ECN Forum Forums Code Related Discussion NEC & other Code issues General Lighting Circuit Voltage Drop

I just had a conversation with a friend doing housing inspections. He, on occasion, uses an Ideal circuit tester to measure voltage drop for specific clients. Recently, he found a rather large, new, 600K house with an office/den that had outlets consistantly showing 15% voltage drop when tested.

The tester will be calculating the drop based on current at the rated circuit capacity, in this case, 15 Amps.

It occurs to me that the outlet, being a general lighting outlet will have a calculated load much less than that. . .perhaps 50 to 120 VA, depending upon how many opennings are in the X ft² alloted for that circuit's portion of the General Lighting Load. Hypothetically, let's say 18 outlets on a 15 Amp circuit, or 100 VA per outlet.

To satisfy the minimum of the Code, and, thinking of 210.19(A)(1)FPN No. 4:

Would I measure the voltage drop on the farthest general lighting outlet by applying a 100 VA load (from the above hypothetical)?

I do not know to much about this, but I do not believe that there is anything enforceable in the code about voltage drop.

Perhaps you could use 110.3(B) and say the device must be supplied with the nameplate voltage.

We have done jobs where in the spec we had to have less then 5% total drop.

They checked this at the end of the job with a DMM at each outlet, without any load what that told them I have no idea.

For all practical purposes in a house only a resistive heater will draw much current.

I would ignore it. If you test all of the circuits, you may find most have that much voltage drop. I think it only takes 100' of #14 wire to cause that much drop at 80% load.

Al, i have the same ideal tester

How many lighting circuits are under 100' (as the wire runs) can you pick out in the average house?

How about receptacle circuits? 75' comes to mind.....

(i wire them seperately)

The fact is, you'd need to use a lot more #10 to idealy comply

Bob,

I like it! A no load voltage drop test, an oxymoron I'll bet the quality of your wiring wowed 'em.

Sparky & George,

I did a calc on 5% drop, 120 V, 12 A load (80% of 15) and I get

#14 solid = 81.4' of cable
#12 solid = 129.5' "
#10 solid = 206.6' "

Thinking more, this afternoon, I suppose placing a 100 VA load on each of my hypothetical 18 outlets and then measuring the voltage at the farthest outlet down the cable run would be the most accurate.

I find it interesting that the NEC Handbook says 210.19(A)(1)FPN No. 4 is

I should have mentioned this was a Government Services Administration Electrical Engineer that was checking and the work was for the IRS. (here comes the audit)

Al,
if i apply the 80% rule to a 14 wire, 15A circiut i come up with 1440W, or 8 locations per 220.3.B(11).

or lets just say we have an exclusive lighting circuit using 1440W at 8 locations, this would be fairly hard to run, switch loops and such, to a total lentgh of 81 feet.

besides that, those ideal testers are giving us a reading back to the x-former.

Interesting point about the Ideal tester. . .I've never used one, but as you say it, it is obvious to me that the electronics in the device would be comparing no load voltage to load voltage, and this would be the drop through the total impedance back to the transformer. Thanks.

Sorry about the 80% thing. That's a red herring. I'm trying to describe a non-specific circuit with luminaires and receptacles that would be calculated through 220.3(B)(10) which flips back to 220.3(A). I've been taught that (in dwellings) the 180VA / outlet figure does not apply for this circuit, and that, in fact, there is no maximum number of outlets.

You're right, 81 feet is rediculously short. The more I think about this, the shorter the distance gets.

Yes Al, i was getting a tad specific, sorry.

The tester itself induces, and holds a load correct? Or is it a phantom load or multiplier used?

This then makes me wonder if, theoretically every circuit in a dwelling where to have said test done simutaneously (so much for diversity) would the VD rise?



[This message has been edited by sparky (edited 03-20-2003).]

Inquiring minds want to know

I imagine the electronics in this device do quick samples of the voltage, quick enough that the 60 Hz sine wave would appear flat for the instant of each sample. One sample would be with no load, and the second would be with 15 amps flowing. The difference of the two voltage samples would then be represented on the tester display.

This tester 15 amp sample current would be on infrequently and for very short duration, just long enough to overcome the "average circuit capacitance", that might reasonably be expected, before taking the voltage sample.

Also, the load pulse is short enough and infrequent enough to limit the heat that the tester electronics must dissipate to manageable levels.

So, it seems to me, with all of a house's circuits having simultaneous testing happening, the individual tester reading would be unreliable if the timing of the load pulse is not identical tester to tester. One tester would be providing a load while another tester is trying to sample the no load voltage. The Vdrop on the panel feeder and/or service conductors would be seen by all the testers.

If the tester electronics do trigger the load pulse at exactly the same instant, then the pulse Vdrop of multiple testers would be additive in the feeder/service conductors. This would be the worst case, IMO, resulting in erroneously large Vdrop readings. Let's say there are 40 branch circuits tested, 16 at the 20 amp level and 24 at the 15 amp level, evenly divided on either side of the panel. The service conductors out to the transformer would see load pulses of 340 amps each.