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I was recently gifted a Champion brand, 3550 watt, 120 v generator. I had intended on hooking it up to my electrical panel via generator plug inlet box and interlock kit, but after doing some research I've come to realize I've got a complex problem.

In theory, I could rig something up to jumper the single hot into two separate hots and power both legs of my panel. But, I have several MWBCs in my house, and my understanding is that would create a situation where I am overloading the shared neutral because the generator is only producing a single phase of 120v power. Is this correct?

What are my options, other than trying to exchange or sell for a 240 v generator?

Thanks!

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  • Are there inexpensive 120v to 240v with center tap transformers out there? They may be a substantial fraction of the cost of a new generator...
    – kgutwin
    Jul 23 '20 at 15:02
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    @kgutwin Yes, you often see 120/240-240/480 5 KVA transformers for $100-ish on Craigslist that would be about the right ampacity. OP would have to ignore the 240/480 primary, and wire the 120/240 secondary to synthesize the other leg. For thermal reasons it could only synthesize half transformer ampacity. Jul 23 '20 at 18:52
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Yes! That is correct. Well done with the MWBC investigation.

The cheap/fast answer is simple: Since you are familiar with MWBC, it is safe for you to rearrange your loads to place all your critical loads on one phase. For instance, that would mean powering only 1/2 of any MWBC. Attach the 2-pole generator breaker in the usual way, but only connect your "hot" to one leg.

For the sole purpose of researching which leg a load is on, it is OK to temporarily pull 1 wire off a 2-pole MWBC breaker, or defeat the handle-ties. Just put it back when you're done :)

In fact make sure they are handle-tied while you are in there.

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  • Thank you, Harper. Just to be clear, if I get my hands on a 240v generator the MWBCs require no special treatment, correct? I should be able to backfeed my panel and run whatever 120v loads I want (so long as the generator can handle it)
    – clwhoops44
    Jul 23 '20 at 19:56
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    @clwhoops44 Yes, in fact I would wire your interlock and inlet for 240V even for this generator, and make an adapter cable to take you from this gen to the correct leg of the 14-30 or whatever inlet you are using. Jul 23 '20 at 20:19
  • I see the potential for a problem if a 240V appliance is switched on while only one leg of the panel is energized. Some of the current would be introduced to the other leg at a reduced voltage. I don't know how it would affect everything, but it sounds like a situation to be avoided. I think you'd see some of the same effect if you try to power half of a MWBC, wouldn't you?
    – mrog
    Mar 2 at 21:27
  • @mrog yes, if the MWBC also had 240V loads on it (MWBCs can concurrently support 120V and 240V loads -- if the breaker is common-trip for that very reason). Unfortunately for OP, there is no good answer. The least bad bet is that the OP knows the generator is 120V so hopefully will shut off 240V loads. Whereas if the 120V generator fed both hots, you'd never know you were overloading neutral. Mar 3 at 19:16
  • @Harper-ReinstateMonica Thanks. So, is it fair to say that if there are no MWBCs and 240V breakers are turned off, that it doesn't matter to safety or legality whether a 120V generator is connected to just one leg or both of them?
    – mrog
    Mar 3 at 21:05
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You could transfer some of the 120V circuits to a box that can be powered from the generator via a transfer switch.

Otherwise, you are correct — trying to run both legs of your power will create problems for all of your 240V circuits, and you can consider a MWBC a 240V circuit with a neutral for this purpose. Much cheaper to buy a 240V generator than to deal with these issues.

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  • I'm not looking to power any 240v circuits, just 120v.
    – clwhoops44
    Jul 23 '20 at 12:54
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    you can consider a MWBC a 240V circuit with a neutral for the purposes of this answer Jul 23 '20 at 13:50
  • Can you elaborate on the potential problems with 240V circuits when powering both legs with a 120V generator? Could it break things, or would the appliances simply not turn on?
    – mrog
    Mar 2 at 21:42
  • ahouseonarock.com/midlothian-home-inspector/what-is-an-mwbc Apart from that, 240v is simply 2x 120v 180deg out of phase. However, when both legs are powered from the same 120v, they are in phase, so the voltage would be 0, and your 240v appliances would not turn on.
    – gbronner
    Mar 3 at 2:26

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