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I'm running a double circuit breaker from the house at 40amps with 8ga for both hots, the neutral and the ground out to my shed, which is roughly 20 feet. I've got a sub box in the shed where I want to run a 20a circuit at 240 for occasional equipment, and separate 120v circuits for outlets and lights.
Since the neutral is unbroken and separate, what's the max 120v 20a circuits I can run safely only sharing the hot busses? I wouldn't be using the 240 and 120 at the same time except for a couple chest freezers I will have out there, so too many tools won't be an issue. Is 3 too much to ask?

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    You are wiring the neutral to the sub panel neutral, right? – Harper - Reinstate Monica Sep 13 at 14:08
  • I have a 75a panel mounted in the shed, room for three or 4 breakers I think. (Haven’t looked since I put it in). 2 per phase. 40 2-pole breaker in the house, #8 for both phases, #8 white taped neutral ran to the neutral bus and a #8 green ran to the ground buss I added in the sub box. One wasn’t included. And ground ran from ground buss to copper rod buried outside the shed (wasn’t much fun in rocky clay). I have a 20a 2-pole for the240 outlet I’ll put in for machinery and a couple GFI outlets for separate 120 circuits. Two chest freezers and three LED shop lights on one. I should be good. – Joshua S Sep 14 at 22:14
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To add to JACK's answer, keep in mind...

A 120/240V subpanel of 40A has two separate 120V "legs". Each leg is capable of 120V@40A.

As you can see, without even having to think about it, we know we can supply four 20A 120V circuits. That was easy LOL.

And if you specifically know you will be running certain combinations of tools at the same time, you can crunch the numbers (considering which leg the tool is on), and see how you are loading up each leg in that state-of-operation. Don't exceed 40A obviously. Tools that meet certain conditions for running "continuously" need to be provisioned for 125% of their actual load - so a 12A heater is treated as 15A. A 240V load counts as its normal current on both legs at once.

Mind you, we're balancing loads, not capacity. It's perfectly OK to "oversubscribe" - put 150A of breakers on one of your 40A legs - if you've looked at your expected usage and do not expect to overload them.

By the way, if your wiring is THWN wire in conduit, you're allowed 50A on #8.

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The number of circuits isn't really the issue. You could run as many as the panel will allow, number of breaker positions. The issue is amps to the panel. You've got 40 amps to use and as long as you don't exceed that for any length of time (you'd trip your 40 amp breaker)' you'll be fine. I'd say three would be fine Think of your house panel; you've got a 200 amp service but if you total up the breaker amps, you could get 450. This is OK because you don't run everything at the same time and the circuits aren't fully loaded.

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    Likewise, if use is balanced across the legs, "40A at 240V" will run 80A at 120V due to the nature of 120/240 service on the North American model. – Ecnerwal Sep 13 at 15:22
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This is really the same issue as most people have in their main panel - you can oversubscribe because, at least in typical residential applications, most circuits are not used to capacity and most circuits are not used at the same time. For example:

  • 120V 15A or 20A lighting circuits in the past might use a few Amps - e.g., 10 x 60W incandescent = 600W = 5A @ 120V. Now with LED lighting you can easily light a big room with < 1 Amp.
  • Heating and air conditioning are rarely used at the same time, so if you have electric resistance heat and an air conditioner, they won't run simultaneously.
  • Power tools are typically run for short periods of time.
  • Dishwashers and many other appliances only need their full power, which is often in the range of 10A - 15A on a 20A circuit, for only part of their cycle (e.g., for a dishwasher, only during the water heating or drying parts of the cycle - during the actual washing they use relatively little power).

There are some exceptions - e.g., clothes dryers, water heaters, ovens, cooktops, etc. are expected to run for long periods of time - but then they are limited to 80% of the capacity of the circuit (except for short-term startup).

The end result is that 40A @ 240V is actually a lot of power unless you are running a clothes dryer, water heater or similar appliance.

Since 40A @ 240V is really 80A @ 120V (provided the loads are balanced), you can easily do something like:

  • 20A or 30A @ 240V as needed for tools, with the proviso that you manage things reasonably well. If it is a one-man shop this should not be an issue as you can only do so many things at once.
  • 15A or 20A circuits for 120V - easily 4, potentially 6 or more. The key is that some circuits (like lighting as noted above) really only use a tiny fraction of the circuit capacity, so for overall usage calculation, think of that 15A or 20A lighting circuit as a 2A circuit (but wire it as if it could someday have 15A or 20A). For power tools you can expect that a 20A circuit will draw 10A - 15A continuously, but as long as you aren't running the table saw, dust collector and 240V welder at the same time, that is not a problem.

There are other advantages to multiple circuits. If your lighting and receptacles are on the same circuit (like 1/2 my 1950s house...) then either a problem (GFCI breaker trips due to a problem with an appliance) or a deliberate action (turn off circuit to replace a receptacle) means the lights are out too. Similarly, if your refrigerator receptacle is on the same circuit as other receptacles then it goes out when something else goes wrong.

There are real calculations available for provisioning power to a full house. My understanding (but I am not an expert) is that a shed/workshop/etc. is a bit more lenient - more rules kick in when you start including kitchen, bathroom, etc.

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  • Thank y’all. Looks like I’ll be fine. Room enough for three breakers so one double for the 240 and a single and skinny twin for 3 120v circuits should be enough. I wish I’d ran a #6 neutral for breathing room but I’ll make do. Last I’d remembered I’d thought #8 thwn was good for 45a in open air but it’s underground pvc so wasn’t sure. Next time I’ll upsize it. – Joshua S Sep 14 at 2:52
  • Sounds like a very small box. Keep in mind that you may need to have GFCI on those 120V circuits, which may mean the "skinny twin" won't work - or that you'll have to put in GFCI receptacles instead - not a big deal but just to keep in mind. If you have not yet started installing that little box, you may want to trade up to a bigger box. – manassehkatz-Moving 2 Codidact Sep 14 at 2:58
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    The neutral wire doesn't need to be larger than the hot wires, as it will never carry more current than one of the hot wires. The neutral current from 120V loads on one hot wire will be cancelled by the neutral current from loads on the other hot wire. If you have a 20 Amp 120 V load on each hot wire, there should be no current in the neutral. – Peter Bennett Sep 14 at 3:44

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