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#85796 - 08/06/03 07:33 PM Voltage drop?
wired14961 Offline

Registered: 11/27/02
Posts: 13
Loc: Massachusetts
According to article 210-19(A)(1) FPN No. 4 the voltage drop on a branch circuit at the farthest outlet can not exceed 3 percent and 5 percent on both feeders and branch circuit.
My question is, how to test this?
The code does not mention anything about the circuit being loaded or unloaded.
A basic test would be to use a volt meter at the main panel and then take a reading out at the receptacles, compare the readings and see if you exceed the limits.
Or, Ideal makes a meter called a Sure Test Circuit Analyzer that uses a load and gives you the voltage drop.
So, is the first meathod of just using a voltmeter (and no load) acceptable to the code?
Any comments or suggestions would be greatly appreciated.

Thanks in advance.

2014 / 2011 NEC & Related Books and Study Guides
#85797 - 08/06/03 08:14 PM Re: Voltage drop?
resqcapt19 Offline

Registered: 11/10/00
Posts: 2209
Loc: IL
First the code does not have a voltage drop "rule". This information appears in a fine print note and is not an enforcable rule. Second, if you have a measurable voltage drop without a load you have serious wiring problems. The only way to test for a voltage drop is to use some type of known load. You must remember that when you are using a voltage drop tester you are measuring the voltage drop all the way from the utility transformer and not just on the branch circuit.

#85798 - 08/07/03 03:44 AM Re: Voltage drop?
wired14961 Offline

Registered: 11/27/02
Posts: 13
Loc: Massachusetts
Good point about the FPN, I overlooked that and was taking it for a "rule". I also agree that if there is a significant drop without a load (other than the resistance of the wire) then there probably would be serious problems. However, any type of load is going to dip the voltage and that Ideal tester I mentioned gives the voltage drop at full load (80% of circuit ampacity). Is that what the FPN is refering to? 3%-5% at full load?
Another interesting thing you mention is that the measurement goes back to the utility transformer, so a reference needs to be made a the service enterance before you take a reading at the farthest outlet. Another good point, thank you.


#85799 - 08/07/03 12:39 PM Re: Voltage drop?
triple Offline

Registered: 04/01/02
Posts: 182
Loc: Wisconsin
I'm probably missing something (usually do) but are you checking for voltage drop AFTER you have ran all wiring? 98.76% of the time, voltage drop needs to be considered far in advance. How else do you know what size wire to run for a given load at a given distance?

#85800 - 08/08/03 04:02 AM Re: Voltage drop?
wired14961 Offline

Registered: 11/27/02
Posts: 13
Loc: Massachusetts
Triple, I agree voltage drop needs to be considered before the conductors are run. Usualy, at least my expierence has been that you would consider voltage drop when you are going to have a long run and the application is usualy industrial or commercial work and maybe house wiring when your service latteral or feeder circuit is a long run. But have you ever thought of it while running branch circuits in a single family dwelling? Unless it was an extremely long run I would not give it a consideration. However, after wiring a house, passing inspection and everything was fine, the homeowner decided to have a "home inspector" check out their house a year after they moved in. The inspector plugged in a Sure Test Circuit Analyzer and it showed a 7% voltage drop at full load on a 20 amp branch circuit (#12 wire of course and runs not longer than an average run, about 8 outlets per circuit). I got my hands on one of those testers and checked the outlet next to the panel and that also showed a 7% drop (I wouldn't think there would be a drop within 5 ft of the panel). My opinion is this tester gives false readings, and that is the reason for my questions.


#85801 - 08/08/03 02:54 PM Re: Voltage drop?
Tom Offline

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA

You've just pointed out the problem with using a piece of test equipment whose accuracy has not been certified.

As far as a 7% voltage drop in a residential setting, so what? In my experiance, non-motor residential loads will not show any noticeable lack of performance at this level.

As Don pointed out, the voltage drop being measured includes all wire & connections back to the transformer. I don't think there are many home inspectors out there that would realize that this is the case.

Few things are harder to put up with than the annoyance of a good example.

#85802 - 08/08/03 03:59 PM Re: Voltage drop?
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Conservative ANSI voltage spans agreed on by electric utilities and appliance manufacturers are about +5% and -9% [and up to +6% and -13% in some cases.]

No personal offense is meant to anyone, but some home “inspectors” seem to have vested interests in trying to define what are problems, and tend to want to report findings that favor their existence, fee structure and wonderful service they offer to save the world.

In electrical measurements with various modern digital devices, resolution foolishly gets interpreted by some as accuracy. They are two different characteristics that may not turn out to be the same.

#85803 - 08/18/03 03:48 AM Re: Voltage drop?
wired14961 Offline

Registered: 11/27/02
Posts: 13
Loc: Massachusetts
Tom and Bjarney,
Thanks for your response, I apolgize for my delay in response but I was on vacation for a week. I agreee with both of you. The response given to the home owner was that all is well and good to look out for your best interest but the "home inspector" is a home inspector not a licensed electrician, and we are confident that the wiring in the house perfectly fine.

Thanks again for everybodys input.


#85804 - 08/18/03 06:15 AM Re: Voltage drop?
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Just for anyone who's curious, in Britain the rule in our "code" is that voltage drop should not exceed 4% from the main service terminals to the farthest point of the circuit under full load.

Cable sizes are determined with this in mind, although for residential work its very unusual to have to increase size just for voltage drop considerations.


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