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

I just went to see this project and a physical measure came to over 800 feet. I hope this guy has a good paying job.

Bob, one could further point out in your example that 50 amps at 240 volts is 12,000 watts. At 13,800 volts, 12,000 watts is only 0.87 amps. That gives a voltage drop percentage of just 0.0038%.

Unless you're feeding an aluminum refining plant, voltage drop at 13,800 volts is essentially zero.

http://www.elec-toolbox.com/calculators/voltdrop.htm

OK I know how to figure voltage drop. To calculate it you need voltage, distance, amperage, and the resistance of the wire. So I have filled all the the equation exept for the amperage. What do you use here?

A. Caculated load using the standard method or the alternate method. By the way this gives you two very different answers.

B. 200A.

C. Some other gohstly number

I need to size this so it dose not go crazy silly on the cost and the guy goes bankrupt. Nor do I want to get called back because the guy cant read the paper by a 100W bulb wile taking a **** . One size differance will make a huge price change. It needs to be accurate.

IMHO the above determination _can't_ be made accurately. It is a design issue, one which will depend as much upon the taste of the home-owner as physics. Part of what you are going to have to do is educate the home-owner on the problem of voltage drop, and explain that the issue will range from problematic, to safe but esthetically disturbing, to now a problem at all but expensive to implement, and offer a range of choices.

1) You have to determine what 'acceptable voltage drop' is. The NEC suggests a _total_ of 5%, with 3% for feeders, but note that this is a suggestion in a FPN, not a requirement or a guarantee of success. If every appliance in the house used 'universal switching power supplies' (such as are on laptop computers), then a 15% voltage drop might be acceptable. On the other hand, a 5% drop in voltage to an incandescent lamp means a 17% drop in light output, which might be disturbing, especially if it were sudden and momentary.

2) To determine the expected 'normal' voltage drop, I would use the smaller of the calculated demand values. Often the NEC estimates pretty high in terms of actual demand.

3) To determine the expected _worst case_ voltage drop, I would find the largest momentary load, eg the motor with the biggest starting current, and add the starting current to the demand load. Motor starting is probably going to cause most of the problems with things like flickering lights. I am presuming that with this big long run for electric, water comes from a well. I would be on the lookout for significant voltage drop each time the pump starts.

4) It may be cheaper to deal with the momentary loads by changing the load characteristics (eg. using VFDs or soft starters) rather than increasng the size of the service conductors.

5) It may pay to design for the future by doing the install in conduit, so that if the lights do flicker too much, the wire can be increased in size without the cost of digging.

And finally: the above is all about design theory, not my personal installation experience
-Jon

Jon, nice post.

I looked at sgreany's post


early this morning and did not have time to answer.

My answer was going to be it is imposable to be accurate. There is no 'right' answer to the question.

It is reassuring to me that my answer is in the same ball park as yours.

Bob

Sgreany-
I would use the 200A number.Thats what my local POCO does in their "green book" that we use to size the conductors.can the POCO bring in conductors overhead and set a pole closer?That would help some.Can the place get away with a 150A service? Either way the cost is going to be alot,I don't think there is anyway getting away from that,whether you use 200A or 150A as your load.If you go with 150Amps as the load a 350 MCM copper will have a VD of 3.67% according to my calcs.I would find out if the homeowner has thought of living "off the grid" and doing a solar/battery/generator set up(I know some folks who do it this way)price with the underground.
More importantly.
I would fiqure out a price on these numbers and see if the customer is serious or not about spending the big $$$ to get this service in.Otherwise your spending a bunch of days "banging your head" over this estimate just to have the customer say "I was only planning on spending X amount of dollars" or something along those lines.Don't forget to include a conduit(s) for phone / cable .Best of luck in getting the job.(I'm not available to pull 800 + ft. of wire with you Sorry. LOL) Andy

For reference, about ten years ago we were looking at building a house that would have been 1,200' from the power line. The breakdown on cost was:

Trenching and backfill (5 feet deep): $18,620
PG&E costs (Supply and place HV wire): $14,000

Total: $32,620

This was a little higher I would expect it to be in most places, because 1) The distribution voltage was 22,000 volts, and needed to be buried 5 feet deep, and 2) For some reason, they needed to "loop" the HV, and were actually going to run six CIC cables up to the transformer.

The maximum distance PG&E would allow from the xformer to the house was 200'. We were going to be right at the limit, because the reels of HV cable are 1000', and if we were over 1000' we were going to have to pay for a vault in which to splice the cables.

According to local rules here, PG&E had to supply and pay for the transformer; we were responsible for the pad, and for the cost and installation of the wire.


[This message has been edited by SolarPowered (edited 12-23-2004).]

As you can see from solar's post ,it can be costly to bring the POCO primary in from the street to a padmount transformer. I did one 600 feet and it cost $15000...and that was just the primary cost.

To this day I wonder if I would have been better off to oversize the secondary and eliminate the unsightly transformer in the guys front yard.It would have cost less also.

Try to price it this way...run two 3inch PVC in parallel with a 350MCM AL URD cable in each pipe.Put a 320 meter can at the house to terminate.Drop into the house with 2 load centers.Oversize your wiring to the air conditioning and motor loads also.IMO this cost less to install and large enough to compensate for voltage drop.

shortcircuit

You _might_ be able to convince the POCO and AHJ to let you run two separate services.

Instead of running a single 200A service, you would put the ordinary general lighting loads on a 100A service and any large appliance loads on a separate service sized appropriately. Then design for different voltage drops on these two services.

On the general lighting service you would size to get 3-4% VD at the calculated demand. When everything in the house is on, the lights might be a little dim, but they won't be flickering.

Then put things like the well pump, electric range, heat pump, etc. on the other service. Make sure that each of these components is suitably rated to tolerate large voltage drop. Motors draw large inrush current when starting, and voltage drop can make it difficult for the motor to start under load, which means more time spent in the high slip, low efficiency, high heating operating state. But when the load characteristics are suitable, 'reduced voltage' starting is a common technique used to reduce the inrush current going to large motors. IMHO if the large appliances are carefully selected, the startup voltage drop might be a good thing. Electric heat won't have a problem with voltage drop; the heating element will produce less heat, but the thermostat will keep it on longer.

You'd have to convince the AHJ to permit this under 230.2(D), different voltages Seriously, read 230(2).

You'll want to make sure the meters are placed at the house if the POCO will allow this; otherwise the customer will be paying for the electricity burned in the voltage drop.

-Jon

WOW. Thank you all for the great resonses. I just want to say how helpfull everyone has been and how impressed I am.

After speeking with the utility, they quoted the first 150 feet free and $13/ foot after that for overhead. I think I can convince the customer to go three poles (450 feet)That brings me under 500 feet which also saves on trenching costs.

Also, I already have been awarded the project. The service is going in as a change order.

This seems to be a good question and perhaps some engineer should come up with a calculation that should be in the code. Just a thought.