Voltage drop real world vs calculated

[tortuga]

Greetings all, we have a agricultural project wrapping up where a greenhouse building got replaced.
I just tested a few of the 36 IN fans, about a 1HP motor and they were struggling to start and wanted to rule out any voltage drop as its on a long feeder.
The fans are new and I tested the caps there good.
Its a preexisting feeder to the greenhouse that's about 175 feet of conductor that I am re-using to refeed a 200A panel, the calculated load is around 150 Amps.
There is about another 30 feet of conductor from the service to the transformer pad, so about 205 feet of 250 kCMIL AL total.
By my calculations and https://www.southwire.com/calculator-vdrop seems like 250kCMIL should be good for about 240 feet at 3% drop.
However when testing the feeder with my Sure Test I am getting around 10% drop. I visually checked the connections at the service and they all look good.
Does anyone find this kind of discrepancy when testing for voltage drop with a sure test?
Could the main breaker be adding that much?
I plan to go back with a better dummy test load Tuesday.
Thanks

 

[mopowr steve]

Sure that your existing underground conductors don’t have some damage where corrosion has caused a reduction in circular mil., bad spot?

 

[electrofelon]

Greetings all, we have a agricultural project wrapping up where a greenhouse building got replaced.
I just tested a few of the 36 IN fans, about a 1HP motor and they were struggling to start and wanted to rule out any voltage drop as its on a long feeder.
The fans are new and I tested the caps there good.
Its a preexisting feeder to the greenhouse that's about 175 feet of conductor that I am re-using to refeed a 200A panel, the calculated load is around 150 Amps.
There is about another 30 feet of conductor from the service to the transformer pad, so about 205 feet of 250 kCMIL AL total.
By my calculations and https://www.southwire.com/calculator-vdrop seems like 250kCMIL should be good for about 240 feet at 3% drop.
However when testing the feeder with my Sure Test I am getting around 10% drop. I visually checked the connections at the service and they all look good.
Does anyone find this kind of discrepancy when testing for voltage drop with a sure test?
Could the main breaker be adding that much?
I plan to go back with a better dummy test load Tuesday.
Thanks

How exactly was the response performed? What is a "sure test"? There will be voltage drop thru the transformer and from the utility side of things too, did you take that into account? The Southside app assumes 90% PF so it should already be somewhat conservative, or at least about on for motors.

 

[hillbilly1]

Where is the fan in relation to the controller and panel? If it is at the other end of the greenhouse, it may be the circuit conductors are too small, not the service feeder.

 

[hillbilly1]

The OP must be using one of these, I'm unfamiliar with the accuracy of these.

https://idealind.com/content/dam/electrical/assets/TestMeasurement/SureTest%20Circuit%20Analyzer/61-164-suretest-circuit-analyzer-instructions.pdf

 

[tortuga]

Yep thats it, Ideal Sure Test.
I have some alligator clips on a lead to test equipment.

Sure that your existing underground conductors don’t have some damage where corrosion has caused a reduction in circular mil., bad spot?

No, thats what I am afraid of.
There is 2-1/2 IN PVC the whole way

Where is the fan in relation to the controller and panel? If it is at the other end of the greenhouse, it may be the circuit conductors are too small, not the service feeder.

Good point its on the same wall, but it does go thru a contactor panel, so probably 60-70' conductor length #12.
I could rewire the motors to run at 240, might have to get different caps though.

 

[hillbilly1]

Yep thats it, Ideal Sure Test.
I have some alligator clips on a lead to test equipment.


No, thats what I am afraid of.
There is 2-1/2 IN PVC the whole way

Good point its on the same wall, but it does go thru a contactor panel, so probably 60-70' conductor length #12.
I could rewire the motors to run at 240, might have to get different caps though.

Shouldn't have to change the caps, just the connections in the motor, unless they are two speed, can't remember if the two speeds can be changed, I don't think they can.

 

[hillbilly1]

Being a 1 HP, it may already be 240, and thats why its struggling, wouldn't be the first time the prints don't match whats installed.

 

[tortuga]

Being a 1 HP, it may already be 240, and thats why its struggling, wouldn't be the first time the prints don't match whats installed.

I checked the wiring in the pecker and its wired for 120, but it was getting late Friday so ill check it again.
I was pretty skeptical about the plans calling for 120 motors to begin with.

 

[ActionDave]

Yep thats it, Ideal Sure Test.
I have some alligator clips on a lead to test equipment.


No, thats what I am afraid of.
There is 2-1/2 IN PVC the whole way

Good point its on the same wall, but it does go thru a contactor panel, so probably 60-70' conductor length #12.
I could rewire the motors to run at 240, might have to get different caps though.

You don't have to change any caps when you switch the connections from 120V to 240V

 

[hillbilly1]

Also, how many motors are starting at the same time? A commercial controller should stage them if designed correctly, but I did have an engineer to not design that into large distribution center, twice in my career in fact. All of them off one thermostat coming on at the same time would trip the main to a MCC on one, tripping the main to the panel board on another. The latter was just last year, engineer wanted to upsize the panel and breaker, when the load was only 25% of the existing panel. Could have even turned up the magnetic trip, but he didn’t want to do that, so split the starters up into four stages. Same engineer used smart breakers to control led lighting that had inverter battery back up. Kinda hard to do when you shut off the whole circuit. Abandoned the smart breaker control, and put contactors in. LOL!

 

[ptonsparky]

Stage the motors as suggested. Should have used the 240v. Get your toys out, IIRC, you have them.

 

[mivey]

The Sure Test gives total drop, not just the feeder drop. Measure at the feeder source then at feeder end and take the difference. Do the same with the impedance values from the Sure Test.

The Sure Test amps may be different than your load amps.

If you have a transformer with 3% impedance and your load is 50% of transformer you will get about 1.5% volt drop just with the transformer. Assume 2% transformer drop, another 2-4% service/main drop and you have bit off a good bit of the 10% before you get to the feeder.

Also, don't forget you should calculated the drop at starting load also, not just running load and that may be 6 times worse. Motors can handle more drop at start but there is a limit. Try to keep drop at start-up amps at 15% or less if you can. If you are 3% at run load then starting drop is probably going to be marginal without soft-start.

 

[ptonsparky]

I've never used a Sure Test, but have used VD calcs to determine one set of DB paralleled cables was bad. Voltage at the motor during startup indicated I only had half the cm installed.

 

[tortuga]

You don't have to change any caps when you switch the connections from 120V to 240V

The caps in there say 250V I think one is a 35 MFD and one is a 200 or 210 MFD
I usually see 440V caps in these fans when they wired for 240V.

Also, how many motors are starting at the same time?

Its two at the same time both about 1HP.

The Sure Test gives total drop, not just the feeder drop. Measure at the feeder source then at feeder end and take the difference. Do the same with the impedance values from the Sure Test.

The Sure Test amps may be different than your load amps.

If you have a transformer with 3% impedance and your load is 50% of transformer you will get about 1.5% volt drop just with the transformer. Assume 2% transformer drop, another 2-4% service/main drop and you have bit off a good bit of the 10% before you get to the feeder.

Also, don't forget you should calculated the drop at starting load also, not just running load and that may be 6 times worse. Motors can handle more drop at start but there is a limit. Try to keep drop at start-up amps at 15% or less if you can. If you are 3% at run load then starting drop is probably going to be marginal without soft-start.

Thats good info thanks.
The alligator clips I made were just a hack job, so they could be introducing errors, ill make some better ones up.
I have been using the sure test forever for older residential / commercial stuff and its usually clear cut;
a few outlets will have 80% voltage drop and the rest will have like 6%, find a bad splice or breaker job done.

Several years ago I used it to settle a little debate between an elevator guy and one of our guys.
Our guy thought it was a bad motor the elevator guy said bad wiring (of course).
I cant recall the reading but it was high enough that we puled a new feeder one size up and it solved the problem.
Ever since then I have been checking 120 - 240V motors with it, and got a few soft starts and larger feeders since.
I dont frequently see over 10% drop in residential / light commercial but large homes with lots of 14awg I will see like 18% in bedrooms and I see percentages in the teens often on larger commercial / industrial.

I first took a reading at the motor, then at the main breaker in the greenhouse and there was not much difference between the two, so thats why I honed in on the feeder.
First I am going to have them convert the motors to 240V and pull 10's to the motors.
Then I'll take a fresh readings at service, feeder main and motor.
Thanks again for all your replies.

 

[Cow]

Wire all of the motors so half of them on are the opposing phase. Start two at a time, one being on each phase.

 

[ActionDave]

The caps in there say 250V I think one is a 35 MFD and one is a 200 or 210 MFD
I usually see 440V caps in these fans when they wired for 240V.....

I would strongly suspect the 35 MFD would be a running cap and I'm surprised it's not 370 or 440V for a dual voltage motor.

 

[tortuga]

Well this is a little embarrassing, our lead electrician whom was not out there working Friday looked at the motors yesterday and discovered the run caps had not been connected in the pecker head.
Not sure how I could have missed that one.

Probably was out in the sun too long that day.

Thanks again for all your suggestions.

 

[ptonsparky]

Well this is a little embarrassing, our lead electrician whom was not out there working Friday looked at the motors yesterday and discovered the run caps had not been connected in the pecker head.
Not sure how I could have missed that one.

Probably was out in the sun too long that day.

Thanks again for all your suggestions.

Thanks. Glad I’m not alone.