Voltage Drop

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BradM

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I wired a 2400 sq ft house last year. I ran 4/0 au 240' to a 200 amp 42 space panel. Home owner called yesterday complaining that the lights are dimming when the ac comes on. Ac is pulling 35 amps coming on and about 20amps running. I ran #6 copper about 60' for the ac. I had the homeowner turn on ac, hot water heater, stove, and all lights and fans. The load in the panel was about 110 amps. If I calculated right the service load came out to 102 amps when I was sizing the service when I bid it. Anyway voltage is 246v at meter and 231v at the panel with a load of 110 amps. I don't understand why I'm getting 15v drop. According to this calculation wire should be oversized.

2 X 21.2 appr.K X 250' X 110amps/ 12 (vd per) = 97,166 = 1/0 (105,600)

What am I missing here? Homowner has had 2 ref. motors burn out in the last year.

I know it's a long distance to the house but the calc. takes that into consideration. It's also another 200' from the meter to the transformer. Overhead service. I'm not sure what size of triplex. Taking a swag it looks like 2/0 from the ground.

If anyone could help me out here I would appreciate it.
 
The calculation I put up there was to determine the wire size. Which by that it would seem I wouldn't have a problem with 4/0 which is confirmed by your calculation. :confused:
 
Where did you check the voltage at the meterbase and panel?

In other words, which side of the meter and main breaker?

I'd check for voltage drop across the main breaker and meter, but something tells me you already did this.

Let us know what you determine the cause to be.
 
I wil take everyones calculations and measurements as gospel. That being the case there can only be one answer, poor high resistance connections.
 
BradM said:
2 X 21.2 appr.K X 250' X 110amps/ 12 (vd per) = 97,166 = 1/0 (105,600) What am I missing here? Homowner has had 2 ref. motors burn out in the last year.
Click to expand...
Using one form of the single phase formula for CM = 2KIL/VD
Given K=21.16, which varies slightly with R, and VD=5.5, using Tbl.9 notes for Ze in AC circuits, the unbalanced single phase circular mils works out to exactly 211600 or 4/0 AL.

Termination issues may be the most common cause for the extra 9.5 volt drop.

A lesser consideration could be addative loads on the neutral. For example, if the ac & lets say refridg's land on the same panal phase as the lights, and nothing else is running on the opposite phase, say hot-water heater, stove, or fans, to balance the neutral, heavy loads on one phase can ad on the neutral and drop more voltage there than otherwise expected.

Addative loads on the neutral will compound voltage drop further if the neutral is smaller than the phases, but whatever the cause a 12 volt under voltage likely over heated those 120vac refridge motors .
 
maybe this is the case of disimmilar metal connections, aluminum conductors are known to exhibit high contact resistance for mechanical or set screw lugs suitable only for copper materials .

check for for pitting or rusted marks on the terminations of the aluminum conductor at both ends
Replace if necessary

236 volt reading on the pnl line side is good but what destroys the motors is the intermitent flow of power due to deteriorating connections. Frequent Motor starting will eventually burn the coils due to inrush currents

davidv
es
 
It seems to me that if the fridge is running and the A/C compressor comes on, the inrush for the A/C motor could pull the voltage down and eventually damage the fridge motor. I know that the numbers add up, but 200 feet of 2/0al from the transformer to the meter, 220' of 4/0al from the meter to the panel and 60' of #6cu from the panel to the A/C pump sounds like a long way to me. I would check the voltage to the fridge as the A/C pump starts and see what kind of drop there is. The problem could also be with the A/C system itself. If the compressor is starting slow due to head pressure, etc., it could be the culprit.
steve
 
Let me see if I'm getting this straight. Wire size from transformer to meter is 1/0 ran 200'. Wire size from meter to house is 4/0 ran 250'. To calculate voltage drop on the 1/0 would I use the overall length (450')? That's the way I see it. Calculator says it would drop 17 amps. In my mind the 1/0 needs to be at least 3/0.
 
BTW thanks for the response. Poco is on their way out to discuss it with me.
Another thing is that at load meter voltage is 231v while the panel is 230v as of this morning with a 110amp load.
 
Using 5 x FLA for the starting amps I get
1/0 AL 200 ft = 7.8 volts
4/0 AL 250 ft = 5.2 volts
total = 13.0 volts not including the other load.
 
Met with poco. He agreed that the 1/0 was undersized after arguing that it was good enough for 200amps because it was overhead. I asked him what that had to do with length and he decided to try 4/0 overhead. They are also going to install a 28k transformer. Not sure what is there now but it's smaller than that.
 
The OP had this.
"Anyway voltage is 246v at meter and 231v at the panel with a load of 110 amps."


BradM said:
BTW thanks for the response. Poco is on their way out to discuss it with me.
Another thing is that at load meter voltage is 231v while the panel is 230v as of this morning with a 110amp load.
Click to expand...

What changed between these two scenarios?
 
I made mistake in the first time I checked voltage drop. I checked the voltage at the meter before I had a load. Then when I got the load at 110amps at the house I checked the panel voltage. But didn't check the meter voltage. Yesterday morning I checked the meter voltage under load and thats the readings I got. 231v at the meter dropped to 230v at the panel. It would have been clear that the problem was in the 1/0 if I would have tested voltage properly the first time.

I guess I was assuming that voltage wouldn't drop at the meter for some reason. Mistake on my part.
 
Brad
I bet the undersized transformer was as much the problem as the undersized feeder wire. I've seen this "dimming lights" thing happen with very very short feeders when there is an undersized transformer feeding them. I even have this problem at my own home. I have a 200A service and it is fed by a little 15KVA transformer on the pole.
So I bet your problems will disappear with the Utility finishes the upgrade!
 
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