Voltage drop

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jj19897

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Does the NEC anywhere define "voltage drop" and specifically, how is it measured?

I am told by an electrical contractor that voltage drop is not measured under load. As an engineer, this flies in the face of my training.

The application is a new single family residence. Supply voltage (Measured at the service entrance with a calibrated digital voltmeter.) is 127VAC. Voltage at furthest outlet from panel (130 feet) is 123 VAC.
Both of the above with no load.
This same outlet under load (1800 watts) measures 114VAC. I feel there is a problem with undersized wire, defective connection, or?
Can anyone refer me to appropriate sections of NEC which apply? Many Thanks jj19897
 
Re: Voltage drop

Strange how these things come in threes. :)

Voltage drop is not regulated by the NEC. Therefore, the contractor is not required by code to observe it.

Your training about the nature and calculations regarding voltage drop are most likely correct. Your 11.4% voltage drop is not very good.

What load do you have that draws a full 15 amps? I would hope this is on a 20 amp circuit?

In general, why has this come up? What's the "rest of the story?" :D

(Edit to add: Given that you lose four volts under zero load, there could be some inferior connections contributing to it.)

[ May 22, 2005, 02:51 PM: Message edited by: georgestolz ]
 
Re: Voltage drop

Without a load there will be no voltage drop. Voltage drop is the results current(load) verses resistance (circuit conductors).
If you are geting differant voltage readings at two points in a circuit there has to be a load somewhere? The meter if a DVM should not be able to load a circuit enough to produce this ether. You can put a 1k ohm resistor in line with a DVM and as long as the meter is the only load it would not show a voltage drop. except for the very little that the meter draws.
 
Re: Voltage drop

In response to your question concerning load - Iused a hair dryer appliance rated at 1800 watts. The reason this has come up is that 2 separate computer printer failures were diagnosed as failed SCR's in power supplies brought about by low line voltage. I began to test various branches and found large numbers of duplex outlets, entrance chandeliers (2 containing 6 40 watt lamps) powder room lighting, hallway lighting and convenience outlets and external security lighting (160 wats)
all sharing the same branch circuit which serves my office.(and the printers which failed. The home is new, the local summer temperatures are often above 100 degrees F, which drives the attic crawl space temperatures to 130+. The wiring for most of the structure is bundled loosely and traverses this attic space. Wiring appears to be Type NM-B and the maximum size is 12 AWG. My concerns are that this wiring is sized inadequately given the number of outlets and lighting fixtures surveyed on just this one circuit and given the linear distance to the breaker for this particular circuit of over 120 feet. Other circuits are up to 140 feet from the breaker panel. Many thanks for your reply.
The builder has asked for my assistance with his subcontractor as he shares my concern. Perhaps my question should address the requirements for proper wire sizing and not voltage drop as you say that is not controlled by NEC. Thanks, jj
 
Re: Voltage drop

Is this circuit a shared neutral circuit? If it is then there could be a problem if a moterized appliance is starting up on the other circuit that shares the neutral as the voltage drop will cause a spike in the circuit feeding your office. I have a hard time of beliving that a voltage drop on just over 11% will cause a failure if the SCR's in the printers? I have all my equipment on a Triplet voltage regulator/line conditioner as well as surge suppressors, then dual conversion UPS to the equipment That I want backed up. I've been hit with a long term surge from the POCO and didn't have one failure. What I mean about long term is There was a fault between a HV primary line to a MV primary line and it cause a 950 something voltes to surg for about 15 seconds befor a OPCD opened. Whith all the TVSS I have it caused my main (200 amp) to open.
But either way it sounds like the circuits should have been split up better.
 
Re: Voltage drop

114V doesn't really sound like the end of the world, especially to an appliance that is fed from a plug-in transformer, IMO.

My printer has a 30V trasnformer. So, at 114V on the primary, it would be running at 28.5 volts. That doesn't sound terrible, but I am not an expert.

Wayne wrote:
I have all my equipment on a Triplet voltage regulator/line conditioner as well as surge suppressors, then dual conversion UPS to the equipment That I want backed up.
Click to expand...
Between your van and your house, I don't know what I envy more! :D
 
Re: Voltage drop

No, this is not a shared neutral circuit. The printers are not connected via transformers. They are older laser printers with thermal fusers. They did not fail at the same time as they were not simultanously powered. The 114VAC measurement with a 1800W load applied was on a longer branch circuit, not the one serving the printers. I cannot determine actual voltage at the time of failure as I don't know the precise time of failure or how many lights and outlets were in use at the time. Again, my concerns are that the wire ga selected for the long runs (140 ft., (one way) are undersized for the wire type, tempersture,etc, Breakers are 20A. I am attempting to acquire the EC's load calculations.

Thanks again
 
Re: Voltage drop

114 volts is not good; however, all electrical equipment made for the U.S. is made to work on as much as 10% voltage drop which you do not have. I suspect your problem lies somewhere else than with the apparent voltage drop in the circuit (motor starting load?).

Most public service commissions permit the electric utility to have 5% over or under nominal at the meter, the electrical contractor is expected to have another 5% and still have reasonable performance. :D
 
Re: Voltage drop

First, the measured voltage drop is 10.2%, not 11.4%: (127 ? 114) /127.

Secondly, household devices are rated at 120 volts. 114 volts is 95% of this value, and should give satisfactory performance. The problem would come up if the utility were to supply less than your one-time measurement of 127 volts. However, you did not state that you measured both the 114 volts (at the outlet) and the 127 volts (at the service) while the hair dryer was running. In other words, we cannot calculate a voltage drop unless all measurements were taken under the same conditions.

Third, as far as the original construction contractor is concerned, voltage drop is not measured, it is predicted. That is, the selection of wire sizes should be made with the intent of maintaining less than a 5% voltage drop from the service point to the farthest point, given the load that that farthest outlet would be expected to see. This is not an NEC requirement, but is ?suggested? in a Fine Print Note.

Finally, the bottom line is that you do appear to have a problem. The computer equipment would best be served by a circuit that is separate from all the other loads that you listed. But if you are looking for a way to fault the original construction contractor for a bad design or bad installation that caused the equipment failures, then I don?t think you will find any help in the NEC. The code gives minimum for safety, and does not guarantee that equipment will function in an acceptable manner. Whatever else comes from your investigations, I think you will wind up having to pay for an ?upgrade.?
 
Re: Voltage drop

JJ19897

Charlie is correct in saying that you should add some more circuits. A 1800 watt hair dryer at 120 volts draws 15 amps. But if you had a run of a 140' using # 8, A 20 amp load would only drop your voltage down 4.2 volts, A 15 amp load would drop your voltage down 3.2 volts. Your have to remember that you need to take 140'x2=280' You have to figure your feed and the return when you do a voltage drop problem like this. If you used #10 on this same circuit A 20 load would drop your voltage down 6.8 volts and A 15 load would drop you voltage down 5 volts. When reading your first thread you said you had 127 volts at the panel and with no load you checked your 140'run and got 123 volts with no load on it. It wouldn't hurt to check all your connections thru out that circuit. You shouldn't have any voltage drop on the end of that run if you don't have a load on it. Hope that will help some.
Jim
 
Re: Voltage drop

I have experienced this same problem. The problem was that the power co. never crimped the neutral splicing device at the service drop.
I believe there is a problem with the neutral connection, probably between the utility transformer and the branch circuit overcurrent device. If you have a loose connection on the line side of the service, even though there is no load on the suspicious branch circuit, and there are loads on the other branch circuits, the voltage will be unbalanced between L1 and L2 and the neutral. Therefore, you can read a lower or higher voltage than the nominal voltage, depending on what the loads are on the other branch circuits.
 
Re: Voltage drop

Many thanks to all who responded. In order to get a better handle on this, I'll conduct a more comprehensive survey noting: load currents, branch circuit lengths, simultaneous voltage measurements of service entry, and branch at varying loads, etc. I sincerely appreciate all the comments and suggestions and will perhaps re-visit this issue with more data. - JJ
 
Re: Voltage drop

Originally posted by georgestolz:

Voltage drop is not regulated by the NEC. Therefore, the contractor is not required by code to observe it.

]
Click to expand...
Voltage drop appears in fine print notes under 210.19(A) and 215.2(A). Fine print notes are not "required" but it would certainly be considered poor design to ignore them. The contractor isn't required by code to observe voltage drop strictly, however he could still have his self dragged into court over ignoring it altogether. An inspector could still reject his installation as well.

In the manufactured housing industry, in some jurisdictions, voltage drop tests are a required step in certification of a house.
 
Re: Voltage drop

Originally posted by lile001:
Voltage drop appears in fine print notes under 210.19(A) and 215.2(A). Fine print notes are not "required" but it would certainly be considered poor design to ignore them. The contractor isn't required by code to observe voltage drop strictly, however he could still have his self dragged into court over ignoring it altogether. An inspector could still reject his installation as well.
Click to expand...
How could an inspector fail an installation based on voltage drop?
:confused:

[ May 25, 2005, 07:23 PM: Message edited by: iwire ]
 
Re: Voltage drop

Yes, once again, NEC is not a training or instruction manual. The NEC does not contain every thing one needs to know about the art/science of electrical technology.

Voltage drop, like many other things, is a reality & must always be given at least a passing consideration.

I hope I am speaking from the standpoint of a practical, professional. It is not just about whether it passes inspection or not.
 
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