voltage drop ???????

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windtech007

Member
Location
New Mexico
I'm somewhat confused about voltage drop calculations and actual drop.
Seems like most of the time when I check voltage at the main service panel I'll get readings like 245.2v or 123.2v. So when the branch circuit calculation indicates excessive voltage drop where should you start subtracting the drop?
Example, on a 120v ac, 20a, #12awg branch that runs 115' VD is 4.8v or here in New Mexico excessive. But at the main I get 124.2v with my Fluke 87 so can I expect 119.4v at the receptacle? On a 120v circuit 119.4 isn't a problem is it?
With the price of wire now days I don't want to up size if not truly needed.
I just threw the numbers out there for example, not actual.
I appreciate any help you guys can offer.
Thanks,
Phil
 

iwire

Moderator
Staff member
Location
Massachusetts
Phil IMO calculations should be done using the 'nominal' voltages found in 220.5.

The NEC does not normally address VD so my remark above is just my opinion and not an NEC requirement.

But I am curious what load do you assume on a 20 amp circuit?
 

haskindm

Senior Member
Location
Maryland
I agree with Iwire. You must first determine the ACTUAL load on the circuit, you can't assume that a 20-amp circuit will be loaded to 20-amps. In fact, if it is a continuous load, it should not be loaded to more than 16-amps. Very few 20-amp circuits should see a 20-amp load, especially in residential installations.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Remember that voltage drop is a design issue.

One part of the design issue is the actual voltage delivered to the load. If the supply is 'hot', then the voltage drop might actually bring you right to your desired operating voltage. Some loads can tolerate a wide range of supply voltages, others are more limited. If the load can tolerate a wide voltage range, then you don't need to worry as much about voltage drop.

Another part of the design issue is variability. If you have a load that _changes_, then the voltage drop will change, leading to things such as lights flickering every time another load on the circuit changes. Some loads are very sensitive to voltage variations (lights) others are not. It can pay to separate such loads, eg. separating lights and receptacles.

-Jon
 

jghrist

Senior Member
Another part of the design is efficiency. High voltage drop means high I?R losses. Keeping the voltage drop to 3% or less, regardless of the terminal voltage, will result in reasonable losses.
 
haskindm said:
I agree with Iwire. You must first determine the ACTUAL load on the circuit, you can't assume that a 20-amp circuit will be loaded to 20-amps. In fact, if it is a continuous load, it should not be loaded to more than 16-amps. Very few 20-amp circuits should see a 20-amp load, especially in residential installations.

This is true. However unless you have a single, unchanging load on a circuit such as only light fixtures maxed to the 80% loading level you should calculate your VD to the maximum rating of the OCPD.
 

windtech007

Member
Location
New Mexico
iwire said:
But I am curious what load do you assume on a 20 amp circuit?

The circuit is a single outdoor GFCI recpt. Customer needed to upgrade meter loop / service for a small sub well pump. It is fifty miles from town out on a ranch and will be used for livestock water. The only load might be a elect fence charger or hand tools used to service the well.

Seems like we would make calcuations using actual voltages instead of nominal.

I wasn't sure so I asked.
 

broadgage

Senior Member
Location
London, England
As others state voltage drop is a design issue/engineering judjement and not a code requirement.
3% is a good guide, but a higher voltage drop may be acceptable.

It may however be unwise to count on the voltage at the main panel being high, what if the serving utility drop it to the lowest permitted? as they might.

In the case of a well pump starting curent needs to be considrered in addition, on very long cable runs the voltage on startup may be insufficient, resulting in burn out of the motor.
 
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