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#26322 - 06/06/03 03:26 PM Voltage drop?
scjohn Offline
Member

Registered: 07/04/02
Posts: 76
Loc: charleston, SC, USA
Question, did a load calc on a dock, were a 2/0 is needed to stay within the 3%. Normally I would tap off the existing house meter with double lugs...inspector, for whatever reason, wont allow it. Ok set a seperate meter at the dock, have the poco tag it....zoning wont allow another meter in this single family neighborhood. They are basically saying I have to come off the house panel. Asked the inspector how do I get a 2/0 on a 30, 50, or even a 100 amp GE breaker, this panel has no feed through taps. He told me to caculate the load, 30 amps, run the #10 for no more than 10 ft from panel, then tap the 10 to the 2/0.
Does this sound right??
Thanks John

[This message has been edited by scjohn (edited 06-06-2003).]

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#26323 - 06/06/03 03:40 PM Re: Voltage drop?
txsparky Offline
Member

Registered: 05/25/01
Posts: 578
Loc: Magnolia,.Texas U.S.A.
Sounds OK to me. How were you planning to terminate the 2/0 wire at the far end?
_________________________
Donnie

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#26324 - 06/06/03 04:01 PM Re: Voltage drop?
scjohn Offline
Member

Registered: 07/04/02
Posts: 76
Loc: charleston, SC, USA
I am going bring it to a main lug panel, than branch from there. I guess there is something I am missing, I have heard of down sizing after so many feet, but not from the start.
John

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#26325 - 06/06/03 05:03 PM Re: Voltage drop?
ElectricAL Offline
Member

Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA
scjohn,

The resistance in the short length of smaller gauge wire will be insignificant when compared to the resistance of the long run of 2/0. Say you run the 2/0 into the house, next to the panel and end at a j-box. From the feeder breaker into the j-box one might run 4' of conductor. Calculate the actual Ohms of that piece and compare it to the resistance of the 2/0.

The voltage drop along the lighter gauge wire will be inconsequential.

Placing the short section of light gauge at the beginning, middle or end doesn't change the IR drop on it.
_________________________
Al Hildenbrand

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#26326 - 06/06/03 06:24 PM Re: Voltage drop?
scjohn Offline
Member

Registered: 07/04/02
Posts: 76
Loc: charleston, SC, USA
ElectricAl, that does make sence, but how do I calculate the vd at the panel under a a signifigant load? Here is the situation, the house is about 600 ft from the transformer, poco supplied the 400 amp service with 250 mcm alluminum service. Do I calculate the demand on the house, then add that to the the calculation? You will have 240volts at that panel with no draw, but when there is a signifigant load then what??
Thanks, john

[This message has been edited by scjohn (edited 06-06-2003).]

[This message has been edited by scjohn (edited 06-06-2003).]

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#26327 - 06/07/03 09:28 AM Re: Voltage drop?
ElectricAL Offline
Member

Registered: 10/10/01
Posts: 615
Loc: Minneapolis, MN USA
Rather than get into that calculation, I'd check the poco's supplied voltage first. I know this varies a lot across the country, but in my experience, the poco's transformer is putting out a higher voltage than what one reads, under load, at the service disconnect terminals.

And, again, in my experience, the supplied voltage under load is higher the 240 volt, much of the time. In my area I routinely measure 247 - 249 volts.

If your client's home is the only load on the transformer, shut off the whole service and measure the line voltage. This will be the transformer open circuit output voltage. If there are other homes connected, however, the reading will be inconclusive unless it is known that the other homes are also disconnected.

As a rule, the voltage drop that we deal with as electricians is from the service disco. The poco is responsible for the voltage drop before the disco.
_________________________
Al Hildenbrand

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#26328 - 06/07/03 07:43 PM Re: Voltage drop?
scjohn Offline
Member

Registered: 07/04/02
Posts: 76
Loc: charleston, SC, USA
You are getting 249 volts? I know that xformers have increase from 210-220-240, the best I get here is 240v. I dont see how you can apply the law, without calculating the poco power/and load. The resistance starts at the secondary of the transformer, not were the secondary terminates.I dont see how you can possibly keep the drop within 3% in certain instances. If the poco does not warrant it, then we are regulated to what they give us. In turn the whole rule is bogus.I think I might be beating this post in the ground.
John



[This message has been edited by scjohn (edited 06-07-2003).]

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#26329 - 06/07/03 09:54 PM Re: Voltage drop?
Bjarney Offline
Moderator

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
The "semi-legally defined" point at where the utility ends and the customer starts is typically at the service entrance {meter in most cases} so far as "voltage" {and voltage-drop calcs} are concerned. For 240V circuits and 230V utilization equipment, the agreed-on acceptable voltage is 208-254 {104-127} volts, preferring 220V {110} minimum if incandescent lighting is served. Some utilities tighten up on these numbers, but also say that it is a “best effort” goal and there may be conditions that do not allow that 100% or the time.
http://www.ari.org/std/individual/110-2002.pdf is an online standard that contains voltage ranges that utilities and utilization-equipment manufacturers agree on in most areas of the US. The pertinent material is §4 and table 1. [ANSI C84.1-1995 is the official standard, but it’s not freely downloadable like the ARI text.]

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