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

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.

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