ECN Forum Forums General Topics of the Trade General Discussion Area Rule of thumb

I'm looking for a rule of thumb on voltage drop for pulling branch ckts. I dont mean to dummydown the voltage drop calculations, but when your out in the field, at what footage do you decide to pull #10's instead of #12's? Do you just usually pull 10#'s to the home run box, and then 12#. I've also heard some people say that if its over 50feet, pull 10's. Just looking for a general rule, if there is one. thanks, -Erik.

I would say that the rule is, that there is no rule. I've seen spec's that call for 75', some for 150', and others for 250'. 210-19 (a) 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, will provide reasonable efficiency of operation. See Section 215-2 for voltage drop on feeder conductors. I would follow the spec's for the job. If there are no spec's then I would judge each case by it's own circumstances. If you are really concerned about a particular installation, pull in larger wire, and go home with confidence knowing that you did the right thing. Remember that FPN's are not enforcable, but good to follow. Good Luck.

Dropped my voltage, and can't seem to find it,
Doc

Engineers can have their own dummy rules for voltage drop. IIRC, the usual division 16 boilerplate on 120V receptacle circuits for 10- versus 12-AWG is ±75-100 feet; ie: “…furthest outlet exceeds 100 feet for 120V circuits use # 10 AWG or larger.”

Doc,

Your voltage is by the current!

WWWaaattt's up!

For a 20 amp circuit with a maximum load of 16 amps this was what the http://www.wescodirect.com calculator provides as an answer:

Length (ft) Min Conductor

20 12
30 12
40 10
50 10
60 10
70 8
80 8
90 8
100 6
120 6
140 6
160 4
180 4
200 4
240 4
280 3
320 2
380 2

thanks for your replys.

The voltage drop calculater is pretty neat. I see that it shows 40feet to be a good rule for 20 amp general purpose branch ckts (lighting and convience recep)
I looked back at some old job spec too and noticed that footages were specified. I guess I just do what I'm told, for now. Pull 10's Erik - Ok brother

One of the "home inspectors" here locally bought himself one of those "Inspector" three-prong electrical testers that measures voltage drop, and boy, has he been making waves! He's got a bunch of the realtors ready for a tar and featherin' party!
Around my area at least, none of the new home electricians seldom pulls anything larger than #12's to basic lighting/receptacle loads.

VD is a social problem, not an electrical one here.

We do service work on houses in the +2000 sq ft range that are all 14/2 NM except where the NEC historically required 12/2 NM (kitchen, dining rm, that sort of thing). 150 feet or more from the back of the upstairs bedroom to the loadcenter in the garage, all in 14 gauge NM cable, is not unusual. They use 14/3 to cut install costs to those long pulls, too. Share that #14 neutral!

Anyways, this home inspector has been noting on his reports that many of the homes he's inspected have an unacceptable voltage drop problem (20% in one case I looked at). And he's right, but how many of these people are going to pay (or can pay) to have nearly their entire house re-wired? They (the realtors) call me and ask for my opinion, and suggested options. I generally give two answers;

1) Re-wire to larger gauge wire, (pretty serious money) or,

2) Sell it with the deficiency noted.

Guess which one is ALWAYS chosen?

By the way, these houses are generally always signed off by the local housing inspectors. There normally is an "Approved" sticker fastened to the loadcenter door, where there are inspections.

Ok, my headache's back.... Doc, oh Doc!..

Dallas, it sounds like the good “inspector” may be taking liberties with {certainly unenforceable} fine-print notes on the 3% & 5% “reasonable efficiency” suggestion. Has there been violation of legal or even ethical business practice?

It’s likely the instrument maker hands out only very general recommendations on interpretation of readings. You might ask for measurements and calculations on division of voltage drop in the branch circuit, feeder, service lateral and transformer. Ask him to respond with the limits established by the serving utility, exactly where they apply and which national consensus standards pertain to his claim of “deficiency.” Ask what other instrumentation was used to verify the accuracy of his reported “problem.” Insist on a current record of calibration and chain of NIST traceability on the instrumentation used to establish his claims.

Eventually a seller is going to question his methods and motives and take action with the help of local real estate and electrical trade. Kinda’ like the small-town cop and judge processing speeding tickets in their 20MPH section of the interstate for “public safety.”

Real estate transactions do seems to have their own rules and customs. The “inspector” sounds like he may be doing this primarily to pad his report and justify his own existence and fees. Might be worth asking what licensing, registration, certification, examinations, and experience he has.

The country needs more self-appointed expert tripe like this.

{Check under the monitor for your pills.}

Shades of Boss Hogg & Rosco.....

I know it's not much help in your situation, but over here our accepted tolerance for voltage drop is no more than 4% from service entrance to farthest point of circuit under normal load. The tolerance used to be tighter, at 2.5% plus 1 volt.

Ditto, Bjarney.

FPNs are not enforceable. I do a lot of Real estate work and on several occasions have had to counter unreasonable "recommendations" with applicable NEC references.
The Home Inspection Agency is a cottage industry that is growing fast. It can good for us because of the additional work. On the other hand, they do seem to be getting more demanding. And this can put a lot of strain on the settlelemnt process.
"A little knowledge can be a dangerous thing"