Best way to run feeder 350 feet underground?

infinity said:
Not sure what your actual load is but for the minimum conductor size I ran these through the Southwire VD calculator and got these numbers for:

120 volts, 350', 3% VD, Copper conductors, (Aluminum conductors):
30 amps-#1/0 (#3/0)
25 amps-#1 AWG (#2/0)
20 amps-#2 AWG (#1/0)
15 amps-#4 AWG (#2 AWG)
10 amps #6 AWG (#3 AWG)
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Are you sizing these at a continuous load rating? Is that necessary considering the wire is undergoing derating and ampacity adjustments?
 
samhutch said:
Are you sizing these at a continuous load rating? Is that necessary considering the wire is undergoing derating and ampacity adjustments?
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No. It's just the minimum conductor size based on the load listed. There is no derating or adjustments required based on the parameters in the OP.
 
For those that don't know...........
A 30A RV has a single A/C unit and is fed from a 30A, 120V, single pole breaker.
A 50A RV has two A/C units and is fed from a 2-pole 50A, 240V breaker. Although it is fed with 240V there are no 240V loads. The reason for the 240V is to divide/balance the 120V loads along with the extra load of two A/C units.
Then there is the fact that a single pole 50A breaker is not a common, easily found, breaker.
 
Little Bill said:
For those that don't know...........
A 30A RV has a single A/C unit and is fed from a 30A, 120V, single pole breaker.
A 50A RV has two A/C units and is fed from a 2-pole 50A, 240V breaker. Although it is fed with 240V there are no 240V loads. The reason for the 240V is to divide/balance the 120V loads along with the extra load of two A/C units.
Then there is the fact that a single pole 50A breaker is not a common, easily found, breaker.
Click to expand...
i'm familiar with the situation. but ....... if it's as you say and just adding a second air conditioner, why isn't it just a 30a 240v receptacle? adding that second air conditioner on the second 120v leg, would fit just fine, right???

(some have electric hot water heaters)
 
infinity said:
Would it make more sense to run a 240 volt circuit and use a transformer at the 120 volt end? That would have a big impact on wire size and voltage drop.
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Maybe somewhat break even if using aluminum conductor vs cost of the transformer for only 350 feet run? Plus there is idle load of the transformer to consider.
 
Master Nater said:
i'm familiar with the situation. but ....... if it's as you say and just adding a second air conditioner, why isn't it just a 30a 240v receptacle? adding that second air conditioner on the second 120v leg, would fit just fine, right???

(some have electric hot water heaters)
Click to expand...
Just having two AC's and little other significant loads probably would be somewhat ideal to have 30 amp 120/240 supply, but some units do have more significant loads so then we would need to have the 5-20, TT30, 14-30 and a 14-50 receptacle available at RV sites and of course even more adapters out there for when you don't have what you need. I'm fine with the choices that are already standard for RV's.
 
kwired said:
Just having two AC's and little other significant loads probably would be somewhat ideal to have 30 amp 120/240 supply, but some units do have more significant loads so then we would need to have the 5-20, TT30, 14-30 and a 14-50 receptacle available at RV sites and of course even more adapters out there for when you don't have what you need. I'm fine with the choices that are already standard for RV's.
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i am too. just stating that if 30a @120v will handle one A/C and the other loads of a small RV (which they do), adding a second 120v leg, to make it 240v should more than suffice for simply adding a second A/C, which is what he was claiming. and that's not the case.
 
infinity said:
No. It's just the minimum conductor size based on the load listed. There is no derating or adjustments required based on the parameters in the OP.
Click to expand...

infinity said:
No. It's just the minimum conductor size based on the load listed. There is no derating or adjustments required based on the parameters in the OP.
Click to expand...
just curious, cause at 30 amps you can get away with a #1 awg and be under the threshold
 
(posting before coffee)
If there is already a branch circuit to the barn, I don't think you can add another one (two supplies to one structure) unless there's an exception, so the new supply would have to be a feeder to a small panel at the barn and then abandon the existing 120 amp branch. The barn would need a panel and GES.
 
infinity said:
I'll have to go back and check. That's why I asked if #1 was good.
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Is your screen shot from a Southwire app that runs natively on your phone?

Apparently the difference in results is attributable to your having chosen "Commercial 75C" and my having chosen "Residential". Investigating a little further, when you choose Commercial and 60C, 75C, or 90C, the answers you get vary with the insulation temperature you provide. The calculator is increasing the resistance/kft with increasing insulation temperature. Which would be accurate if you were specifying operating temperature, but the actual operating temperature won't depend on the insulation temperature. It will instead depend on the current specified and the conductor size; when you are upsizing cables for voltage drop, the operating temperature will go down.

So it seems like the Southwire calculator is oversizing cables a bit by assuming an elevated operating temperature and thus elevated resistances. If the cable size you are using is such that the 60C ampacity is sufficient, I suggest always using the Commerical 60C setting to minimize this effect.

There's also another bug in the calculator (at least the web version, but as the results match, presumably the app too). Which is that the answers are only accurate for PF = 0.9. For other PF, the app messes up the calculation of the effective impedance. But the discrepancy is only significant when the conductor reactance is signficant compared to the resistance, e.g. for 250 mcm and above. I reported this bug to them in July 2024, but they still haven't fixed it.

As to the "Residential" setting, from the info text it sounds like it should match the Commercial 60C setting. They are close but slightly different; not sure what it is doing differently.

Cheers, Wayne
 
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