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Old 11-06-2012, 09:20 AM   #1
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Gen Transfer Switch Feed Line Length


Yet another generator related question, thanks to Sandy.

I bought a 5500 watt (6850 peak) generator. I'm looking to add in a transfer panel and I would like to keep the generator in the garage, and when needed, roll it out on to the drive way to run it. The problem is that the breaker panel is on the other side of the house.



The idea is to put the inlet receptacle at the garage and run a feeder all the way to the transfer switch. The problem is that I will need to snake this feeder through the attic to get to the panel which is at least a 75ft run.



Running the numbers, I will have to run at least a #8 or even a #6 line to keep the voltage drop down. With this generator, I know I'll never hit 30amps, not even at surge, but I want to do the calculations against 30 amps to have room to grow in case I get a larger gen.


The question is how to connect the inlet receptacle to a #8 or #6 line when the terminals on a L14-30 take #10? Do I need a junction box with a terminal block in it and run a short length of #10 between the terminal block and the receptacle. I know there are rules regarding different sized wire in a single run.



Also, I have to check to see if the transfer panel will accept a larger gauge wire. If not, I will probably have to do the same thing there with another terminal block.


Any ideas, suggestions?
Thanks,
Omar

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Old 11-06-2012, 10:49 AM   #2
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Gen Transfer Switch Feed Line Length


At 75' or even 100' you will be fine using #10. Copper is expensive, no need to waste your money. However, if you are dead-set on upsizing the wire, #8 will fit into every inlet I have ever seen. Check out the inlet, it will list the range of wire that it accepts.

The transfer panel will also accept larger wire than #10. If for some reason it doesn't, just wire nut a short pigtail of #10 onto the larger wire.

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Old 11-06-2012, 01:22 PM   #3
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Gen Transfer Switch Feed Line Length


If it is likely you will install a larger generator in the future, use 8 gauge wiring from the generator inlet for 40 amps (9-1/2 kw)and 6 gauge wiring for 55 amps (13 kw).
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Old 11-06-2012, 03:20 PM   #4
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Gen Transfer Switch Feed Line Length


I will soon have the same situation (70 feet)and have determined that #10 wire (@0.999 ohms/1000ft) will drop less than 3 volts at 25 amps, your maximum balanced load.

I will use #10 which I priced at $100. Anything more is overkill and wasting money

Last edited by zeke11; 11-06-2012 at 03:25 PM.
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Old 11-06-2012, 03:24 PM   #5
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Gen Transfer Switch Feed Line Length


Thanks for the info....But....Bare with me here....

If you run 100' of #10 at 30amps, based on the calculations I've run, you get a 6 volt drop which is above the 3% recommended threshold for a feeder and branch circuit. The same goes if I would use a #8 for 40 amps which is about a 4 volt drop or a little over the 3% threshold.

I understand that this 3% value is merely a recommendation and not code, but is it something that should be considered especially if you are running motors (frig, heater)?
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Old 11-06-2012, 04:27 PM   #6
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Gen Transfer Switch Feed Line Length


Quote:
Originally Posted by obadran View Post
Thanks for the info....But....Bare with me here....

If you run 100' of #10 at 30amps, based on the calculations I've run, you get a 6 volt drop which is above the 3% recommended threshold for a feeder and branch circuit. The same goes if I would use a #8 for 40 amps which is about a 4 volt drop or a little over the 3% threshold.

I understand that this 3% value is merely a recommendation and not code, but is it something that should be considered especially if you are running motors (frig, heater)?
I've got 3-4 dozen customers who I have installed interlocks or generator panels with runs of 10-3 over 100' and I never got a single call back with undervoltage issues.
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Old 11-06-2012, 05:03 PM   #7
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Gen Transfer Switch Feed Line Length


Note that the worst case of dropping 6 volts out of 120 volts on the 10/3 line will only occur with 30 amps being drawn on one 120 volt leg and 0 amps on the other. Most likely, the total load and also the total drop will be split across both legs with the likelihood of no more than 4 volts dropped on one leg (and 2 volts on the other). Less of a drop if the heavier loaded leg was carrying less than 30 amps.
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Last edited by AllanJ; 11-06-2012 at 05:07 PM.
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Old 11-06-2012, 09:27 PM   #8
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Originally Posted by AllanJ View Post
Note that the worst case of dropping 6 volts out of 120 volts on the 10/3 line will only occur with 30 amps being drawn on one 120 volt leg and 0 amps on the other. Most likely, the total load and also the total drop will be split across both legs with the likelihood of no more than 4 volts dropped on one leg (and 2 volts on the other). Less of a drop if the heavier loaded leg was carrying less than 30 amps.
Thanks for that explanation. I was hoping someone would give such an explanation.

I sometimes have been accused of over designing stuff (ie. exceed code...which I know isn't necessarily a bad thing). I guess if I ever get near to fully loading both 120V legs, I should probably get a bigger generator. With the generator I have, I'll max out 25 amps.

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