ECN Forum Forums Code Related Discussion NEC & other Code issues UG service VD

How are you guys figuring voltage drop for underground services? Are you using demand load or 200A or what? As you need amps to properly calculate VD and the max amps are variable and undertermiable in a dwelling.

I am prepairing a proposal for a 600 Ft UG service with copper conductors.

In my area the POCO has a spec sheet for what they want the conductors sized for depending on the length of the service.Also,in my area they won't let you go more than 500' UG for a 200 amp service.

There are no NEC rules for this.

If it was my job I would figure the voltage drop based on no more than the calculated load. I would not base it on the service size. Just my opinion.

The calculated load is always higher than the 'real' load.

But 600' feet is a long way to stay within the recommended 3% for a feeder.

Using 150 amps I come up with 350 Kcmil CU or one 600 Kcmil AL (or two 300 Kcmil AL) to maintain 3% or less drop.

If you run the same load but allow 5% drop it gets better.

One 4/0 CU or one 350 AL

By the way just for fun, 200 amps at 600 feet 240 volts with only 3% drop requires these conductors.

One 500 Kcmil CU or one 750 Kcmil AL

Bob

Bob,
You're numbers are right on track and you are correct about the NEC,but my point was (in my area at least) that the POCO has to approve of the size cable and length of run OR they won't hook it up.They will do a site survey and tell you what pole they want you to come from and which side a the house they want you to locate the meter socket on.(At least in my neck of the woods)

Andy

Andy thanks and I was not disputing what you posted.

My answer was based only on the NEC.

Power company's are all different and certainly can have a say in what you do.

The power company's in my area do not tell us wire size but it does not surprise me some power company's do.

So sgreany you better call the POCO or check their Web site for their rules.

Look on their web site for a "Green Book" link. That will often get you a pdf of their installation requirements.

Bob


[This message has been edited by iwire (edited 12-18-2004).]

In this particular area it is my responsibility and if it dont work its my problem. A couple towns and a county over the utility accually supplies and installs and maintains the conductors. 310.15 states flat out a 2/0 is ok but I might go 4/0 just to cover my ass. I dont ever recall hearing about anyone having a VD prob on an UG service?

Well, no, 310.15 doesn't just flat out state that 2/0 is OK. 310.15 states that 2/0 is recognized as safe from the point of view of being unlikely to overheat to the point creating a fire hazard. It says nothing about whether 2/0 is adequate in regards to having an acceptably low voltage drop over a distance of 600 feet.

Something else to consider for Voltage Drop calculations:

How 'steady' will the voltage drop be. If the lights run a little bit dimmer when every thing in the house is on, than that probably is acceptable.

But if every light in the house gets substantially dimmer for a moment whenever the well pump cycles or the heat pump turns on, than that might not be acceptable to the homeowner. Even worse if the voltage dip is enough to cause electronics to reset!

I would strongly suggest finding the load with the largest inrush or startup current, and checking the voltage drop when that load starts up. If you run the bare minimum 2/0 wire for a 200A service, you might easily get >10% momentary voltage drop if there is a large motor load on the property.

Another thing to consider: given the length of the underground run, would the POCO place a padmount transformer near the home, with most of the underground run at distribution voltage?

-Jon

Yes the utility would do the trans. thing if the customer wants to pay for it.

I had another thought on the subject. What about te utility overhead line supplying the service. They just run a # 4 solid aluminum that could go for a thousand feet to the transformer with seven other houses attached to it. I know its in free air, but the rules of VD should still apply. The reason I say that is the customer may op to let the utility set a couple poles to reduce his cost, Whats the diference if I go the extra 400 feet with a 2/0 copper or let the utility run a #4 aluminum?

The only real difference is who will be looked at as 'the bad guy'.

If you size the conductors large enough to prevent voltage drop and the house still has problems the only blame will be the POCO.

If you do not size your conductors for voltage drop you will be the 'bad guy' in the customers mind.

Another thing to consider is that solid 4 AWG is probably running at 13.8 KV the higher the voltage the less problems with voltage drop.

Here is an example

13,800 volts, 50 amp load, 600' 4 AWG AL.

That results in only 0.221% voltage drop

Now change only the voltage to 240.

The voltage drop is now 12.5%!

The POCO usually are required to be within 5% up or down of the service voltage. They can change taps on a transformer if the voltage drop is unacceptable.

Bob