RE: Volage Drop

[BradM]

RE: Volage Drop

Well the poco came out and changed transformer. But did not change out the 1/0. They told the homeowner that their calculations for it showed it was fine. Lights still dim when a load is started such as ac. I need some definite answers.

Under this scenario is the service ok? There is 1/0 triplex ran 200' overhead from the transformer to the meter. 4/0 URD ran 250' underground in conduit
to panel. The total house load is 110 amps. Before a load there is 246v at the meter and 241 at the panel. When a 110a load is online there is 231v at the meter and 230v at the house. Shouldn't the vd calculation for the 1/0 include the distance from the meter to the panel?

Does running overhead make any distance on length calculations? When I learned to calculate voltage drop nobody made any distinction between in conduit or overhead when it came to voltage drop because of length.

I'm going to talk to the poco again tomorrow to see if it was just a mixup with their linemen or what. I need all the ammunition I can find.

 

[charlie tuna]

if the a/c unit is the new scroll design --- it has a very high starting current. your customer may have to live with this dimming issue. and the power company is probibly within it's required wire size for the service. mike holt had this problem when he built his house......................

 

[ctrane]

Not sure if I am right, but check this out. Using the technique in the examples following table 9. You have 200' #1/0, and then 250' #4/0 to a 110A load. For the #1/0 you have: V(d) = .12*(200/1000)*110A, which equals 2.64V. For the #4/0 you have V(d) = .062*(250/1000)*110A, which equals 1.71V. Total Voltage Drop = 4.35V. But, this is Voltage Drop Line to Neutral. Since this is 240V/1 Phase, and the two Phases are 180 degrees apart, to get Line to Line drop, you multiply Line to Neutral by 2 (I think). So, then you get 4.35V*2, which equals 8.7V. Subtract this drop from service you get: 240V - 8.7V = 231.3V. This number seems to jive with your readings. I don't know, usually when things work out nicely is when something is wrong! Anyways, I would greatly appreciate your feedback to see if I had done anything wrong, and if not, I hope it helps.

 

[winnie]

Every length of conductor has its own voltage drop. _Every_ conductor in the circuit, including both the service triplex to the meter and the underground conductors from the meter to the panel. The supply transformer also has its own 'voltage drop', caused by its impedance.

At 110A 240V, I would expect about 8V of drop in the 200 feet of 1/0, and 5V drop in the 250 feed of 4/0. It appears that you are seeing a total of 16V drop, which is in line with these numbers once you include the transformer drop. The only thing that doesn't really fit this picture is the fact that such a large fraction of the drop shows up from transformer to meter, not from meter to panel.

Something that I have _proposed_ for this sort of situation, but never used (I'm a researcher, not an electrician) is to run a _separate_ service for the 240V equipment loads, under the 'different uses' provision of 230.2(D). One service is characterized by small loads and low voltage drop, the other service by large loads with significant voltage drop


-Jon

 

[hardworkingstiff]

Before a load there is 246v at the meter and 241 at the panel. When a 110a load is online there is 231v at the meter and 230v at the house.

This is an interesting situation. With no load you should not have a 5-volt drop. I think this might indicate a problem with the conductors or connections between the meter and panel. But then with a load the voltage drop from the meter to panel changes to only 1-volt which indicated the conductors are fine.

It seems the information stated needs to be confirmed.

 

[malin_joe]

I remember from Chemistry that different metals, when in contact results in one to corrode much quicker. I think aluminum corrodes when in contact with copper. Maybe the corrosion is increasing the resistance, and thus higher voltage drop?

 

[winnie]

Dissimilar metals in contact can create voltages due to thermal gradients; this is the essence of the thermocouple. However these thermal voltages are measured in millivolts, and are DC, and thus should not cause any sort of voltage drop.

Dissimilar metals in contact with each other and with an electrolyte can induce corrosion by galvanic action. Essentially the two metals and the electrolyte form a 'battery' that produces a small bit of electric current. This current causes the flow of metal ions and thus causes accelerated corrosion. This sort of corrosion only happens if there is electrolyte around.

If there is an oxide coating between two conductors, then it could be a semiconducting or insulating coating which could cause a couple of volts drop until significant current flow 'punches through' the coating. This could cause a voltage drop that goes away under load, and indicates a loose connection somewhere.

-Jon

 

[subtech]

Any secondary service that is more than 200 feet long is just too long, period.
Whatever it takes, you and your customer need to convince the PoCo to move the transformer closer to the home in question and get that service shortened up.
I'm a lineman, and I've seen this foolishness many times. The answer to this delimma is always the same, move the trans. closer to the house, STAY with the 4/0 service wire, and make sure all connections are made properly.
Best of luck with your "situation".
Regards

 

[mario]

yo subtech ... had to say ... up here inthe wilds of Alaska, rule of thumb is 300 feet before poco will up the wire size ... why do you say 200 ft ... m

 

[LarryFine]

Whatever it takes, you and your customer need to convince the PoCo to move the transformer closer to the home in question and get that service shortened up.
I'm a lineman, and I've seen this foolishness many times. The answer to this delimma is always the same, move the trans. closer to the house, STAY with the 4/0 service wire, and make sure all connections are made properly.

No problem. Now all we need to do is convince the POCO to not charge the customer for such work. Will you call them and explain "this foolishness" to them?

For some strange reason, they'd rather run a secondary drop than a primary tap up someone's driveway. Isn't insulation still cheaper than conductor? Isn't a #2 ACSR cheaper than 3 x 4/0?

 

[iwire]

Any secondary service that is more than 200 feet long is just too long, period.

I don't see the power company buying that argument as there are likely tens of thousands of services that have circuits must longer than 200' back to the transformer and are not problematic.

 

[BradM]

Update: I'm getting two different opinions from the poco depending on the person I talk to. One says that the reason they aren't going to usize the wire from the transformer to the meter is because "we don't keep 4/0 in stock and we would have to buy 1000' and we would never use the remaining 800' so we ain't going to do that." The other says there isn't a real problem. I would have to differ with the last guy. Two fridges burning in the last 6 months would say different.

I've gone about as far as I'm willing to go with the poco. I'm going to have a hard start kit installed on the ac to help with the lights dimming when it starts and let the customer fight it out with the poco.