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Old 02-07-2013, 09:18 PM   #1
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Which gauge wire from portable generator to transfer switch?


Hi Folks,
I have a Honda eu3000is which is rated at 2800W (3000W max for up to 30 mins) and am consider a 90-100' run to the 30A, 120V single phase APC UTS6H (.PDF) transfer switch that's connected to my main panel.

I originally envisioned a 10' run, but the 100' run would place this generator in a spot that's much easier for refueling and help keep it shielded from bad weather.

I've been trying various calculations of voltage drop that support 23.4A (i.e., 2800W / 120V) up to 100', but am not sure my numbers or assumptions are correct.

If I looked at AWG size resistances on Wikipedia, I see things like:

AWG Size Resistance (Ohms/ft)
10 0.000999
8 0.000628
6 0.000395
5 0.000313
4 0.000249
3 0.000197
2 0.000156
1 0.000124
1/0 0.0000983
2/0 0.0000779
3/0 0.0000618
4/0 0.000049

So, I thought Voltage drop could be calculated as:

Voltage drop = (Ohms/foot) (length) (current)

Which, in my case would give examples like:

10: 0.000999Ohm/ft x 100ft x 23.4A = 2.34V
8: 0.000628Ohm/ft x 100ft x 23.4A = 1.47V
6: 0.000395Ohm/ft x 100ft x 23.4A = 0.92V
5: 0.000313Ohm/ft x 100ft x 23.4A = 0.73V
4: 0.000249Ohm/ft x 100ft x 23.4A = 0.58V
. . .

Looking at other discussions, I may be doing something wrong with these numbers - plus, I don't know if there would be any problems using lower gauge wires with my (essentially) 30AMP run and connectors.

Figured I would use THNN/THWN through outdoor nonmetallic conduit from the outlet to the transfer switch, if that meets code.

My target is to minimize voltage drop for this little generator across the 100' span.

Thoughts or ideas welcome.

- wader

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Old 02-07-2013, 09:34 PM   #2
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Which gauge wire from portable generator to transfer switch?


#8 copper will give a 2.76% voltage drop over 100' @ 24 amps @120 volts.

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Old 02-07-2013, 09:35 PM   #3
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Which gauge wire from portable generator to transfer switch?


Hope you do not plan on running much more then a few lights.
Before doing anything I'd suggest you take the time to figure out the loads on what you plan on running to see if it will even work. Adding on that 100 ft. is really pushing it.
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Old 02-07-2013, 09:41 PM   #4
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Which gauge wire from portable generator to transfer switch?


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Originally Posted by joecaption View Post
Hope you do not plan on running much more then a few lights.
Before doing anything I'd suggest you take the time to figure out the loads on what you plan on running to see if it will even work. Adding on that 100 ft. is really pushing it.
Yeah, I know - that's why I was hoping that heavier gauge wire would help minimize the impact of this distance.

I'm only looking to power the house furnace blower, refrigerator, gas water heater and some odd lights - not more than the basics. Plus, this transfer switch allows me to configure different circuits as enabled for turning off up to x minutes in a given period of time, so it can dynamically load balance when one circuit has a suddenly large draw, etc.

- wader

Last edited by wader; 02-07-2013 at 09:50 PM.
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Old 02-07-2013, 09:51 PM   #5
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Which gauge wire from portable generator to transfer switch?


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Originally Posted by brric View Post
#8 copper will give a 2.76% voltage drop over 100' @ 24 amps @120 volts.
Thanks brric - maybe my calculations were off?

I should have mentioned that I'm planning ahead for parallel generators someday (twice the amps, essentially - neat feature of these models, and with a compatible transfer switch), so would like to wire for that eventuality.

Should I be considering 6 or even 4 gauge, then?

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Old 02-07-2013, 09:59 PM   #6
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Which gauge wire from portable generator to transfer switch?


Why would you want to have to deal with twice the maintaince, noise, fuel consumption and not just buy one bigger generator with a 30 amp outlet?
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Old 02-07-2013, 10:01 PM   #7
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Which gauge wire from portable generator to transfer switch?


What you need to do is put the generator 10' away as you planned. With the money you save on the wire you can buy another generator.
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Old 02-07-2013, 10:10 PM   #8
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Which gauge wire from portable generator to transfer switch?


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Why would you want to have to deal with twice the maintaince, noise, fuel consumption and not just buy one bigger generator with a 30 amp outlet?
This inverter already worked well (using extension cords, which I'm getting away from), is very quiet at night (important due to the neighbors), has been easy to service and is great on gasoline. I would add on a parallel generator only if it was really needed - since that's possible, it was something I considered as a no-brainer growth path.

Sure, another option is that I could sell this unit through eBay and go for a 6500W 120/240V inverter model that's a little more money than two of my current models. Guess I'll cross that bridge when I get there, thanks.

So, on to my question about wire gauge to consider - any thoughts?

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Old 02-07-2013, 10:12 PM   #9
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Which gauge wire from portable generator to transfer switch?


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What you need to do is put the generator 10' away as you planned. With the money you save on the wire you can buy another generator.
Yeah, wire is expensive, I'm finding.

Still, I really want the better location and am considering a future upgrade to twice the power if this generator doesn't match up to my load expectations - so, I'm willing to at least consider how much it might cost for a future growth path before writing off the idea.

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Old 02-08-2013, 03:38 PM   #10
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Which gauge wire from portable generator to transfer switch?


Looking back, maybe I should revise this request for advice by just asking about wire gauge that would be appropriate a 100' run from a 54A generator to the transfer switch. Perhaps that's more straightforward to focus on.

I'll consider upgrading my generator ASAP if this makes sense to invest in.

We're in a storm now and the 10' solution is workable, but not easily so compared to the 90-100' location. My understanding is that you ideally want a voltage drop less than 5%, but again, I could be wrong.

Sorry, I'm just learning as I go along.

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Old 02-08-2013, 04:18 PM   #11
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Which gauge wire from portable generator to transfer switch?


How do you plan on running this 100ft of wire? I think it would be much better to put the generator closer.
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Old 02-08-2013, 04:35 PM   #12
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Which gauge wire from portable generator to transfer switch?


I really think you should consider moving the generator closer to the house. With motors starting (such as a fridge), you need all the power you can get, and you want as little voltage drop as possible.
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Old 02-08-2013, 04:45 PM   #13
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Which gauge wire from portable generator to transfer switch?


Don't forget that it's a 200 foot run for the round trip.

Shoot for 3% or less voltage drop (3-1/2 volts out of 120) on the line out to the generator. There will be a percent or two additional loss in the wiring inside the house to the receptacles.

At most 3-1/2 volts loss divided by 30 amps means at most 0.12 ohms for the 200' round trip. I come up with 8 gauge copper wire for this run.
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Last edited by AllanJ; 02-08-2013 at 04:56 PM.
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Old 02-08-2013, 06:35 PM   #14
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Which gauge wire from portable generator to transfer switch?


I did this a few years ago, also with a EU3000; only my run was 70'. I used 8/3. The labor is much more important than the material (at least it was in my case) and the /3 would allow an upgrade to a 240v generator twice as large.
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Old 02-08-2013, 08:38 PM   #15
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Which gauge wire from portable generator to transfer switch?


Quote:
Originally Posted by AllanJ View Post
Don't forget that it's a 200 foot run for the round trip.

Shoot for 3% or less voltage drop (3-1/2 volts out of 120) on the line out to the generator. There will be a percent or two additional loss in the wiring inside the house to the receptacles.

At most 3-1/2 volts loss divided by 30 amps means at most 0.12 ohms for the 200' round trip. I come up with 8 gauge copper wire for this run.
Thanks AllanJ - good point about the return trip. I'd be interested how you mapped the 0.12Ohms for 200' to come up with 8 gauge wire, but the results is useful in and of itself.

I'd really prefer to use the 90-100' run (through plastic conduit around the outside of the house, adjacent to a plastic conduit that holds the wire run for our garage panel from the main) due to convenience, but might need to just build a mini structure to keep the generator protected from elements - and still allow for exhaust per Honda's operations manual - for the shorter run everyone is suggesting, I am thinking.

With my switch, does the voltage drop effect all circuits equally?

If I want to move up to a generator rated at 45A with 54A max, (and a corresponding transfer switch), would I need to use 6 gauge?

- wader


Last edited by wader; 02-08-2013 at 09:56 PM.
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