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?
Wiring – How much 120 from 240 is too much
120-240vshedwiring
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Best Answer
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.