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Joined: Jul 2004
Posts: 9,677
Likes: 9
There is an article in the IAEI magazine this month advocating that the 3% and 5% in the FPN be made code.
It is no surprise the author is from the copper lobby but he does make a compelling argument that it might be part of an energy code. He even has a poll saying 94% of respondents think it should be in the NEC but that poll may have been taken around the doughnut box at

If you actually put pencil to paper you will find that 310.16 and perhaps even 240.4(D) may not be very useful.
We could be using yet larger wire.
We might actually end up changing the typical topology of most wiring systems with more sub-panels and more use of multiwire circuits ... when the 2 pole AFCIs hit the market.


Greg Fretwell
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Joined: Jan 2005
Posts: 5,316
Cat Servant
Instituting such a rule is pure silliness - and ignores the most important factor: what is the supplied voltage?

For example, NEMA standards require motors to operate within 10% of the nameplate voltage. Let us, for the sake of this discussion, assume a motor is marked "220v"

If that motor is installed on a nominal 208v system, it can operate as low as 198v. The 5% rule would give you 197v. You're outside the motor specs.

If that motor is installed on a nominal 240v system, you're OK - as long as the supplied voltage does not exceed 242v. Since the utility might very well be providing 250v, you're in trouble unless you make sure you have at least 4% drop.

Moreover, about half of that 5% is assigned to the PoCo side of the circuit - the one part you have no control over.

Why are PoCo voltages a concern? Because areas still under development tend to have really high voltages, while older installations have voltages drop as loads increase. That's what transformers do at full capacity: lose voltage.

Therefore, the really important issue is the actual voltage delivered to the motor, and not some percentage of voltage drop.

If you want to check your overall circuit, then test it for impedence. A higher impedence -reflected in voltage drop under load - might suggest a bad connection somewhere. Still, keep in mind the critical voltage range of the appliance.

After all, a motor marked "230" will be fine at 208 - but out of spec at 207!

Joined: Apr 2002
Posts: 7,288
Likes: 4
There are times VD calcs are necessary, however there are many times when they are not.

Comm/Ind here in NJ requires plans, one-lines, etc prepared by a Lic Architect/PE/EE. Instances where this could become an issue are noticed sometimes during plan review, and only 'suggestions' can be mentioned. Changing the FPN to 'code' eliminates 'suggestion' to 'comply'.

As to EC's and electricians being affected by this, in the McMansions it may become an issue. I include VD calcs in one of the classes I teach at Vo-Tech, and it is usually well received.

Joined: Jul 2004
Posts: 9,677
Likes: 9
The point the article is trying to make is excessive voltage drop represents wasted energy.

Greg Fretwell
Joined: May 2005
Posts: 984
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It's mandatory in Chicago; you'd be amazed now many 225A panels need to be fed with 500kcmil to meet the voltage drop limits.

It gets to be interesting when you're trying to save money by using smaller wire and the building owner finds out that he isn't permitted to use the NEC wire sizes, but that he COULD reduce the wire size if his building was 2 miles further West.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Ahh ... the article is using 'voltage drop' as an excuse to introduce 'green thinking' and 'global warming' into the NEC. (And, yes, the article brings up 'global warming').

Humbug! Stop these fanatic missionaries ay the gates! Their message has NO place in a document dedicated to the concept of the ''minimum for the the practical safe use of electricity.' The NEC is all about 'safety,' saving the planet is outside the scope of the NEC.

Voltage drop is both a design issue, and a quality issue.
We have little control or influence over the 2% on the utility side of the equation.

As for 'our' 3%, the calculations may be nice .... but are largely irrelevant.

Irrelevant, first, because they cannot evaluate connections - which is likely to be a greater source of loss than simple wire length.

Second, because the voltage drop depends upon the actual current draw. While it can easily become an issue when you're drawing 15 amps on a 15 amp circuit (or even more if you're trying to start a motor under load), it's meaningless if all that branch circuit is powering is the TV set.

There's the rub: most of our work involves the very circuits that are NOT sized according to the loads to be used - the household 'convenience' circuits.

As to this 'green' nonsense: For convenience circuits, you have a choice: more receptacles on a circuit, or more circuits. Either way, you're going to 'lose," as you either use much more material, or risk greater voltage drop in the calculations.

The 'save the world' crowd will then adopt the same approach they do with regard to parking spaces: fewer receptacles will be their goal. This is in direct conflict with the NEC.

Beware of wolves in sheep's clothing.

Joined: May 2007
Posts: 169
Stop them at the gates? Ha!! They are already in your bedchambers!!!

About 220 miles to your west, Reno, the thinking is likely to be on the "save energy whatever the cost" side. And a great number of misguided people want to adopt CA's standards as those of the entire US.

Include the word green in it somewhere and its as good as sold.

I have always interpreted the VD notes to mean 2% on the feeders from the service entrance and a total of 5% to the furthest branch point and have designed accordingly. Not for green-ness but for the best power transmission and least chance of my wishing I had run larger conductors.

As far as it becoming code, anyone want to bet against it?

Joined: Jul 2004
Posts: 9,677
Likes: 9
I have been told this is already the law in Florida, via the energy code but I didn't include any V/D calculations in my plans when I added onto my house, nor were they part of the process, so I am not sure how it is getting enforced.

Greg Fretwell
Joined: Nov 2007
Posts: 763
Originally Posted by gfretwell
It is no surprise the author is from the copper lobby but he does make a compelling argument that it might be part of an energy code.

Great... It seems like the NEC is apparently just a rump swab for every lobbyist with an agenda or a hip pocket to fill. Nema's little cash cow as well as the more Holy than tho’ do gooders guide to salvation for the plebeian unwashed masses.

Wait... did that come off as too cynical? grin

Joined: Jul 2002
Posts: 717
It will make the home inspectors happy, cause if it is adopted then their cute little VD testers ( Ideal Shure Test ) will suddenly become real.......

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