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I'm building out a shop (within the same building as the main panel) and I'm considering adding a sub panel to make running circuits easier, and to make any future expansion easier.

I'm unsure how to calculate the load for the sub's breaker in the main panel.

I've got the following in mind (120V except where specified):

  • 40A shop heater (240V, 7500W)
  • 20A circuit for outlets
  • 20A circuit for outlets (2nd)
  • 20A "outside" circuit for outdoor outlet and eventual extension to outbuildings
  • 20A circuit for dust collection
  • 20A "large tool" circuit (240V, ~16A max)

Those wouldn't all be at capacity all the time, but I could certainly envision the heater, dust collection, large tool, and an outlet all generating significant load (~100A).

Do I size the breaker and the subpanel feed for what I think I would need (100A) or for the maximum possible capacity (140A+)? Or is there a accepted formula or scaling factor I should use?

Thanks!

(This is in Washington state in the US in case that's relevant for code compliance).

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  • Edited to clarify Washington state.
    – user2446
    Jan 1 at 7:32
  • How many Watts is the shop heater, and how many HP is your dust collector? How many square feet is the shop, for that matter? Jan 1 at 16:08
  • 1
    7500W heater. Dust collection not purchased yet, but will probably be the HF 2HP (20A @ 120V). Shop is about 550 sqft.
    – user2446
    Jan 1 at 17:53
  • One trick you can do is bring out BOTH poles of power (L1 and L2, aka red and black). That gives two banks of 120V each for only 1 more wire run. So 20A of feeder supports two 20A/120V circuits, one on each bank! Jan 1 at 18:12
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    For the loads listed I00A would certainly be enough and not significantly oversized, but whether you should increase to a larger feeder is indeterminable due to future extension to outbuildings. Curious where you came to 140A, not a size listed in 240.6 or 310.16? Jan 1 at 18:12
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Feeder sizing isn't so simple

The minimum rules for service and feeder capacity planning are found in NEC Article 220; while they don't account completely for your usecase, we can use a modified version of those procedures here. We start with the 550sf you gave, and multiply it by 3 to get the lighting and general receptacle load as-if this was part of its parent dwelling unit, giving us 1650VA (volt-amps, similar to watts). We then multiply this by the .35 demand factor for dwelling unit general loads over 3000VA before adding 3000VA to account for the heavy tools circuit and 7500W/VA for the heater as per NEC 220.51. Note that while the demand factor and the heater are both computed as per NEC Article 220 rules, the 3000VA for the tools circuit is an allowance (similar to a pair of Small Appliance Branch Circuits) for the heavy tools circuit due to the unavailability of details about the actual loads found there.

This gives us 11078VA, to which we add the dust collector load. Since we are dealing with a UL listed appliance here, we start with the 20A nameplate amp rating instead of using the tables to look it up, although it doesn't matter in this case anyway since Table 430.248 gives us 20A for the full load current of a 2HP, 115VAC motor. From there, we multiply by 1.25, per NEC 430.24 and 220.50, to get us 25A, which when multiplied by 115VAC gives us 2875VA, for a total load of 13953VA. We then divide this by 240V, giving us a 58A minimum for our feeder ampacity.

While it's possible to wire this using 6AWG THHNs in a conduit or a 6-6-6-6 copper SER cable, that gives us 65A, leaving us little margin for an outbuilding. In NM, we'd have to use 4/3, giving us a 70A feeder instead, but neither NM nor copper SER are cost-effective ways to wire the circuit, at $3.5 to >$4/ft vs the $2.5/ft of materials that using 6AWG THHNs with a 10AWG ground in 3/4" ENT achieves. Nonetheless, we can do better than that by going to aluminum. A 1-1-1-3 aluminum SER cable goes for a mere $1.67/ft or so, and gives us 100A to the shop panel; we can upgrade this to a 1/0-1/0-1/0-2 SER cable to give us a 120A feeder (using a 125A breaker) for just under $2/ft. If that's still too much cost, we can go with a 4-4-4-6 aluminum SER cable for just under $1/ft, but that puts us back at that 65A figure (with a 70A feeder breaker) from the 6AWG THHNs in conduit or 6-6-6-6 copper SER cable mentioned above.

GO BIG OR GO HOME

Now that we've decided on the size of our feeder, we need to figure out what size subpanel to put in, and this is where folks tend to get penny-wise and pound-foolish. You see, ripping out a panel to replace it with a larger one is a big pain in the rear end, and far costlier than what you'd save by using a smaller panel.

Since this is in the same structure, we don't need to worry about a main cutoff (main breaker), nor a second set of ground rods, but we do need a set of separate grounding bars fitted to the panel to land equipment grounding conductors on. Many main lug panels come with these factory fitted, but if yours doesn't, you'll need to buy the correct bars and add them yourself. In either case, you'll want a large panel: a 24-space or 30-space, 125A panel is not at all out of place here, and if you feel like splurging, you can upgrade to a 40-space or 42-space, 200A panel for not that much more. Just make sure that your panel's bonding screw or strap gets pulled out when you install it!

Sidebar: that dust collector may need a bigger breaker

There's one other issue with your setup, and that's your choice of a 20A breaker for the dust collector. You see, motors draw a significant inrush current on startup; for a 2HP, 115VAC, single phase motor, the breaker will have to handle up to 144A for a short period of time. So, there's a decent chance that your 20A breaker will trip on overload when you start the dust collector.

Fortunately, the NEC authors are familiar with this issue, and provide an "out" in 430.52(C), which permits you to size inverse-time (read: regular) breakers protecting a single motor load at up to 250% of the full-load current, or up to 45A for a 20A motor load, provided that the motor has its own overload protection. You probably won't have to go up that high, but having a 25A single-pole breaker for your choice of subpanel handy in case the dust collector won't start on a 20A breaker would be wise (it's the largest value permitted for a 20A motor if the motor doesn't have a built-in overload protector).

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  • Thank you for the detailed answer! I'll look into the 1/0-1/0-1/0-2 option. My experience so far has been limited to wiring copper and haven't used aluminum for anything. Is there anything unique to dealing with aluminum that I should know?
    – user2446
    Jan 6 at 6:27
  • @user2446 -- No-Al-Ox or equivalent antioxidant paste, while not strictly necessary for aluminum wiring connections, is generally considered a good idea by many; more importantly, though, aluminum is somewhat less forgiving of mistorqued lug setscrews than copper is, so you'll want to use an inch-pound torque wrench to tighten breaker and loadcenter lugs to their marked tightening torques Jan 6 at 12:42
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Go big. You can legally feed a 200 amp panel with a 50 amp breaker on 6 ga wire. Not that you'd want to do that, just an illustration of what's possible. In my shop, which is part of my house, I installed a 200 amp 40 space panel (Square D/ QO, not Homeline) fed by a 100 amp circuit from the main panel.

You are smart in that you're installing a sub-panel in your shop. BC it's in the same building, you don't need separate ground rods, but you still need a 4 wire feed and isolate the ground from the neutral. Since it's in the same building, I'm guessing the run from the main panel to the sub-panel isn't too long, so wire cost won't be a huge issue. All of us here always say the same thing, GO BIG! The additional initial costs pale when compared to having to upgrade later.

I also went with Wiremold 4000 raceways for future flexibility. Much easier to pull off a few covers to run more circuits than running more conduit.

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First of all, you want a 240V sub-panel and run 2 hots instead of 1 (plus neutral and ground). This way, you can split the load between legs and get away with smaller gauge wire.

If you don't already have it, get a 20A/240V shop heater instead of 40A/120V. The wiring will be cheaper. In fact, put the tools and dust collector on 240 volts if you can. The motors will run cooler and last longer.

Based on your estimate, go for a minimum of 60 amp/240 volt panel. Both legs share the 20 amp heater. Hot 1 gets the dust collector and one set of outlets. Hot 2 gets the large tool, the other set of outlets and and outdoor outlets.

I'd actually suggest at least 100 amps to give you some growing room.

Updated due to the OP clarifying that the amperages quoted for the heater and large tool are at 240 volts while the other circuits are 120 volts.

So my minimum suggested layout would be heater and large tool across both legs (60 amps total), dust collector on hot 1 and all 120 volt outlets on hot 2 (total of 20 amps). Therefore 80 amps would be minimum feed to the subpanel to allow all of the heater, large tool and dust collector to run and still have 20 amps left for lights and other 120 volt loads.

If it were me, I’d bump it to at least 120 amps to give you future capacity.

As @georgeanderson noted in his answer, go big on the panel. You can use a 200 amp panel and feed it from a 100 amp breaker and rate the wires for 100 amps.

I'll let a pro comment on if you need size up for the official load calculation, though I don't imagine there would be any need since I’m already sizing based on the major circuits all on at once.

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  • The shop heater already exists. It's a 7500W heater running on 240V (~30A). Other amperages figure for 120V except the "large tool" circuit which allows for larger ~16A tools running on 240V (one at a time).
    – user2446
    Jan 1 at 7:36
  • @user2446 That is information you should have included in your original question. An amperage quoted for 240 volts takes twice as much power as the same amperage for 120 volts.
    – DoxyLover
    Jan 1 at 12:32
  • @user2446 Please edit this information into your question so anyone else answering sees it.
    – DoxyLover
    Jan 1 at 12:54
  • Updated. But can you explain why that matters? When selecting wire gauge and calculating loads, my understanding is that current is the criteria. I don't see a reference in any wire gauge charts to power. Am I misunderstanding something?
    – user2446
    Jan 1 at 17:58
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    @DoxyLover Actually drawing 10 amps at 120 is the same amperage as 10 amps at 240, but twice the power (wattage), which is why you get 1,200 watts at 10 amps at 120v and 2,400 watts at 240v. Allows for the use of smaller wires to deliver the same amount of power. Also, commercially, not all 3 phase power is 208. My church is 3 phase, 240. I know this because I supervised a large commercial water heater replacement and specifically told the plumber it was 3 phase 240v. Well he got a water heater set up for 208. Blew some fuses and failed. Had to swap out the elements for 240v elements. Jan 1 at 19:57

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