I didn't check you calculations, but I would just add that youir calculated voltage drop will only occur with a 200 amp load.
5% is the generally accepted total drop, including both feeder and branch circuit. You have already used up 3.8% in the feeder. What is the connected load like? Do you have electric heat, or something else that is going to draw a lot of current?
200 amps at 230 is 46 kilowatts, which is about 157,000 BTU. You can heat a decent sized residence with that much power.
The greater the voltage drop to motors, especially these hermetically sealed motors found in AC systems, the hotter they run. On a hot day, with the units running at max amps, and the local power grid browning out.. voltage drop issues burns the compressors out. There are many reasons for this.
The motor is less efficient with lower voltage so there is more rotor slip, generating more heat in the windings.
These are suction gas cooled compressors...on a hot day, especially if you let the house get warm then turn the AC on... the suction gas is warm... and fails to cool the compressor motors adequately. If you have low voltage wired into your system (small conductors) and the local power grid drops 10 or 15%... you have reached compressor burn out conditions (and expensive proposition).
Note: Most manufactures AC units are not designed to tolerate continous run times with the house temp above 85 degrees F. If you let the house get up to say 100 degrees inside, and you have small AC units incapable of cooling the home below 80 within an hour... the prolonged run time under those conditions, combined with area brown out conditions will burn out the compressors after a few such episodes (these have internal thermal overloads that give some protection, but its marginal)..
an aside: Even if the motors dont burn out that day, as cylinder head temperatures rise under these conditions to 238 deg F or higher, you reach a point where the refrigerant and oil begin to break down, exponentially fast if there is any faint amount of air or moisture in the system. This creates an acid.. depending on the refrigerant used.. that attacks the motor winding insulation, then the compressor burns out later.
My advice if you live in a generally cool climate is to not worry too much about it.. just size your wiring to the NEC code. If you live in a very hot area, or where the AC runs a lot... go one size larger on your wire than the NEC minimum... the reason for this is to eliminate the usual 3% voltage drop in the wiring to the compressors... that can make a difference on brown out days.
I would also make sure the utility company feeders to the house are sized fully for your peak loads. Running the usual
100 amp feeders to serve an upgraded 225 amp panel and AC has made a lot of HVAC contractors rich.
(on days where the utility is warning of brown outs... you can save yourself compressor/ motor damage by not running your AC in those peak times..2 to 5 pm usually.)
Regarding pure resistance loads...lights and the water heater... you can run them with high voltage drops without any liability at all. The motors you can't though. They will overheat.
Phil Scott Mech Engr and HVAC contractor since 1849
I hadnt noticed this before....300' and 9 volt drop... thats too much for the compressors...but wont make any difference to the water heater and lights.. you will have trouble with AC compressors with that set up, adding another 3 v drop in your house wiring. Thats 12 or 13 v drop... another 10 or 15 volt drop for summer brown out conditions added in.. ..and you get AC compressor motor burn out issues.
I would have the utility upgrade the feeders...or use a heat pump for heating the water instead of the instantaneous electric resistance... get the total load down so the voltage drop stays under 10 or 12% worst case with local area grid brown out factored in.
do you really know what your line voltage at the meter is actually going to be? several of my sites that are supposed to be 208 are actually 202 (on a good day) and often less then 200. the power company thinks this is just fine.
this means it take more current to do the same work that is needed. the I squared R losses increase, transformers run hotter, breakdowns are more frequent then they should be.
i wonder if you could get water and air heat in 3 phase versions (and get 3 phase service)?
(the copper and labor for one 400A 3 phase run of 100 ft was $7,000 this summer)
btw your 2 ton ac will draw approx 10 amps at 230 volts...depending on the heat of the day etc. .. and the efficiency rating of the unit. It should be on a 20 amp two pole breaker with size 12 awg minimum, in your situation Id run 10 awg wire.
| I'm hoping someone can give me a an answer to this I've gotten mixed | information. | | I'm wanting to run power to my lake cabin without having to down hundreds of | trees for a power company right-of-way. | | My problem is that my 200 AMP service panel will be about 300 feet from the | power drop and meter base. | | From my calculations, 240 volts (single phase) using 2- 3/0 copper | conductors with a 2/0 neutral will yield a voltage drop of about 9.1 volts | or 3.8% is this acceptable for most household uses? | | Won't I still be getting about 230 volts and 115 volts on my household | loads?
You didn't specify your insulation type. Is it rated for at least 75 C?
I confirmed your calculations at:
based on 30C ambient temperature and full 200 amp load. This results in a conductor temperature of 74.98 C (a rise of 44.98 C over the ambient). So it better be rated for at least 75 C if you're going to be pulling all
But more likely you'll be using far far less than that at a lake cabin unless it's real purpose is for growing lots of illegal plants under hundreds of lamps.
Why would you even have 200 amp service at a lake cabin? Sheesh.
At a still heavy 40 amps, you're looking at a drop of 0.7%, giving you
238.4 and 119.2 volts for 240 and 120 being supplied. And you'll have a miniscule 1.55 C temperature rise.