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Posted By: watersparkfalls volt drop - 02/05/14 01:27 AM
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
Posted By: HotLine1 Re: volt drop - 02/05/14 02:30 AM
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.
Posted By: Tesla Re: volt drop - 02/05/14 03:07 AM
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.

Posted By: SafetyWired Re: volt drop - 02/05/14 03:16 AM
Up your wire size! and stick with 100a
Posted By: gfretwell Re: volt drop - 02/05/14 03:31 AM
Vote me with Tesla. I don't think you have that many coincidental loads. How many workers are in this shop?
Posted By: ghost307 Re: volt drop - 02/05/14 01:32 PM
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.
Posted By: Tesla Re: volt drop - 02/05/14 06:29 PM
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.

Posted By: watersparkfalls Re: volt drop - 02/06/14 06:09 AM
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
Posted By: watersparkfalls Re: volt drop - 02/06/14 09:02 PM
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
Posted By: SafetyWired Re: volt drop - 02/11/14 03:46 AM
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???
Posted By: HotLine1 Re: volt drop - 02/12/14 02:36 AM
I'm confusedtoo.

Someone clue me in on 'URD' please. Not a term I know.

Posted By: ghost307 Re: volt drop - 02/12/14 02:48 AM
URD = Underground Residential Distribution.
It started out as a twisted set of individual MV conductors, but is now gaining populatity for LV as well.
It's generally not allowed inside a structure, so you should check with the AHJ to see if you can run it all the way or if you have to splice to something else before entry.
Posted By: gfretwell Re: volt drop - 02/12/14 04:15 AM
Typically the URD terminates at the meter and they go with a more smoke proof conductor for the service entry into the building. I assume the NEC calls it USE but twisted up in a triplex or quadplex.
Posted By: Tesla Re: volt drop - 02/12/14 06:45 AM
URD started life as a Poco oriented product -- to run at medium voltages.

The scheme morphed over the years... becoming quite economic for low voltage feeders, too.

(In the Poco world, low voltage means 600VAC or less.)

So what had been strictly a Primary Loop conductor came to be accepted by the Pocos for Secondary Service Taps.

If buried directly in the soil, URD carries ampacities of over 200 in 1/0. The industry makes this with or without a reduced size neutral. For our OP, I would recommend staying with a full sized neutral as he does not have any meaningful 240VAC loads. He has to anticipate that the neutral for this installation will see wildly unbalanced return currents.

[The only time that you can safely get away with a reduced neutral is if you have significant line-to-line loads that won't load the neutral -- ever.]

IIRC, the OP claimed that this feeder would be limited to 100A across 240VAC. So the fact that he's feeding a 200A sub-panel is not a deal killer.

I can't say as I'd recommend it: 125A and 100A single phase panels are common as dust and much more appropriate.

With such intermittent loads, the actual panel size is not indicative of the load to be imposed. Watch out for build out at the new location. I'd certainly leave behind documentation as to the limits of the feeder, etc.

I'd also recommend using only high quality splice techniques in the outdoors for aluminum URD connections.

I tend to favor epoxy encapsulation -- or its equivalent with Polaris and its rivals. That's no junction where you'd want to take a risk.

Be sure to lay down caution tape and a backfill with sand around the URD. Sand is dirt cheap, and easy to compact. It won't bite you. It's astonishing what shifting cables and stones can do to the dialectric.
Posted By: HotLine1 Re: volt drop - 02/12/14 04:42 PM
Thanks for the info gentlemen.

Not a common thing around here. Our POCOs do UG distribution (BUD) up to the line side of the meter pans.

When a HO or builder wants 'underground' for a new SFD in an overhead zone, it is the customers responsibility to supply & install conduit & conductors from a designated pole to the meter pan. The POCO has requirements for the conduit larger than the NEC requirements.

Reduced neutrals on the service side are not allowed by the POCO, for customer installed services.

Posted By: gfretwell Re: volt drop - 02/12/14 05:33 PM
FPL will do the lateral for a price and most builders I know go that way. The price is certainly competitive and it is one less thing they have to worry about
Posted By: HotLine1 Re: volt drop - 02/12/14 06:06 PM
PSE&G installs BUD systems within the sub-divisions that have sprung up at a cost to the builder. My Twp has limited 'open space' for any more new sub-divisions. Most that have been built are <10 SFDs.

New SFDs are 'tear downs', and the few remaining empty lots.

A side note to the BUD areas are the people calling wondering how a 'downed power line' is affecting them!!
Posted By: gfretwell Re: volt drop - 02/12/14 07:49 PM
FPL even says in the web site that underground SFD laterals are mostly aesthetic. The MV distribution out on the hard road will be overhead.
They are very aggressive with their tree trimming (to the dismay of a lot of home owners) and we don't see the tree damage you see in other areas.
Posted By: harold endean Re: volt drop - 02/24/14 02:51 PM
Here in northern NJ, we have JCP&L and they are a little lax on the tree trimming. That is one of the major reasons for power outages here. Either a squirrel jumps on the HV wire and blows a fuse, or a tree branch will touch it. The primary wire around here is a bare #6 cu wire running along the top wire from pole to pole. If they kept that wire covered and ran a messenger wire along with it, I bet it would cut out a lot of blown fuse call outs.
Posted By: HotLine1 Re: volt drop - 02/25/14 03:10 AM
Greg:
Since hurricane(s) Irene and Sandy, our POCOs have widely publicized tree work to reduce 'future' outages.

I have the same POCO at my home that Harold has in his area, and yes, they have distribution (MV) that leaves a lot to be desired. Comments from the out of state crews that were here after Sandy were not very good.

They did tree work in my neighborhood; notices in all mailboxes, blah, blah.

One tree at my front yard was 'trimmed' complete on the street side. The end result was I had a tree guy come and take it down, before it got blown into my neighbors house.

Great tree trim job by the utility contractor. No recourse to get reimbursed to have the tree removed.

rant over
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