overkill on panel spaces.

So when you said "150A panel like this one" I thought "30 space panel for a shed? Now you're talking!"

Seriously. 30 spaces is not too many.

So yeah, if you want to get a similar subpanel, go for it. Although that one is a classic "CH" panel, I have 2 myself and they are my brand of choice. They are now wider because of a rule change on wire bending radius. 14.25" so they fit between joists.

If you're thinking "I'll free up THAT panel when I upgrade my panel" I would not do that. I would leave this panel where it is and put a new 200A CH panel right next to it, then have that feed this existing panel. Way less work lol.

Main breaker size in a subpanel

Subpanels don't need main breakers at all. What they need is disconnect switches if they are in outbuildings (no breezeway between them).

Actual disconnect switches can get pretty expensive, and just the way they price service panels, it's cheaper to buy a "main breaker" panel and completely ignore the number on the breaker handle.

So yeah, a 150A main breaker panel will work for ANY feeder size, though a feeder larger than 150A would be wasted.

The feeder MUST be fused

You mentioned 150A feeder to the subpanel. However you can't "tee" off the un-fused, always-energized feed coming off your meter and carry it to the subpanel. The run to the subpanel must be protected by a breaker. And here's the rubaw: the largest branch circuit breaker in the CH line is 125A.

To go off on my tangent about a panel upgrade, one option is to replace the meter pan with a "meter-main". It contains both meter and main breaker (some even have a micro-panel with 8 spaces that is perfect for feeding outbuildings). At that point the wires coming off the meter are fused, and you could "tee" off them at your leisure. It's a complicated job that requires permit/inspection, but the scope of work is small.

So the feeder wire must be behind a breaker, and the breaker decides wire size. Since you're in conduit, we're talking THWN/XHHW individual wires, and those are allowed to run hotter than UF/NM cable, so amps may not be what you expect.

  • 30A -- #10 copper wire
  • 50A -- #8 copper wire or #6 aluminum
  • 65A using 70A breaker -- #6 copper or #4 aluminum
  • 100A -- #1 aluminum
  • 120A using 125A breaker (largest 2-space breaker) -- 1/0 aluminum
  • Full 150A -- 2/0 aluminum for you

You can actually provision full 150A on 2/0 wire (135A normally), because of Rule 310.15(B)(7) which says "no feeder needs to be larger than 83% of the service size", that's 124.5A in your case. Services "round down" on breaker size, feeders "round up" as above.

#6 or larger is where you start thinking about aluminum wire. Much above that, aluminum is the wiser choice. The lore about "dangerous aluminum" never applied to large feeder like this.

Grounds must be #10 for up to 60A, #8Cu/#6Al for 61-100A, and #6Cu/#4Al for 101-200A. That's decided by wire capacity not breaker size! (if conductors are upsized e.g. for voltage drop, grounds must be too).

You aren't going far enough for voltage drop to matter at 240V, I don't even crunch the numbers until 180'. However I also like to recalculate for a single large 120V load, and on smaller feeders (#12-#10) that can become an issue. Even so, you're just on the cusp; I don't even bother crunching the numbers on that until 90'.

Your questions

Is 150A overkill? Yes, until the day you're at the Tesla dealer who asks "do you want the slow charger or the fast charger?" Prior to the existence of EVSEs, most people had little chance to use that much. But there's always another gadget, and always will be.

Is my panel big enough? Always. Panel capacity isn't decided by subpanels or spaces, it's decided by load calculations based on the loads of the house.

How do I choose the best size sub? Spaces: PLENTY. Amps: Any size, at least enough to exploit whatever feeder you're willing to pay for.

Remember you need 6" of cover over top of the conduit, not a 6" trench. Also must be 12" under vehicle drives.

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  • Note that 150-200A CH breakers are no longer made Jan 10 '21 at 21:19
  • Aw heck. I have a PDF of a 2019 Eaton catalog, I thought I saw it in there. Must've been looking at QO. I shouldn't confuse those, I'm a CH guy for Pete's sake. Unless DC is involved, then yeah, QO lol... Jan 11 '21 at 1:17
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Yes, your main panel has breaker space in it that can supply a 240 volt circuit to the sub-panel.

Clearly DO NOT go with a 150 amp circuit to your sub-panel. That is overkill and expensive. You'd have to get pretty large wires, which over that distance gets quite expensive.

Go larger on your sub-panel than you think you'll need, the cost difference is pretty small and much less expensive than swapping it out later. Just because a sub-panel can handle 100 amps, doesn't mean you have to supply it with that.

Your power needs seem rather modest, mostly hand tools? I'd think a 50 amp circuit on 6 ga. wire (copper) or 4 ga (aluminum) would suit you well. Since digging ditches sucks, please consider putting your wires (not cable) in conduit. Oversize it a bit to allow for the future, should your needs change.

Your sub-panel will need to be fed by a 4 wire circuit: 2 hots, a neutral and a ground. The ground and neutral will need to be isolated (floated) and ground rods installed at the shed connected to the grounding buss bar in the sub-panel. The ground from the main panel will also be connected there. Do not bond the neutral to the ground in the sub-panel.

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For the loads listed you may only need a 30A 240v panel, but a 60A breaker to feed the panel would certainly be advisable and would provide plenty of room for expansion.

3/4" Conduit would be large enough for 3@#6 copper wires using the conduit as ground, but would leave little ability to upgrade later. If you decide you use #4 aluminum you would need 1" conduit. That would leave much room to expand later, but 1" GRC is quite a task compared to 3/4". Your judgement call. If you choose to use PVC instead you will need to add #10 copper or #8 Al ground. Sch 40 would not require upgrading size, sch 80 would. Wires smaller than #4 need to have insulation specified green for ground, and white or grey for neutral. Search table 300.5 for NEC requirements for depth of cover for conduit type for the conditions.

You need a disconnect to disconnect both hots in the shed, a circuit breaker isn't required, but a main breaker panel a simple and economical way to accomplish that. The minimum rating of the sub-panel must be at least the rating of the feeder breaker, so 60A, but a 100 amp or larger panel with many extra spaces often is the same price and provides many options for the future.

You will need to drive ground rods at the shed also.

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  • 60A is kind of a funny size. It's correct for #6 UF cable ... but once it's THWN wires in conduit, the choices are #8 (50A) or #6 (70A breaker). That's because of the higher thermal limit on THWN. Further, because #6 is capable of more than 60A, the ground must be capable too, so #10 is too small. Subtle... Jan 10 '21 at 19:10

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