wire size for long feed to house

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goldcoast

Member
Any suggestions for sizing feeders for a 700 foot run from the meter to a house for 120/240 200 amp and also for 125 amp? Does a buck and boost make sence? I came up with 500 mcm al for 200A.
Thanks.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: wire size for long feed to house

Why not calculate voltage drop based on the calculated load and not the service size?
 

rattus

Senior Member
Re: wire size for long feed to house

What is the acceptable voltage droop at full load? I would guess no more than 5%. Remember also that total wire length is twice the length of the run. But you knew that didn't you?

[ December 11, 2004, 08:46 PM: Message edited by: rattus ]
 

tom baker

First Chief Moderator
Staff member
Re: wire size for long feed to house

There is no NEC maximum for voltage drop. A FPN tells us it should be limited to 5%. While the service may be 200A, your connected load will be a lot smaller. Ask the POCO what they use for the average connected load and use that number.
 

jtester

Senior Member
Location
Las Cruces N.M.
Re: wire size for long feed to house

Tom
I would be concerned about using the utility's average demand load number. When I worked for a utility, a house with gas appliances was about 3-5 kva, and with refrigerated air and 2000 square feet, we would use a diversified demand of 10 kva.
Utility demand is often based on a average over 15 or 30 minutes, and loads on transformers for example are averaged over hours.
As an engineer, I would be tempted use the calculated load, however that is clearly very conservative.
Since in general, the maximum voltage drop is not specified by code, I don't think there is one correct answer. The load used is probably based on how conservative you want to be, and how tolerant the homeowner will be to dips and sags.
Jim T
 

250mcm

Member
Re: wire size for long feed to house

I have done 500 ft with 350. I think the voltage drop at 100 amps was 3 volts or so.
Some people like to stay with 3% some 5%.
My opinion is that 500mcm will be fine with a voltage drop of 10volts at 700 ft.4.2% Good luck with a can for that size wire.
 

highkvoltage

Senior Member
Re: wire size for long feed to house

If you going to run 700 ft why not run the primary voltage back to a more reasonable distance and set a transformer closer to the house. Smaller wire, less voltage drop and cost would be about the same. Talk to your local utility and see if they will unstall it. This way if there is ever a problem it's theirs and not the home owner.
 

hurk27

Senior Member
Re: wire size for long feed to house

Highkvoltage while that would solve the problem it also creates one, as the home owner has to give up some of his land for a right of way to allow the utility to run their wires.

One mistake I made once was I had a calculated load of about 125 amps on a 200 amp service but I didn't figure in the start up current of a 5 ton AC unit and the lights would still dim very noticeably. I don't rember the LA of the AC unit but it was over 100 amps.
Just somthing to keep in mind.
 

derekh

Member
Re: wire size for long feed to house

Goldcoast,
If you know the approximate sq. footage of the house, I would calculate the minimum service based on Article 220, then bump it up maybe 15-20% and calculate the feeder sizes for voltage drop based on that. If you have no idea on the size of the house, then I would assume maybe 75% or 80% of the service size, then do a voltage drop calc based on this number. It will still be more than the min. req'd by code, while not major overkill. I will tell you at 700' of 75C cable in PVC conduit, to stay under 5% drop using the second method, you could use 350KCMIL for the 200A service and for the 125A service, 4/0 would work.

[ December 17, 2004, 02:06 PM: Message edited by: derekh ]
 
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