Generator Transfer switch question

russ47November 14, 2010

Hi,

I currently have a 20 amp gentran transfer switch installed that runs 6 circuits off my 100 amp service box. It has a 20amp twist lock plug to connect to my generator. I have a 40ft cable that runs from the switch to the generator. I have just purchased a new generator that is bigger and has a 30amp twist lock output. My question is: do I have to replace my current switch and cable? I'm fine with the amount of backup circuits I've been running and don't want to spend a lot of money. Is there a way I can upgrade the current switch and cable or use a pig tail converter plug. What are the drawbacks?

Thanks in advance.

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kaib

If you have no code issues; if you're happy with the current capacity and if you watch out that you don't add more, your plan should be fine.

I'd upgrade if I could afford it - as the extra capacity may be nice to have...say a bit more heat during the next one week ice storm.

    Bookmark   November 15, 2010 at 8:14AM
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russ47

Thanks. I didn't know I had a plan ;-). So its OK to replace the 20 amp twist lock plug on my cable at the generator end with a 30 amp plug? And this is OK as long as I don't exceed a 20 amp load?

    Bookmark   November 15, 2010 at 12:53PM
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spencer_electrician

Only if the cable is 10 gauge and the switch is rated for 30 amps at 240. Quite likely the case, a lot of the packaged sets come with a 10/4 cord with a 30 amp twistlock but an extra 20 amp twistlock connector is included for people with smaller generators. Now you'll probably be taking off the replacement 20 amp twistlock and putting a 30 amp twistlock back. But make sure it is 10 gauge.

    Bookmark   November 15, 2010 at 5:56PM
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russ47

Thanks. I'll check that. What has me confused is my old generator is rated at 5000 watts continuous and 6250 surge and has a 20 amp out plug. The new generator is a OHV rated the same as the old, (I mistakenly said it was bigger in my OP) 5000/6250 but has a 30 amp out plug. Isn't this the same except for the plug? Am I missing something here?

    Bookmark   November 15, 2010 at 7:08PM
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terribletom

"Thanks. I'll check that. What has me confused is my old generator is rated at 5000 watts continuous and 6250 surge and has a 20 amp out plug. The new generator is a OHV rated the same as the old, (I mistakenly said it was bigger in my OP) 5000/6250 but has a 30 amp out plug. Isn't this the same except for the plug? Am I missing something here?"

It may or may not be the same. Keep in mind that the gensets include overcurrent protection devices (i.e., circuit breakers) for their outlets. Gen #1 may have a 20-amp breaker for its 120/240V receptacle and Gen #2 may have a 30-amp breaker for its 120/240V outlet. In the case of Gen #1, you might only be able to use the full capacity if you have simultaneous loads on the 120/240V receptacle and one or more of the 120V receptacles.

If the new genset has 30-amp overcurrent protection on the 120/240V receptacle, then you really should have no smaller than a #10 cord, as Spencer suggests.

All of which gets us to the issue of the transfer switch itself. Since it appears to be rated as a 20-amp switch, it probably has a 20-amp breaker at the generator feed point. If so, it's a mixed blessing -- a plus because that's the mechanism that'll limit the current drawn to 20 amps (except, perhaps, for very brief surge intervals); a minus because it'll prevent you from using the peak capacity of the generator.

    Bookmark   November 16, 2010 at 11:10AM
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wayne440

The "mechanism that'll limit the current drawn to 20 amps" is more likely the generator itself, if the subject generator is a currently sold consumer grade set. The stated 5000w continuous rating is 20.8A at 240V, and said rating is probably realistic only in the eyes of the marketing dept.

The chances of such a generator producing enough current to thermally damage a quality #12 cord and L14-20 inlet is just about nil, the generator end will smoke, or engine slow to the point that the user will reduce the load well before a good 20A cordset will fail, and probably before a 20A breaker will trip except in the event of a sudden large overload.

The NEC and its wire ampacity tables are nearly sacred at times on this board. So nobody on here (including me) will tell you to just stick an L14-30P plug on your cord and use it. Perhaps you would have been better off to have never asked.

    Bookmark   November 16, 2010 at 2:37PM
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brickeyee

"The NEC and its wire ampacity tables are nearly sacred at times on this board. So nobody on here (including me) will tell you to just stick an L14-30P plug on your cord and use it. Perhaps you would have been better off to have never asked."

Cordage uses differnet rules than permenent structural wiring.

The first thing is that the insulation is often higher temperature than routine NM, and is NOT CONCEALED.

Having a cord overheat and fail lying on the floor is a lot different than having wiring concealed in walls overheat and fail.

Cords are also not usually used in a confined space, so their ability to dissipate heat is much better.

NEMA sets what few standards there are.

    Bookmark   November 16, 2010 at 4:13PM
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wayne440

Well said.

Notwithstanding that timely and relevant information, I doubt the OP will see a reply consisting of "stick an L14-30P plug on your cord and use it".

    Bookmark   November 16, 2010 at 4:28PM
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russ47

OK. Thanks for all the good responses. Because of the potential safety issues I'm now looking at a generator with the same 5000/6250 rating but has a L14-20R output. My cable is 12 gauge and has a L6-20P at the gen end (with the green wire taped off). My old gen had a 3 prong outlet. The TS cord end has a L14-20P (the green wire is inserted in the plug) that matches the TS inlet. So my new question is: do I just get a L14-20P for the gen end of the cord? I see a green wire coming out of the TS to my service box so assume its wired OK at the TS end. Any issues with this plan?

Thanks

    Bookmark   November 16, 2010 at 5:00PM
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wayne440

Do you have an ohmmeter? If so, you could use it to verify continuity between the housing of your breaker box, transfer switch housing and your cord's green wire. Otherwise, if you feel safe going forward on assumption, you are doing fine.

    Bookmark   November 16, 2010 at 6:02PM
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