700' run of 120V

[Kadgarth]

I looked for an introduction forum and couldn't find one. If theres one and I missed it I apologize.

Heres my issue:

I need to get ~40A about 700' from the nearest transformer.

Throwing conventional wisdom out the window, is there any actual reason I can't use the following cable to get it done:

1/0 Al 5kv THHN

I can bump up the transformer a couple taps if need be too.

The load will be an AC/Heater unit, a few lights, and a couple receptacles.

 

[iwire]

I come up with 8% drop which is kind of high.

Bumping the taps is really not the way go as the things like lamps will have over voltage until the heater load comes on.


I would at least bump the voltage to 208 or 240 depending on what is available and place a transformer at the load end to drop it down.

 

[Kadgarth]

I'm in Afghanistan currently so I'm trying to do this with parts on hand as buying ANYTHING takes (almost literally ) an act of congress.

The rated amperage on the AC/Heating unit is 16.9 amps though it only pulls about half that when running (I don't have literature so I can't find the LRA).

If you take that out of the equation and then assume only about a 15A constant load theres still a 3% Voltage Drop. In thoery bumping a tap up 2.5% shouldn't make a huge difference then (I think).

I just feel like theres something I'm not accounting for and its driving me insane *_*.

 

[gar]

110425-0851 EDT

Kadgarth:

What is the typical voltage at your source with no load, typical min and max values? What else loads this source? How much does the source voltage change with a 40 A load?

At the load end how much current does the AC/heater require? What kind of light bulbs will be used and how much power requirement for these? What kind of loads will be on the receptacles?

Suppose the usual source voltage unloaded ranges from 120 to 125 V. If I ballpark your resistance at 0.3 ohms, then the voltage drop is about 12 V at 40 A. Depending upon the series equivalent source impedance of your source this drop could be slightly larger.

Does this 12 V drop from 120 - 125 V introduce any significant problem at the destination? An incandescent bulb will dim some. Is that critically important? If so this can be minimized by using dimmable GE CFLs that have fairly constant light output vs voltage down to about 100 V and then just quit. Some other non-dimmable CFLs, might be better than the incandescent. But do note that at startup of the AC much more than 40 A will be drawn. Thus, there might be 30 to 40 V drop for a few line frequency cycles.

Do the things that maybe connected to the receptacles care much about their input voltage?

Change the conditions to a source line voltage range of 105 to 110 V. For various reasons, one of which is the AC compressor voltage requirements, this 12 V or 30 to 40 V drop could be a problem.

Do you really gain a lot by using 240 vs 120 and stepping down at the load end. The transformer itself may provide a 5% impedance and to this you add the voltage drop at the transformer primary.

Can you change the AC/heater to a 240 V unit, supply 240 to the destination, no transformer for the AC/heater is required, then a much smaller transformer from 240 to 120 for the lights and other loads? Also use CFLs for the lights.

.

 

[gar]

110425-0941 EDT

Kadgarth:

You greatly changed the conditions, 17 A vs 40 A.

You have no problem. If your nominal voltage is about 120 V do nothing. If it is about 105 boost it to maybe 115 V.

.

 

[Kadgarth]

Gar,

Thank you for your very informative reply.

I'll head out tomorrow when its light out and check out the nominal voltage. It should be good since its only about 800 meters from the power plant but we'll see.

Again, thank you for taking the time in writing so much. If nothing else I learned something

 

[iwire]

If it is about 105 boost it to maybe 115 V.

.

Gar, that is IMO very poor advice as that will cause an over voltage to the small loads when the higher current loads are off.

It maybe how things are done in other parts of this field but it is not how things are done under the NEC by electricians.

 

[broadgage]

I would agree that 240 volts should be considered for the heating/air conditioning unit.
Then either use a small transformer for the lighting, or a 3 wire 120/240 feeder with lighting and small appliances divided between the two hots.

Or even 240 volts for everything, in line with European practice.

If the voltage is low then it might be worth altering the transformer taps to partialy compensate for the drop in the feeder.
There is however a limit to this, or the voltage will be too high at low load.
125 volts off load and 105 volts on load is probably better than 110 volts off load and 90 volts on load.

 

[don_resqcapt19]

You could use a constant voltage transformer. This one keeps the output ?1% for a input voltage of - 20% to +10%. I have no idea what they cost, but it would eliminate the overvoltage problem when using a booting transformer and the load current it low.

 

[gar]

110425-1031 EDT

iwire:

Most equipment today is designed around a nominal voltage of 120 V. If for whatever reason the supply at a given location has a nominal value of 105 V instead of 120 V and it normally varies within a 5 V range and source impedances are comparable, then why should you not boost that voltage somewhat?

Gar, that is IMO very poor advice as that will cause an over voltage to the small loads when the higher loads are on.

I believe that you meant to say when the higher loads are off. Assuming that is what you meant, then if one boosts an unloaded nominal low voltage to a somewhat higher voltage, more near what the nominal voltage should be, how does this create an excessive voltage for the light loads when the heavy load is removed? The relatively unloaded supply voltage just rises to near a more correct unloaded nominal voltage.

Basically as a first approximation for a linear circuit you can represent a circuit as a series circuit consisting of an ideal voltage source (means it is invariant), a series source impedance that is constant, and a load impedance that will vary. If there are no resonance effects, then the load voltage will never be larger than the ideal voltage source. That equivalent source impedance is made up of everything from the internal impedance of the real voltage source, and all the distribution system impedances from the voltage source to the load.

At a user point in a large distribution system if you measure the noload voltage, then this is a good approximation of the ideal source voltage. The internal impedance of the pole transformer and the wiring from the transformer to the load is the source impedance. The transformer internal impedance is usually large relative to the impedance of the primary supply to the transformer when that primary impedance is reflected to the secondary of the transformer. So you can probably ignore the primary source impedance. This is because on the primary side of the pole transformer it is a large system designed for many loads.

Only if the supply voltage was temporarily low would there be a problem with boosting it, and no way to automatically unboost it when the voltage returned to normal.

.

 

[gar]

110425-1122 EDT

Don:

Ferroresonant voltage regulators (Sola) are very expensive, and inefficient, but very reliable.

Following are some values from a 120 VA rated 120 V Sola transformer.

Input Load (both in watts)
26 .... 0
37 .... 8
83 .... 55
134 ... 105 .... 78 % efficiency

In the OP's application I really doubt there is any need for real good voltage regulation. Most important is the startup voltage on the A/C compressor is not too low, and that maximum unloaded voltage is not too high.

.