underground service

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lile001

Senior Member
Location
Midwest
Re: underground service

The NEC recommends in 215.2 FPN #2 that voltage drop not exceed 3% on feeders. You'd need to know the demand load for the building to really calculate this.


If you are using a #3/0 copper wire, and you pretend the demand load on the residential building is 100 amps, and you use Table 9, effective impedance for 0.85 power factor, for PVC conduit (similar to free air) you find an impedance of .088 ohms/1000'

if you have 409' from the transformer to the service entrance, on a 240V system, then you would see about 3% voltage drop in this situation.
 

sparks1

Senior Member
Location
Massachusetts
Re: underground service

Suppose I pretend and take it at a full 200 amps demand, which is a little over kill and run 250 mcm copper.
How far can I go now ?
Will the meter terminals take 250 mcm? I think they will...
Can you provide formula for the above.
Thanks

[ July 18, 2005, 01:45 PM: Message edited by: sparks1 ]
 

charlie

Senior Member
Location
Indianapolis
Re: underground service

In general and considering cost, what would be maximum distance the poco would allow.
We stop at the cash register (meter) and what is done beyond that is the customer's responsibility. If we have a voltage complaint, we are responsible for maintaining the voltage to the service point which is basically at the cash register. If you want to run 200 amperes, 500 ft with #10 copper and it is behind the service equipment, it is OK with us. By the way, under that scenario that is a 500 ft long fuse if you can actually pull 200 amperes through the run. :D
 

sparks1

Senior Member
Location
Massachusetts
Re: underground service

Originally posted by charlie:
[QB]
If you want to run 200 amperes, 500 ft with #10 copper and it is behind the service equipment, it is OK with us.

Come on charlie do you really think so
It wouldn't be ok with the inspector!

Here are some ballpark numbers for a 200 amp service.
You could run 250 mcm cu to deliver a demand load of 100 amps for a 120/240v 200 amp service a distance of 480'@5%vd .

[ July 18, 2005, 08:22 PM: Message edited by: sparks1 ]
 

tx2step

Senior Member
Re: underground service

Originally posted by sparks1:
Here are some ballpark numbers for a 200 amp service.
You could run 250 mcm cu to deliver a demand load of 100 amps for a 120/240v 200 amp service a distance of 480'@5%vd .
The Vd calculator at electrician.com gives the following: for #250 Cu, 240V single phase, circuit length 480' one way, 100 amp load = a 5 volt drop = 2.1% voltage drop (Vd).

The same conditions but at a 200 A load = 4.1% Vd

Edited to add:

Of course, that's with the load pretty well balanced between the 2 phases. If you have a widely unbalanced load on just 1 phase, then the Vd goes up.

[ July 18, 2005, 08:33 PM: Message edited by: tx2step ]
 

sparks1

Senior Member
Location
Massachusetts
Re: underground service

Originally posted by tx2step:


Edited to add:

Of course, that's with the load pretty well balanced between the 2 phases. If you have a widely unbalanced load on just 1 phase, then the Vd goes up. [/QB]
I based my calculation on the one way distance given from the link , assuming this means out and back to the load. I used 120v for maximum unbalance on the neutral not 240v since these loads are balanced.
Correction the distance should be 580'

[ July 19, 2005, 07:21 AM: Message edited by: sparks1 ]
 

sparks1

Senior Member
Location
Massachusetts
Re: underground service

Originally posted by sparks1:
Originally posted by charlie:
[QB]
If you want to run 200 amperes, 500 ft with #10 copper and it is behind the service equipment, it is OK with us.

Come on charlie do you really think so
It wouldn't be ok with the inspector!

Here are some ballpark numbers for a 200 amp service.
You could run 250 mcm cu to deliver a demand load of 100 amps for a 120/240v 200 amp service a distance of 480'@5%vd .
Correction 580'
 

growler

Senior Member
Location
Atlanta,GA
Re: underground service

The question is not right to start with if you are going to try and use it on a test ( just guessing ). It should read, what is the maximum distance and wire size for a 200 AMP UG residential service to operate at reasonable efficiency. NEC 215.2 FPN #2 , 3% voltage drop on the feeders is not Mandatory. Even with reasonable efficiency thrown in you would have to provide a load value. Around here the power company runs to the meter and the meter is on the house and I've never seen them worry about it even when the service is up graded. I am surprised to see that one POCO runs bigger wire than the others ( they also have higher rates, there must be a connection ). Note* I have tested a few houses locally ad very few pass the 5% recommendation at the end of branch circuits because the state allows #14 gauge Romex. It doesn't appear that anyone over-sizes to keep the voltage drop down in residential.
 

james wuebker

Senior Member
Location
Iowa
Re: underground service

Charlie, you wrote
(Even with excessive voltage drop and #14 wire, it is still a design issue and not a safety issue.)
Maybe I not understand. I think it is a safety issue and I don't understand why It's a design issue. Please explain a little.
Thanks!
Jim
 

sparks1

Senior Member
Location
Massachusetts
Re: underground service

Originally posted by growler:
[QB] The question is not right to start with if you are going to try and use it on a test ( just guessing ). It should read, what is the maximum distance and wire size for a 200 AMP UG residential service to operate at reasonable efficiency.

Its a trick question for the students to go figure. You can't have reasonable efficiency with excessive voltage drop. So lets use 4/0 cu
3% vd to the meter and a demand load of 100 amps
this will allow us a distance of 490'.
 

growler

Senior Member
Location
Atlanta,GA
Re: underground service

My point exactly Charlie, How many times is a home owner going to have the last receptacle on a branch circuit loaded to the full 15 Amps. When they used a space heater. When they plug in a TV or stereo they don't have much of a voltage drop. Ever since home inspectors started using Sure Test I have had to explain that the 5% drop is only a recommendation and not a rule. I haven't found may places where every receptacle will meet the recommendation. I figure if voltage drop is ever taken as a serious issue the will put a "shall" in front of it.
 

charlie

Senior Member
Location
Indianapolis
Re: underground service

Jim, let's assume a voltage drop of 10% from nominal. A light bulb will not be as bright, electronic equipment is on the edge of being able to operate, and motors may or may not work and the overloads will open the circuit. All of this means that the customer will have very poor service but what is not safe?

Let's assume another 10% from nominal for a full 20% drop in voltage. A light bulb will not be as bright, electronic equipment will not be able to operate in most cases, and motors will not work and the overloads will open the circuit. All of this means that the customer will have unacceptable service but what is not safe? :D
 

james wuebker

Senior Member
Location
Iowa
Re: underground service

Charlie, I understand where your coming from but what you said in your last thread assuming the owner of the house didn't know a lot about electricity. You have let's say a space heater, a vac, a computer,TV and a stereo. Let say your between your 10% & 20% VD. You start blowing breakers. The owner keeps resetting them and now your wire overheats causing insulation break down. That leads to shorts in your wire which leads to possible fire. Electronics can overheat due to low voltage causing transformers inside to heat up and again possible fire. I don't think it's a design issue due to the fact that we can run #12 to these recep. if there's a possible big voltage drop. To me there's a safety issue hear. I do run mostly #12 on a circuit but not all the time. But I do see what you mean.
Thanks for your reply!
Jim
 

charlie

Senior Member
Location
Indianapolis
Re: underground service

Jim, circuit breakers that are use in homes are thermal devices. If there are overloaded circuits and good workmanship, it doesn't matter that the circuit breakers are immediately reset, they will immediately trip again until they cool off. While they are cooling off, the wire is cooling off. Also keep in mind that the small conductors are actually rated for much more current than the circuit breakers will permit.

The bottom line is that the system works, it is still a design issue. Also, it is still poor design and needs attention. :D
 
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