voltage drop calculation

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electric wrangler

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ky
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electrician
Hi everybody, I've got a customer that needs power ran from the house panel to a 60 amp sub panel for an RV located 300' from the house panel.
My questions:
What percentage should I use for my VD calculations? Is 5% too high?
When I use 5% the corresponding wire size is #2 AU
2-2-2-4 URD is a little less than $4 a foot.
Would like to get some guidance on this topic before I buy 300' of wire.
Thanks,
Mark
 
I would probably go 3% but 5% should work. If it is 120 volt load then at 5% you would see 114 volts. At 240 v you would see 228v.
 
Since you show 3 CCC I'm assuming this is a 120/240 50 amp outlet. I would use 3%. Also I would figure the drop using 120 volt as you will often have an 120 volt A/C unit starting when there might not be any load on the other leg.
 
Traditional rule-of-thumb has been 2% for the feeder, 3% for the branch circuit. But being an RV, the branch circuit will be negligible in length so I'd go with 5% for the feeder.
 
Thanks for the responses! The sub panel would be 60 amp 120/240. However the RV (actually little trailer from the 80's) has a 120v 30 amp cord that runs lights and an AC unit that has not been installed yet. I suggested to the customer that we install the sub for possible future use.
 
texie said:
AFAIK they are all 120/240 with all load connected line to neutral and there are no 240 loads. That's why 208 volt parks are OK.
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Same thought here. Those with 50 amp supply cord often have multiple AC's but they are all 120 volt AC's

You also don't want reduced neutrals if 208 volts.
 
texie said:
Since you show 3 CCC I'm assuming this is a 120/240 50 amp outlet. I would use 3%. Also I would figure the drop using 120 volt as you will often have an 120 volt A/C unit starting when there might not be any load on the other leg.
Click to expand...
If the AC starts across the line it will always introduce pretty significant voltage drop during the starting time period. Unless you want to do VD calculations based on the locked rotor current, but even then the source impedance can possibly still cause VD during starting.
 
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