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Old 01-27-2015, 02:27 PM   #1
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Generator transfer switch sizing


Hi All,
I bought a Champion model #41537 generator. It is listed as 7500 running watts/9375 starting watts. I am going to install a manual transfer switch/panel and a power inlet box on the house in a few months when it warms up outside.
I am doing my research now looking at some of the usual types: Generac, Reliance, etc.
The generator is listed as roughly 30 amps (7500 watts/240 volts=31.25 amps).
1. Should I size the transfer switch/panel at 30 amps or use the starting wattage 9375 watts/240 volts= 39.62 amps, and up size the panel to a 50 amp model?
Not sure where I am going to put the power inlet box. I could put it on the house and have about a 25' run across the cellar to the panel or I could mount it on the exterior of the detached garage and use a 1" pvc spare conduit that I laid in the ground years ago. The garage option would be approx. 100' to the panel. I calculated the voltage drop @ 30 amps to be about 2.69% with #10 copper wire.
2.Should I up size to #8 copper or is this an acceptable voltage drop?
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Old 01-27-2015, 03:08 PM   #2
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You will be fine with 30 amp equipment.

I would choose the shorter routing from inlet to panel. Remember that you also have a generator cord X feet long from the inlet to the Generator.

Your generator has a 30 amp breaker for the 240V receptacle. It will trip if you exceed the 30 amps for too long.

The surge capacity is primarily intended for a brief overload caused by motor(s) starting.
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Old 01-27-2015, 03:10 PM   #3
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You can always "upsize" the transfer switch and associated wiring. Then if you ever get a bigger generator you're all set. The generator is protected by it's own breakers.

If you use a 50 amp inlet and only have a 30 amp receptacle on your generator, make a transfer cord that will "adapt" the two.

Don't plan on running a portable generator at full load for an extended period or it won't last.
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Old 01-27-2015, 03:17 PM   #4
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Quote:
Originally Posted by AandPDan View Post

If you use a 50 amp inlet and only have a 30 amp receptacle on your generator, make a transfer cord that will "adapt" the two.

.
Is that legal? lol....
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Old 01-27-2015, 04:33 PM   #5
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Quote:
Originally Posted by stickboy1375 View Post
Is that legal? lol....
You don't need a generator to put in a transfer switch.
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Old 01-27-2015, 04:44 PM   #6
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Quote:
Originally Posted by stickboy1375 View Post
Is that legal? lol....
Yes it is ?



Other way round could be another story thou !
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Old 01-28-2015, 08:40 AM   #7
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Thanks all for the help.
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Old 01-28-2015, 11:38 AM   #8
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Quote:
Originally Posted by Oso954 View Post
You will be fine with 30 amp equipment.

I would choose the shorter routing from inlet to panel. Remember that you also have a generator cord X feet long from the inlet to the Generator.

Your generator has a 30 amp breaker for the 240V receptacle. It will trip if you exceed the 30 amps for too long.

The surge capacity is primarily intended for a brief overload caused by motor(s) starting.
Is 30 amp rated equipment rated for over 30 amps if not for too long ? Especially 10 amps over
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Old 01-28-2015, 01:54 PM   #9
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If it isn't, everyone is in trouble.

What (approximately) do you think the trip time is on a 30 amp 240V breaker with a 40 amp load ?
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Old 01-28-2015, 07:59 PM   #10
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If it's not much additional work, put it closest to where you will be able to leave the generator running. You don't want to be stepping over an extension cord for a week if you have power go out.

Plan for the carbon monoxide buildup and, depending on your neighborhood, on having something to lock the generator to.
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Old 01-28-2015, 08:48 PM   #11
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Quote:
Originally Posted by Oso954 View Post
If it isn't, everyone is in trouble.

What (approximately) do you think the trip time is on a 30 amp 240V breaker with a 40 amp load ?
HMMMMMMMMMM... do you know the answer ???... I'm gonna make a WILD guess at......hmmmmmmm..............5 seconds
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Old 01-29-2015, 12:10 AM   #12
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It could be that quick, or longer. There isn't a one right answer as there are variables involved.

But, if I use the 5 seconds, that makes the exceeding the rating time 5 seconds before the generator breaker opens or the OCD in the transfer panel opens.

The breakers are doing their job to remove the overload, before the safety margin is gone.

The generator surge ratings are there for two reasons, to let motors start and the marketing departments find that the bigger the number you put on the side of it, the better it sells. Most purchasers do not realize how short of a time that surge power is really available.

In practice, the OP is going to find that he will have to keep loads below 30 amps or a breaker will trip. And when running big loads (or a combination of loads), you sometimes have to kill the breakers on refrigerators, A/C or other thermostat controlled loads to keep them from auto-starting while the other big loads are running. Otherwise you are making a lot of trips to reset breakers.
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Old 01-29-2015, 04:38 PM   #13
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"If you use a 50 amp inlet and only have a 30 amp receptacle on your generator, make a transfer cord that will "adapt" the two".

I was thinking of using a 50 amp set up in case I go bigger in the future. Instead of making the adapter cord now I can use a 30 amp inlet and run #6 copper to the transfer switch. Either way it will cost about the same to buy the receptacle end or buy a new power inlet box if and when that time comes. If I decide to get a bigger generator in the future then I will only have to change out the power inlet and the wire will be all set. Another question: Do I need to use service type cable from the power inlet to the transfer panel or can I use 6-3 nm copper? I can also pipe it with pvc or emt with #6 thhn/thwn.

In practice, the OP is going to find that he will have to keep loads below 30 amps or a breaker will trip. And when running big loads (or a combination of loads), you sometimes have to kill the breakers on refrigerators, A/C or other thermostat controlled loads to keep them from auto-starting while the other big loads are running. Otherwise you are making a lot of trips to reset breakers.

I know that I am going to have to manage the loads closely while on generator power. Luckily the history of power loss in my area has been of relatively short duration, usually 3-5 hours with the longest I remember being about 12 hours in the last 12 years that I have lived here. Of course there is always that future storm that could change all of that which is what I am preparing for.


Last edited by teamo; 01-29-2015 at 04:43 PM.
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