Allowable voltage drop on a circuit

mkcookNovember 14, 2006

Question for the gurus of the National Electrical Code

We have a new home being built. As engineers (oh, no collective groans, please :-) ) we are trying to ensure the home is built well and safe.

We have a couple of circuits which are showing excessive voltage drop under 15 Amp load (we have a tester). These circuits are showing greater than 10% voltage drop, RMS, under the 15 Amp load.

One of these circuits also has 5 duplex outlets, a switched duplex outlet, 2 multi-bulb switched light fixtures, and a switched bathroom fan all on the same 15 amp circuit.

The builder says his electrician insists the wiring was done to code. Yes, the actual wiring was assuredly the correct gauge, etc., but what about performance to code?

This is not an inexpensive home and we don't understand the pushback. We are concerned about fire safety and equipment warranty.




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That circuit is most definitely wired within code and not overloaded by any means.
There is no "performance code", and the 3%/5% voltage drop is only a suggestion. Albeit a strong suggestion.

How long is this circuit? The main reason there would be that extreme drop is if the circuit were unusually long.

Fire safety is NOT an issue with regard to this. Just performance.

FPN No. 4: Conductors for branch circuits as defined in 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 the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation.

    Bookmark   November 14, 2006 at 7:32PM
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What type load are you placing on the circuit? At what point are you measuring the drop?

    Bookmark   November 16, 2006 at 1:48PM
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We are using an Ideal SureTest 61-155 tester, which I believe puts a 12-12.5 Amp load on the circuit which the tester extrapolates for the 15 and 20 amp testing. We are testing each duplex receptacle on the circuit, starting at the first and going to the last. The voltage drop gets worse the further down the circuit you go, but is >10% at the first outlet.

This is new construction and there are no loads on the circuit other than the tester when we are checking it.

We suspect the problem could be the number of splices in the circuit and the type of connections made at the receptacles, but we have not pulled one out of the wall to verify the connections since we don't yet own the house.



    Bookmark   November 16, 2006 at 5:07PM
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The number of splices has nothing to do with it.

How long is the run?
Something does not sound right.

    Bookmark   November 16, 2006 at 6:00PM
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There should be no voltage drops through proper splices and receptacle hookups. You only get voltage drops through a load. All circuits in a home are wired in parallel, so that all points see the same voltage. I'd suspect the testing tool. Have you tried another test tool?

    Bookmark   November 16, 2006 at 6:15PM
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A bad connection will give a high VD reading on these instruments. If you get this reading at the first receptacle, remove it and check the connections. I have seen this before and usually find a either a screw not tightened or a poor splice.

    Bookmark   November 16, 2006 at 6:46PM
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Exactly. Poor connections heat. Poor connections mean resistance. Resistance causes voltage drop which is the energy that does the heating. Someone may explain it more eloquently, but a voltage drop means that electrical energy is converted to some other form of energy.

    Bookmark   November 16, 2006 at 7:01PM
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Don't forget that you are measuring voltage drop all the way back to the transformer. The cause of the voltage drop is most likely in the branch circuit to the outlets, but could be anywhere from the outlet back to the transformer.

    Bookmark   November 16, 2006 at 7:38PM
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The original post mentioned that two circuits have the voltage drop problem. If the observed drop is confined to those circuits only, the transformer is not the problem.

    Bookmark   November 16, 2006 at 8:14PM
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If I may clarify further - we are seeing proper voltage at each outlet when we aren't applying a load. It's only when the test tool applies a test load to check for voltage drop on some circuits (the couple in question), we see a greater than 10% drop under load, and then at each duplex outlet on that circuit.

In each case, the outlets are not the only connections on the circuit, one of them has a light in a completely different part of the house wired in (we have asked for this to be changed) and the other has at least 2 light fixtures plus an exhaust fan as well.

Our thoughts are exactly what "normel" and "bus driver" are echoing - we think it's a connection quality issue and the high impedance connection is causing voltage drop (and energy loss elsewhere).

It could also be the length of the circuit from the distribution panel. Because this is new construction we are trying to get the builder and the electrical contractor to investigate but they are refusing to. I was hoping to find someone in the trade who had experience in this area and might help us communicate our concerns (damaging electronic equipment due to low voltage excursions is one) more convincingly.

Thanks, everyone, for your ideas and questions.


    Bookmark   November 16, 2006 at 11:04PM
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For a circuit wired with #12 cable to show that much voltage drop at the first receptacle (over any drop at the main panel, which is apt to be small), it would have to have at least 200 ft of cable from panel to first recept. If that's not the case in your house, you most likely have a loose connection. It could be in the panel or at the receptacle. It might just be a screw that needs to be tightened.

In any case, the VD you're seeing is excessive, and your electrician should fix it. This is not good for appliances and it wastes energy.

    Bookmark   November 17, 2006 at 1:18AM
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Hi ... I am an engineer like you and I am in the process of challenging my builder as well for the SAME IDENTICAL ISSUE. I agree its a problem that needs to be rectified. My builder has agreed to contract an independent 3rd party to work with me (sounds much better than your contractor) even though the electricians are saying no problem exists.

Can you please let me know how your situation turned out. Also, when I am through with my process I will share my outcome with you. Please advise ... I look forward to hearing from you.

    Bookmark   March 26, 2009 at 8:45AM
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Attempting to find out if your voltage drop issue was ever resolved in 2009. It is 2014 now and I am having the same issue. The builder and electrical contractor are stone walling me. Even the city does not want to get involved. I need help in finding a solution to this issue.
I am using a Extech CT70 AC circuit load tester. I have 50 electrical receptacles that exceed the 5% national code limit for voltage drop.

    Bookmark   April 23, 2014 at 1:37PM
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On any conductor, the voltage drop is directly proportional to the load and directly proportional to the length from the source to the load. Provisions of the code seem to me to assume that a 15 amp circuit would be loaded at not more than 12 amps continuously. So voltage drop of itself does not have much meaning unless the amperes load and the length are also known.

    Bookmark   April 23, 2014 at 5:43PM
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"I am using a Extech CT70 AC circuit load tester. I have 50 electrical receptacles that exceed the 5% national code limit for voltage drop."

If you read the posts in this thread you'd see that there is NO "5% national code limit for voltage drop".
Voltage drop is a performance suggestion in the NEC and other codes.

    Bookmark   April 23, 2014 at 9:01PM
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If you want to know the length take the circuit off the neutral bar and breaker then tie them together, goto the last outlet and test the resistance (ohms).
Take the ohms reading and divide it by 2 and then divide it by .003 if its 14 gauge or by .002 if its 12 gauge. This should get you a general idea of how long it is. Wire has a resistance per foot and that's what causes the voltage drop.

    Bookmark   April 25, 2014 at 11:34PM
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