I am doing a job for a guy at church.He has a shed 169 ft from his house.He has a few flourescent lights and a welder 21 amps.Me and a buddy have done the calculations but I wanted your opinion.Not taking the future into account,which the owner says don't worry about(:)),should I not pull 10 to save him money?Also,not downsize nuetral?

I'm not sure if the Code will allow this, but I would want to run two lines out there, one for the welder, and one for everything else, so that you don't have the lights dimming up and down (as much) from the welder.

assuming 240 volts, 3% VD would be 7.2 volts
VD = 2 x I x L x R / 1,000, using the R per 1,000 feet in Table 8, Chapter 9.
Transposing: max R = 1,000 x 7.2 volts / 2 x I x L
L = 169 feet
I = 21 amps (or 25 amps with lights?)

so max R would be 1.1 to .85 ohm which means a #8 copper conductor would be required. (.809 ohm per 1,000 feet)
Neutral could be downsized for 240 volt load.

If you are running 120 volts, then maximum VD would be 3.6 volts.
and max R would = 1,000 x 3.6 / 2 x 25 x 169 = .42 ohm, which corresponds to a #4 copper conductor.

If you run less, you risk low voltage to appliances. Problem??? maybe, maybe not.

As usual,I left out info:(..The welder is 230 volt,3 prong outlet(I am pulling 2 hots,nuetraland ground....thks

I am trying to figure how you are going to same him money, he needs a 18 in deep trench and a lot of PVC and wire, hope he is getting a permit for this, if the job is done shabby and it leaks current to the neighbors property watch out. I would not even be a thing of saving money, but doing it right.

yeah,LK,WE have a permit and there is an existing conduit.1000' no 8 270.oo,1000 no 10 140.oo.I intended to pull 8 from the start but thought I'd discuss the load and wire size among friends....Leaking current???????????

I think the welder is such intermittent service I would not get too crazy about it. When you have your mask down you won't see the light dim anyway. If he really thinks he will be adding a lot more load, then go with the #8 but 10 should be fine.

I would really consider a second circuit for the light, if he runs the welder too long and pops the breaker, he will be in the dark with a really hot piece of metal in front of him.

Whatever you do he Will outgrow it, like a gold fish people expand to fit the size of their surroundings, I would present him with the prices and lrt him make the call. Personally I feel bigger is better, I would try to sell him some 6awg wire just to have the headroom. Greg is right on the nose with the welder, I have one that Hobart says needs a 50a breaker to run, been running it on a 30 for 3 years and never had a problem. Low amps and light duty cycle equals low required current.

Leaking current better known as stray, comes from various sources, poor substation grounding, bad power distribution designs, shabby underground work.

Every year we get calls I am getting a shock when I walk in the back yard, or when I touch the chain link fence, usually when we get to the job we may find a backyard shed that has been wired by a DIYer, and it usually had shabby wiring with faults causing us to get stray readings.

Hope this helps you understand, leaking current.

If there is a welder he will use a chop saw and or a grinder, just a thought.

Yeah,pulled 8.Along time ago ,15 to 20 yrs,someone gave him some 2 conductorand ground overhead aluminum entrance wire(bare ground).He was getting 20 volts on one leg to ground.Used the old wire to pull.Old wire had one strand in tack on ground-nuetral! and one hot blown in two,may have been in tack some before the pull.He made the comment that his lights looked BRIGHTER now:)His panel is NOW bonded and nuetrals sep.With the old 3 conductor,do you thing it was good that they DIDN'T bond panel to ground to grt hit off return? or not really

Being budget-minded, I would suggest the inexpensive ACWU cable for that application. (ACWU is the Canadian name for a light-duty teck-type cable with alum. conductors.)

Probably a #6 - or possibly larger as some sizes are cheaper because they are made in much larger quantity.