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volt drop #212697 02/04/14 08:27 PM
Joined: Mar 2005
Posts: 236
watersparkfalls Offline OP
Member
I know the NEC volt drop is a FPN, however I need a little feedback on a particular future install. The customer has a shop which he will weld and use a air compressor along with lights and a couple small fan motors. The distance is 260 foot from panel to new 100 amp sub panel. My calculations @ 100 amps is a 16.6 volt drop (yikes) reduce the breaker to 60 amps and I get 9.9 volt drop. My question is what would you think the max volt drop should be?

My math: 2*K*I*L/CSA
2*21.2*100*260/66,360=16.6

Thanks H2O

Last edited by watersparkfalls; 02/04/14 08:27 PM.
2017 / 2014 NEC & Related Books and Study Guides
Re: volt drop [Re: watersparkfalls] #212698 02/04/14 09:30 PM
Joined: Apr 2002
Posts: 7,225
HotLine1 Offline
Member
IF I had a true 240 volt at the source panel, and I had no equipment that was 'fussy' for 240...I would be happy with 3-5% VD. (232 or 228).

Keep in mind the starting currents, although brief, will cause additional drop, brown-out, flicker, etc.

Why not just increase the feeder?? Cost??
Having to possibly 'do it over' is not a savings.


John
Re: volt drop [Re: watersparkfalls] #212701 02/04/14 10:07 PM
Joined: Jun 2004
Posts: 1,273
T
Tesla Offline
Member
Modern welders use internal logic to control voltage output -- and a whole slew of tricks in between. A 7% voltage dip will mean nothing to it.

Small fan motors won't notice voltage drops, either.

Modern lights use the same voltage insensitive tricks that the welder uses. They won't be affected at all by 7%.

The only load that would object might be the air compressor. You have omitted any statistics for it. Some units, pitched as 2hp are actually 1/2hp as NEMA would define it.

Lastly, your calculations assume that all of the loads are pulling full -- and at the same time. For the bulk of the time, voltage drop for this set-up will stay way below 3%. No-one welds all of the time!

The air compressor is an intermittent load, for sure.



Tesla
Re: volt drop [Re: Tesla] #212702 02/04/14 10:16 PM
Joined: Jul 2013
Posts: 44
SafetyWired Offline
Member
Up your wire size! and stick with 100a

Re: volt drop [Re: watersparkfalls] #212703 02/04/14 10:31 PM
Joined: Jul 2004
Posts: 9,556
G
gfretwell Offline
Member
Vote me with Tesla. I don't think you have that many coincidental loads. How many workers are in this shop?


Greg Fretwell
Re: volt drop [Re: watersparkfalls] #212704 02/05/14 08:32 AM
Joined: May 2005
Posts: 984
G
ghost307 Offline
Member
Remember that just because 'modern' welders can tolerate a voltage dip doesn't mean that the shop will have one.
I've seen lots of new shops where the Owners bring their old stuff and don't replace it with something new until they can't get repair parts anymore.
Personally I'd run the 100A; even for a 1 man shop.


Ghost307
Re: volt drop [Re: watersparkfalls] #212706 02/05/14 01:29 PM
Joined: Jun 2004
Posts: 1,273
T
Tesla Offline
Member
If you really want to get picky:

Do a load calculation -- from the demand side.

The OP never was explicit as to the size of his conductors... or even if they were Cu or Al.

(I'd only consider Al for such a run, Cu is just too pricy.)

And it's only implied that this is a 240/120 three-wire scheme.

BTW, changing breakers is not going to magically throttle shop demand... unless you intend to keep throwing the breaker into a tripped condition.

Ghost...

Even ancient arc welders will perform okay with 10% voltage drops. The welder merely adjusts his stroke.

Other than the lights, may be the fans, all of the loads cited are intermittent. They are not going to stress the feeder.

The wet side of Washington State doesn't have a climate that requires air conditioning. (A de-humidifier might be desired.)

One should look to whether the shop is going to be built out with additional tool loads. I'd size it with that in mind.

If this is going to be ditched, then the pipe should be deliberately oversized by at least 'one bump.' { 2" instead of 1 1/2" } The money is in the trench, not the PVC. You don't want to be ditching twice.


Last edited by Tesla; 02/05/14 01:31 PM.

Tesla
Re: volt drop [Re: watersparkfalls] #212707 02/06/14 01:09 AM
Joined: Mar 2005
Posts: 236
watersparkfalls Offline OP
Member
Thanks...your input is always GREAT!!! I will talk with the owner and give him the option of feeders. BTW it is just a one man shop and you are right Tesla about the 2HP. Thanks again my fellow electricians!

H20

Re: volt drop [Re: watersparkfalls] #212708 02/06/14 04:02 PM
Joined: Mar 2005
Posts: 236
watersparkfalls Offline OP
Member
Thanks again,
decided to use 1/0 URD in the trench, since it is the longest part of the 260 foot run (155 to be exact)its going to land in a 200 amp main breaker (sub panel) I wont have to fight the over sized wire into a 2 pole breaker. I needed a Jbox anyway to switch from SER #2 to the URD, so I figured split bolts and tape save the day. I will reduce my volt drop to a total of 6.55 volts by only using 105 feet of SER #2 from the main panel to JBOX which will be less than 3%.

H20

Last edited by watersparkfalls; 02/06/14 04:04 PM.
Re: volt drop [Re: watersparkfalls] #212769 02/10/14 10:46 PM
Joined: Jul 2013
Posts: 44
SafetyWired Offline
Member
Originally Posted by watersparkfalls
Thanks again,
decided to use 1/0 URD in the trench, since it is the longest part of the 260 foot run (155 to be exact)its going to land in a 200 amp main breaker (sub panel) I wont have to fight the over sized wire into a 2 pole breaker. I needed a Jbox anyway to switch from SER #2 to the URD, so I figured split bolts and tape save the day. I will reduce my volt drop to a total of 6.55 volts by only using 105 feet of SER #2 from the main panel to JBOX which will be less than 3%.



H20


I am confused, or maybe I missed a post...but you are going to run 1/0 to feed a 200a MB panel...and on top of that change wire sizes in the run???

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