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I have a work shop currently powered with a 20 amp breaker coming from my main panel. I want to dedicate that to a small hot water heater, and run more power for other appliances. I was thinking of running more power from my house's subpanel. I’d like to also power a small heat/ac window unit, microwave, plus some receptacles and lights. Would one more 20 amp circuit be enough, or should I run two 20 amp circuits?

  • Describe better your panels and the CB configuration Main Panel Feeder (is this the 20amp CB) to Sub Panel CB, Sub panel CB (is 20Amp?) etc.. – Ken Jul 10 '18 at 10:52
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    What make and model is your electrical panel? Is this workshop attached to the house, or in a separate outbuilding? What size are the wires running to the workshop, and are they run using a cable or individual wires in a conduit? – ThreePhaseEel Jul 10 '18 at 11:36
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    Is the shop a separate building? – Harper - Reinstate Monica Jul 10 '18 at 13:22
  • one is pushing it if you want to AC+microwave at once; two more 20 amps should be fine. – dandavis Jul 10 '18 at 17:08
  • how far away is your shop from the panel? do you have to worry about voltage drop? – CD Bosak Feb 10 '19 at 4:27
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  • power a small heat/ac window unit [ How much current will that use ? I am thinking at least 20 AMPS ]

  • A microwave - fixed (like over range or in wall) it must have a dedicated circuit - moveable countertop models 20 Amp

  • Receptacles 20 Amp

  • lights 15 AMP

Will one more 20 amp be enough ? No - you will need at least 3 more depending .

  1. Do you have a sub panel that feeds your shop or in the shop ?
  2. Do you only have a 20 Amp run from the main to the shop sub panel ?
  3. What CB Amperage is in the main panel feeding the shop sub panel ?

If you have 20Amp CB in your main panel feeding the Sub panel in your shop - you will need to upgrade the sub panel feed CB upgraded, Wiring from Main to Sub panel upgraded, Sub Panel might also need to be upgraded.

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    where does one get a 2,200 watt microwave or AC (20A)? my biggest window AC uses 800w (7A), and a typical BIG counter microwave uses ~1500w (13A) – dandavis Jul 10 '18 at 17:13
  • @dandavis depends on your microwave. Mine is internally equipped with a Slow Blow 20 amp fuse. In the NEC you will see 20amp. Circuits are de-rated so a 2200 watt device requires more than a 20Amp breaker, 1600 Watts derated is 20 Amps. Regardless the NEC code is what matters and a 10 amp breaker is not going to be acceptable for the window AC ; Part VII of Article 440, 440.32 210.23(a) 210.23(b) . Most likely it will have two outlets and op will surely attempt to use the other outlet - BTU rating and other factors - 20 Amp is a practical expectation. – Ken Jul 10 '18 at 20:38
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    better safe than sorry for sure; i just think the estimates are a bit pessimistic in the energy star age... – dandavis Jul 11 '18 at 17:36
  • I have not ever seen a microwave for residential use with a 20 amp plug the internal fuse has nothing to do with its rating. Better to understand the loads than chime in. Most AC units are also 15 amp plugs with a few I have seen at 20a. We don't know if it is a detached building or not. – Ed Beal Dec 16 '18 at 19:19
  • @EdBeal read the ops question again - note op is not asking to run one unit, but multiple units a 1000 watt microwave and a 1500 watt heater / or 1200 watt ac unit is over 2200 watts - as typical with these things people ask about the majors and start plugging in extras as well. The op will need more than a 20 amp circuit. – Ken Dec 18 '18 at 5:09

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