Voltage Drop for an Outdoor Receptacle Circuit
Voltage Drop for an Outdoor Receptacle Circuit
(OP)
How does someone consider a voltage drop for long runs of an outdoor receptacle circuit? The NEC shown below says you need 3% maximum voltage in your feeder. I've been given two different approaches to this and would like the opinion of this forum. One engineer has told me he just uses #10 wires and doesn't calculate anything.
Another told me to use the demand for a typical outlet (180w @120v = 1.5a) and calculate the voltage drop for each leg and sum it up.
For example:
A 5 outlet circuit w/ 250ft to first outlet 7.5a total with the remaining four circuits having 75ft between them you calculate the voltage drop for each leg and sum it up and design for 3% max total.
(VD%=2*250ft*7.5a*0.51ohm/kft/1000ft/kft*100%=1.59%VD)
(VD%=2*75ft*6.0a*0.51ohm/kft/1000ft/kft*100%=0.38%VD)
(VD%=2*75ft*4.5*0.809ohm/kft/1000ft/kft*100%=0.46%VD)
(VD%=2*75ft*3.0a*0.809ohm/kft/1000ft/kft*100%=0.30%VD)
(VD%=2*75ft*1.5a*0.809ohm/kft/1000ft/kft*100%=0.15%VD)
total VD of 2.89%
Alternately:
Would you want to design the cable size considering the voltage drop with a max load of say 1.5kw (12.5A) plugged into the final receptacle (VD%=2*550ft*12.5a*0.201ohm/kft/1000ft/kft*100%=2.3%VD)
I would like to hear your thoughts on how to design for this situation.
Thanks,
CKB
The NEC says:
Conductors for branch circuits as defined on article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent.
(0.201ohm/kft #2, from Table in Chapter 9 NEC)
(0.51ohm/kft #6, from Table in Chapter 9 NEC)
(0.809ohm/kft #8, from Table in Chapter 9 NEC)
Another told me to use the demand for a typical outlet (180w @120v = 1.5a) and calculate the voltage drop for each leg and sum it up.
For example:
A 5 outlet circuit w/ 250ft to first outlet 7.5a total with the remaining four circuits having 75ft between them you calculate the voltage drop for each leg and sum it up and design for 3% max total.
(VD%=2*250ft*7.5a*0.51ohm/kft/1000ft/kft*100%=1.59%VD)
(VD%=2*75ft*6.0a*0.51ohm/kft/1000ft/kft*100%=0.38%VD)
(VD%=2*75ft*4.5*0.809ohm/kft/1000ft/kft*100%=0.46%VD)
(VD%=2*75ft*3.0a*0.809ohm/kft/1000ft/kft*100%=0.30%VD)
(VD%=2*75ft*1.5a*0.809ohm/kft/1000ft/kft*100%=0.15%VD)
total VD of 2.89%
Alternately:
Would you want to design the cable size considering the voltage drop with a max load of say 1.5kw (12.5A) plugged into the final receptacle (VD%=2*550ft*12.5a*0.201ohm/kft/1000ft/kft*100%=2.3%VD)
I would like to hear your thoughts on how to design for this situation.
Thanks,
CKB
The NEC says:
Conductors for branch circuits as defined on article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent.
(0.201ohm/kft #2, from Table in Chapter 9 NEC)
(0.51ohm/kft #6, from Table in Chapter 9 NEC)
(0.809ohm/kft #8, from Table in Chapter 9 NEC)






RE: Voltage Drop for an Outdoor Receptacle Circuit
Remember more load leads to more drop. Simply using #10 will not solve the problem if the load is more than a few amps. If the load is say 16Amps (assuming a 20amp breaker), then you'll need larger wire. Possibly #4 or #6 (would have to do the calc to know for sure).
www.electrician.com has a free VD calculator if you want to save some time. Depending on your company procedures you may need to verify the software calc.
Hope this helps.
Sense
RE: Voltage Drop for an Outdoor Receptacle Circuit
CKB
RE: Voltage Drop for an Outdoor Receptacle Circuit
http:
RE: Voltage Drop for an Outdoor Receptacle Circuit
thanks
ckb
RE: Voltage Drop for an Outdoor Receptacle Circuit
Don
RE: Voltage Drop for an Outdoor Receptacle Circuit
I would suggest sizing for voltage drop at at least 80% of breaker size or more.
RE: Voltage Drop for an Outdoor Receptacle Circuit
If, for instance, the maintenance crew tried to use a pressure washer at the last receptacle the starting current may be over 50 amps. On a 500+ foot feeder motor starting will be difficult even if the circuit is sized for voltage drop at 12 or 16 amps.
yours
RE: Voltage Drop for an Outdoor Receptacle Circuit
Keith Cress
Flamin Systems, Inc.- <http://www.flaminsystems.com>
RE: Voltage Drop for an Outdoor Receptacle Circuit
RE: Voltage Drop for an Outdoor Receptacle Circuit
If you expect a bunch of loads, all at the same time, use different circuits to break up the voltage drop.
RE: Voltage Drop for an Outdoor Receptacle Circuit
RE: Voltage Drop for an Outdoor Receptacle Circuit