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#61204 - 01/18/06 08:54 PM Voltage Drop
HoseB611 Offline

Registered: 12/24/02
Posts: 16
Here in New Mexico they have started implementing a voltage drop rule in residential wiring. Anything over 75', the wire has to be upsized. No more than 5% VD. My problem is that I think homes have been working just fine for the past 100 or so years. It is really going to jack up the price of wiring a home. Any thoughts?

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#61205 - 01/18/06 09:48 PM Re: Voltage Drop
trollog Offline

Registered: 10/02/04
Posts: 273
Loc: San Diego California USA
and with all the varied turns and curves a run of romex takes in wending its way though a structure.. are you supposed to cut it @ 75 feet and just see where the wire runs out? I am still of two minds on this one.. yes houses have worked well for 100 years, but uses for electricity (as well as electricity usage) in homes has also changed in that time. Might be overkill, but why not build them to the best standards that the times allow? At the turn of the century a single 20A circuit was deemed adequate to supply an entire household, back in the days when consumer appliances consisted of: 1.incandescent lights, 2. incandescent lights, 3. incandescent.. there were no plasma tv's, kitchenaid mixers, conair blowdryers, garbage disposals or garage arcwelders (there weren't even cars to park inside.. garages were for horse carriages..) I think it is a lot cheaper to build in the capacity before the drywall goes on. Whatever it costs it is still cheaper than adding it years after or even hours after the drywall goes up.

As an aside, I am wondering if in 100 years our present power usage in homes won't be looked upon as just as quaint, not for how little we built into the building but for how much we built in. With more efficient and lower power consuming products coming out every day, as well as all the essentially DC stuff like computers & PV Cells.. who knows, maybe they will say in 100 more years that Edison was right with his DC power (!?!?!)

But to get back to the original point.. I don't think you can ever go wrong building it good and building it right, and keeping voltage drop under control is part of "building it right". What has me scratching my head a little is how this will be enforced by inspectors- seems a bit time consuming and opens a can of worms for determining what work on whose side of the line is at fault..I mean how does one accurately measure the lenght of a run once it is on the frame of the house? And if the VD is over the limit, you could also argue it a few ways to keep the issue going round-and-round, to no resolution. Sounds like a good idea on paper and also a goal worth striving for as the electrician, but I also see lots of gray area.

#61206 - 01/19/06 04:14 AM Re: Voltage Drop
WFO Offline

Registered: 09/03/05
Posts: 206
Loc: Cat Spring, TX
I think the old REA (now RUS I understand) used to recommend no more than 5% drop in the service entrance. By "entrance" they were defining it as transformer (including the transformer) to the meter. If you have a 2.5% impedance transformer, at full load you have already lost half your allowance.
Considering that some primary line voltage isn't that great to start with, they may have a point.

It's a little off topic, but we've always had problems with customers not wanting that "ugly" old transformer anywhere near their pretty new house, so they insist on a 250 foot service drop. A week later, they're complaining about their lights dimming when the AC comes on.

#61207 - 01/19/06 05:24 AM Re: Voltage Drop
Larry Fine Offline

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
In my opinion, if a house is large enough to have 75-foot runs, a sub-panel is an option to be considered. With load diversification, a single feeder will exhibit less voltage drop than individual home runs.
Larry Fine
Fine Electric Co.

#61208 - 01/19/06 06:53 AM Re: Voltage Drop
George Offline

Registered: 02/23/02
Posts: 380
My refigerator draws 2.5amps peak. Foolish to have a dedicated ciruit. More foolish to upsize the circuit.

I use a 50amp subpanel for every 600-700sqft in a residence. Keeps the voltage drop down. Makes it easy to NOT work on live circuits.

#61209 - 01/19/06 07:06 AM Re: Voltage Drop
pauluk Offline

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
In the U.K., we have a requirement in our wiring rules which states that the voltage drop from service entrance terminals to the farthest point of utilization on a circuit under normal load should not exceed 4% (i.e. 9.6V on a 240V supply).

In fact it used to be even tighter, and at one time specified 2.5% plus 1 volt (7V on 240V).

#61210 - 01/19/06 02:45 PM Re: Voltage Drop
HotLine1 Offline


Registered: 04/03/02
Posts: 6792
Loc: Brick, NJ USA
VD is addressed within 210.19 (FPN)3% branch circuits, and 2% feeders, hence 5% total.

I was and am under the impression that (FPN's) are 'non-enforceable', and only advisory. On that note, did your location write a local amendment with the "5%"??

As an EC (commercial/ind) VD is an issue that I address 'as required' for my clients, not as an NEC requirement. Further, within plans and specs, the EE/Architect addresses his/her VD requirements, and they MUST be adhered to, unless they create an NEC conflict.

As an AHJ, I have to enforce the plans and specs, as they become part of the Electrical Permit, upon review and approvals.

The areas that I am familiar with are comm/ind; a 'large' building can be 700K SF, and yes, VD is addressed. (By the EE/Arch, EC, and AHJ)

As to resi....; sub-panel(s) are common, a lot of #12, and it has never become an issue that I am aware of.

BTW; we do a volt check at the service entrance, and base calcs on the 'available voltage', not the 'rated voltage'.

I also include 'basic' VD calcs in my class at the Vo-Tech.


#61211 - 01/20/06 07:58 AM Re: Voltage Drop
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma

Can you give us more information on how this voltage drop rule is enforced or evaluated? Is it an amendment written into the code? Or an inspector enforcing the FPN that has been in the code for ages?

Do they mandate no more than 5% VD at the trip rating of the circuit? Or at the expected load on the circuit? How are short duration overloads (motor starting) evaluated. Must the circuit be sized sufficiently to start a large motor without 5% drop?

IMHO there is already an implied voltage drop requirement in the code, as a result of the 'effective ground fault current path' requirement. Soares interprets this as meaning that a short circuit must cause sufficient current flow to reach the 'instantaneous trip' range of the breaker. This means that at the end of the circuit, you want about 8-10x the trip rating of the breaker, implying a maximum voltage drop at the trip rating of about 10%.


#61212 - 01/20/06 08:14 AM Re: Voltage Drop
Radar Offline

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
Quote --------------------------------------------
As an AHJ, I have to enforce the plans and specs, as they become part of the Electrical Permit, upon review and approvals.

I'm a little surprised at this - I pretty much thought the local building inspector was strictly code compliance and not concerned with contract compliance.
There are 10 types of people. Those who know binary, and those who don't.

#61213 - 01/20/06 12:22 PM Re: Voltage Drop
gfretwell Offline


Registered: 07/20/04
Posts: 9025
Loc: Estero,Fl,usa
It sounds more like the AHJ just bought a suretest
Greg Fretwell

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