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Wiring the workshop, it'd be simplest to use the 220V circuit to also supply the shop's 110V outlets. I know some machines/appliances do that internally; can I expose it on the wall?

(I'm never sure whether to call it 110/220, which is what I heard as a kid, 120/240, or 115/230.... or if they're all recognised as nominal so it doesn't much matter except when planning wattage capacity.)

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    The technical term is "120/240V". – Tester101 Aug 30 '15 at 17:30
  • Are you talking about supplying both 120 volt and 240 volt loads, on the same circuit? Or do you want to split a 240 volt circuit, into two 120 volt circuits? – Tester101 Aug 30 '15 at 17:32
  • The question is whether I can cheat and use it both ways at once depending on what's plugged in and turned on, subject to current limits. – keshlam Aug 30 '15 at 18:27
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There are a few concerns:

Neutral and Grounding

First and foremost to do any of this would require a neutral conductor and a non current carrying conductor known as a ground/earth.

Undersized Grounding

Most double pole branch circuit grounds are only sized at #10. This is only good for up to 60 Amps according to the NEC. The total Amps gained with the 120 V outlets may exceed what the code allows for #10 grounding. If the existing double pole is rated at 50 Amps, than you have 10 Amps to spare before having to increase the size of the ground to a #8 If you plan on removing the double pole receptacle all together than there shouldn't be a problem with a undersized ground.

OverCurrent Protection

If the above criteria has been met and the existing double pole receptacle is protected by a double pole 15 or 20 Amp breaker then you can safely tap off the conductors at the junction box to fed other 120 V outlets.

However if the double pole receptacle is protected by a breaker greater than 20 Amps, then one solution would be to change the the double pole receptacle to a small main-lug sub-panel. Then branch off the sub-panel so each branch circuit is protected by its own breaker.

  • Understood re it being a four-wire circuit. Good point re beefing up ... did you really mean safety ground, or neutral? I believe the existing 240V circuit has a 30A breaker in each leg, back at the box, so yeah, subpanel is probably required to meet code. Darn; I was hoping to simplify a bit. – keshlam Aug 30 '15 at 18:38

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