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I have a 50A 220v box outside for use during a power outage to feed the breaker box on both bars. Since this is very rarely used, i'd like to use it to install a 'dryer plug' for a new shop heater. The shop heater requires 30A 220v service.

What's the best way to splice into the 50A line to feed the 30A plug for the heater?

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    We're going to need a bit more info. All generator installs require a transfer switch to be installed to prevent whats called islanding where your generator can back-feed the grid causing all sorts of problems and hazards for utility workers. This would hamper your install attempt. If you don't have a transfer switch, your generator outlet/inlet was improperly installed. – Mister Tea Nov 29 '16 at 23:39
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Don't try this (at home, or anywhere else for that matter)

You'd have to defeat the function of the transfer device to actually power the heater (unless you wanted to use your generator to power it without throwing the transfer device over to the generator position). Furthermore, once you did that, the generator inlet would be live, with live prongs sitting there waiting to zap unsuspecting folks with 240V.

And if you're planning on doing this because you don't have some sort of transfer device (transfer switch, breaker interlock) installed, then your electrical installation deserves a red tag from the local inspector, as backpowering the grid is a great way to accidentally zap unsuspecting linemen who are out there trying to turn the lights back on.

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You could just do a home-run of 10/2 from the service panel to the heater receptacle, and your problem goes away. Here I consider only options cheaper than that.

Here's what I'd do. It's tedious, inelegant and involves messing with wires in the rain. But it works and is safe, assuming you are competent.

TLDR: manually re-wire your house every time you want to switch.

Install a transfer switch in your service panel if you don't already have one. You need that regardless. Near the generator's side of the transfer switch, install a 30A 240V breaker in the panel. By "near" I mean close enough that the wires could be moved over to it.

Install the largest junction box you can, at whichever point the heater line diverges from the generator line, that is, where you would change the wire splices so it goes to the heater instead of the generator.

To run generator, alter your wiring to use it for a generator:

  1. main power off and generator breaker off
  2. open up your service panel and move the wires to the generator breaker
  3. open up the junction box and splice the wires to go to the generator inlet
  4. make sure you did everything right
  5. generator breaker on
  6. light up the generator

To run heater:

  1. main breaker off and generator breaker off
  2. open up the junction box and move the wires to the heater receptacle
  3. open up your service panel and splice the wires to go to the 30A breaker
  4. make sure you did everything right
  5. main breaker on.

Take your time, make sure you did everything right.

You could also do it with two DPDT switches, but you'd absolutely have to have those interlocked so they throw together. That is possible, but is more expensive than a 10/2 homerun. "cheap" is the operating principle here.

  • Egads, what a kludge! (Also, do panel interlocks let you set both interlocked breakers OFF?) – ThreePhaseEel Nov 30 '16 at 3:57
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    Terrible kludge! This is what cheap looks like, and "cheaper than a 10/2 run" is a pretty low bar. Most panel interlocks I've seen allow both to be shut off; the simplest is a rod between alongside breaker handles which is too long for both breaker handles to be pushed inward. – Harper Nov 30 '16 at 5:31

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