Voltage Drop for an Outdoor Receptacle Circuit

[dspfun]

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)

 

[Senselessticker]

From a practical standpoint in your case: It will be most helpful to know the exact load and kind of equipment that each receptacle will see. Assuming 180VA is not a good idea in this case.

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).

http://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

 

[dspfun]

They will be outdoor receptacles for a Down Town streetscape improvement project. Typical loads would be Christmas lighting or maintenance equipment.

CKB

 

[cuky2000]

Try the enclose link

http://www.nexansenergy.com/egy/tecdoc/Corflex/old.html

 

[dspfun]

Actually calculating VD is not the question. I don't need any programs to do it. I would like guidance on what assumptions to use in this case.

thanks

ckb

 

[resqcapt19]

Note that the voltage drop information in the NEC is not a code rule. It is a FPN (fine print note). Fine print notes are informational only and are not enforceable as requirements. It is a good practice to follow but not required by the code.
Don

 

[waross]

Hello dspfun

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)

Typical loads would be Christmas lighting or maintenance equipment.

I would suggest sizing for voltage drop at at least 80% of breaker size or more.

 

[waross]

Hi again;
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

 

[itsmoked]

The NEC really doesn't look at peak currents. What happens if the user plugs a vacuum cleaner or an air compressor or a tungsten load (Christmas lites) in at the last outlet..? We get people in here asking why their compressor blows the breaker at the last but not at the first outlet .. I like the "I use 10AWG and call it a day" suggestion. It's a good one.


Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[Senselessticker]

Using #10 will work fine if the load will be limited to a couple of amps at 120V. If the circuit will be loaded to say 10 amps, you will need #4 to stay under 3% drop at 500 ft.

 

[tommom]

You need to look at the intended use - if they are for "convenience", just take voltage drop with the largest expected load at the last outlet. If your panel is close to the source panel (ie panel feeder has low voltage drop), you could increase your allowable voltage drop accordingly, as high as to 5 or 6 percent and still be within standards.

If you expect a bunch of loads, all at the same time, use different circuits to break up the voltage drop.

 

[dspfun]

thanks for your comments.

 

[stevenal]

Is three phase available? For a given amperage you can double the copper and halve the voltage drop for single phase. For the same amperage going to balanced three phase, you can again halve the voltage drop by doubling the copper. To achieve the the same amperage, though, you've tripled the loading. Of course your receptacles are not three phase, I'm just suggesting that you pull three circuits off different phases and use a shared neutral. The Christmas lights in particular could be balanced among the phases. Same theory if pulling two hot legs from a 120/240 system. Still have that maintenance device at the end of the line causing a single phase two way voltage drop.