240 vac 7.5hp air compressor voltage drop question

[artieb]

Hi, I have a question concerning voltage drop an my air compressor.
I'm trying to use a 240 leg for the coil of a 120vac relay to break the compressors pressure switch circuit.
I had an episode where the 120 relay chattered. The relay drop away voltage is 85%.
I checked the 120 side of the 240 feed and found that it dropped to 90 / 180 vac.
What is the acceptable voltage drop? The compressor has been wired this way for 18 years with no issues.
http://www.garagejournal.com/forum/attachment.php?attachmentid=422295&stc=1&d=1425599544
Here should be a pic of my motor tag.

 

[nehog]

Open neutral/poor neutral connection.

 

[alfredeneuman]

What are you connecting the side of the coil that's not on the 240 leg to?

I hope it's a neutral, and not the ground

 

[Aceman]

Hi, I have a question concerning voltage drop an my air compressor.
I'm trying to use a 240 leg for the coil of a 120vac relay to break the compressors pressure switch circuit.
I had an episode where the 120 relay chattered. The relay drop away voltage is 85%.
I checked the 120 side of the 240 feed and found that it dropped to 90 / 180 vac.
What is the acceptable voltage drop? The compressor has been wired this way for 18 years with no issues.
http://www.garagejournal.com/forum/attachment.php?attachmentid=422295&stc=1&d=1425599544
Here should be a pic of my motor tag.

Your line to line voltage is too low. It could be anything from an undersized power company transformer, undersized service, undersized wire or too long of a run feeding the compressor, etc etc.

Open neutral/poor neutral connection.

His line to line voltage is too low, how do you figure it's a neutral problem?

 

[artieb]

Thanks and I'll look tomorrow, as to the wire run length, and gauge. It has always started a little sluggish.

 

[wyliesdiesels]

Sounds like voltage drop...let us know the wire size and length! At a minimum u should br using #8 THHN or #6 NM-b!

 

[nehog]

...
I checked the 120 side of the 240 feed and found that it dropped to 90 / 180 vac. ...

...
His line to line voltage is too low, how do you figure it's a neutral problem?

I read his 90 / 180 as being one leg to neutral, and the second leg to neutral. If I misread, sorry. If it is one reading across the entire 240 volt circuit, then absolutely you're right: voltage too low, probably a bad connection or splice.

 

[Rookie2]

is the load on the panel balanced? maybe switching legs will help but you may have other problems in the motor/ wiring.

 

[artieb]

Thanks and I went and confirmed wire gauge 10 awg stranded 3 conductor and about 80'. I'm sure that's where the drop is. I'll go out and check the voltage at the breaker when starting. I'm glad to have a good capacitor bank for the start up, THANKS! Artie

 

[sberry]

They will run on an 8 but a 10 is way too small for a 7.5 especially at 80 ft. which wouldn't be a problem with a bigger wire.

 

[wyliesdiesels]

I read his 90 / 180 as being one leg to neutral, and the second leg to neutral. If I misread, sorry. If it is one reading across the entire 240 volt circuit, then absolutely you're right: voltage too low, probably a bad connection or splice.

180v line to neutral ?

Thanks and I went and confirmed wire gauge 10 awg stranded 3 conductor and about 80'. I'm sure that's where the drop is. I'll go out and check the voltage at the breaker when starting. I'm glad to have a good capacitor bank for the start up, THANKS! Artie

#10 is WAY too small!

 

[justsam]

The voltage drop you are seeing is far too great, even for the undersized wire.

You are an order of magnitude off, you should see about a 5 Volt drop but are seeing more like 50 Volts.

Either the motor, while in running mode, is drawing far more than 32 Amps or there is electrical or mechanical resistance some where. You need to check at head end of circuit to confirm there is 240 VAC. Double or triple the current draw and you still should not see a 50 Volt drop.

Since you mention 3 wire to the compressor, I assume 2 hot and a ground. As has been asked, where do you get the 120VAC since neutral is not present? Are you using Ground as neutral?

I would want to resolve what has changed in the 18 years of "no issue" operation, and then proceed to wire properly.

 

[sberry]

I agree, while the wire should be upsized it did run for a long time. The wire size did not help especially if this saw heavy service but can contribute as well to a connection failure.
While the wire is marginal its not directly the cause of the problem.

 

[sberry]

Just out of curiosity what is the wiring method, pipe, cable or cord and what breaker is on it? Go to a plug?
The real demand is a pinch above 5 hp, way closer to 5 than 7.5 If the connections were secure wouldn't have started a fire even if it ran continuous.

 

[artieb]

I know that the wire gauge is toooo small. At the breaker, in a cold shop, the voltage dropped to 220 from 238 volts, no load. I need to buy some #6 or #8 gauge

 

[justsam]

I know that the wire gauge is toooo small. At the breaker, in a cold shop, the voltage dropped to 220 from 238 volts, no load. I need to buy some #6 or #8 gauge

If you dropped 18 Volts at the breaker, with load and no load, you have an issue with whatever is feeding the breaker. Is this a subpanel? Your "undersized" wire is not even in the equation at this junction, so measure at the source feeding the breaker.

 

[sberry]

You shouldn't have any drop no load.

 

[alfredeneuman]

I know that the wire gauge is toooo small. At the breaker, in a cold shop, the voltage dropped to 220 from 238 volts, no load. I need to buy some #6 or #8 gauge

That makes no sense at all. 220-238 is an increase, not a decrease.
Are you sure it didn't drop to 238 from 240? If it did, it's just normal fluctuation.

EDIT: According to the Southwire Voltage Drop Calculator, you'd only need to run (3) #8s and a #10 ground for that load at 80'

 

[justsam]

I know that the wire gauge is toooo small. At the breaker, in a cold shop, the voltage dropped to 220 from 238 volts, no load. I need to buy some #6 or #8 gauge

I read that as 220VAC under load and 238VAC with no load, both measured at the breaker. OP will need to clarify.

First you need to determine why the voltage at the breaker is sagging some 18Volts. That is too much.

That leaves about another 30, being lost on your 10 ga to compressor. Still too much drop even for the undersized wire. Check connections all along the circuit beginning with what ever feeds the breaker.

 

[alfredeneuman]

I read that as 220VAC under load and 238VAC with no load, both measured at the breaker.

The voltage should be the same at the breaker whether it's with a load or not. The drop should occur only at the load and nowhere else.
If not, I'd suspect a bad breaker.

Either way, you'd need to run #8s for the compressor motor.

 

[Charles (in GA)]

Run #6 and get it done to last.

 

[wyliesdiesels]

I know that the wire gauge is toooo small. At the breaker, in a cold shop, the voltage dropped to 220 from 238 volts, no load. I need to buy some #6 or #8 gauge

You dropped 18 volts with NO load? Sounds like u either have a problem with the feeder(if this is at a subpanel) or if in the main service panel, u have an issue with the service entrance or transformer feeding your house.

We really need more info to better help you...

That makes no sense at all. 220-238 is an increase, not a decrease.
Are you sure it didn't drop to 238 from 240? If it did, it's just normal fluctuation.

EDIT: According to the Southwire Voltage Drop Calculator, you'd only need to run (3) #8s and a #10 ground for that load at 80'

I think u misread what he said. He said it dropped to 220V FROM 238v, meaning the voltage WAS 238v and it dropped to 220v...

 

[Rookie2]

I would check the incoming panel voltage to see if the drop in voltage is at the main lugs also . Check for corrosion on the buss , an aluminum buss can corrode where the breaker attaches. Yes the wire is too small.

 

[alfredeneuman]

I think u misread what he said. He said it dropped to 220V FROM 238v, meaning the voltage WAS 238v and it dropped to 220v...

I did misread it. Sorry.
Nonetheless, he said the readings were taken at the breaker. It should have been equal AT THE BREAKER, regardless of the load on the other end.

If that's the case either the breaker is at fault, or something upstream on the line side,

 

[ichabod]

food for thought. voltage drop is based on the conductor length, size, type of material, the load served and supply voltage.

In motors circuits as the voltage goes down the current (load) goes up.

if you are using a 120 volt coil the voltage drop % would be higher.

either way a #10 conductor is to small for 7.5 hp motor

 

[artieb]

Thanks for all the replies. I'll be out today, checking voltage drops along the circuit. I wasn't aware of an issue, until I wanted to install a NO contact break in the pressure switch circuit. Using a KRPA-14AN-120 relay. This when I became aware of the voltage drop.
My plan is to get materials to do 3/4" EMT and use 6 thhn. Do I want a 10 in the conduit, for the ground?
Presently, my ground and neutral are the same conductor.

 

[CNGsaves]

OP . . . . . . What country are you located in ??

Now would be good time to Update GJ Profile with City/State/Country.

Is your 80 ft electrical wire run to compressor inside shop, or buried ??

Fixed orientation of your motor pic using free software Irfanview.

 

[artieb]

Thanks CNG, usually I have that info, got it now. All wiring is inside the shop.

 

[justsam]

The relay you specified has a 10Amp contact rating, and cannot directly switch the motor in series with the pressure switch. It is 3 pole, but putting contacts in parallel is not always a good idea, and it till would not deal with motor inrush.

Are you using a magnetic starter or some other motor current relay that is fed by the one you specified?

 

[wyliesdiesels]

Thanks for all the replies. I'll be out today, checking voltage drops along the circuit. I wasn't aware of an issue, until I wanted to install a NO contact break in the pressure switch circuit. Using a KRPA-14AN-120 relay. This when I became aware of the voltage drop.
My plan is to get materials to do 3/4" EMT and use 6 thhn. Do I want a 10 in the conduit, for the ground?
Presently, my ground and neutral are the same conductor.

When was this wired? Youre ground and neutral unless pre-2008, should NOT be the same wire...

 

[Danno63]

If the motor is 7.5 HP, this means about 5625 Watts (1 HP is about 746 watts). If you are running 220V, the steady state current will be about 5625/220=25.6 Amps. With an 80 foot run (loop is 160 feet), the voltage drop will be about 4V with #10 wire, 2.57V with #8, and 1.47 V with #6. Also, the start current is significantly higher than running current, but is momentary. You WILL see line voltage drop, no way around it, the question is how much can you live with. 10 gauge wire is rated for about 30 amps, so the current should not be the issue, the issue may be line length. Sounds like there is something else wrong to me. If you can measure (AND THIS COULD BE DANGEROUS, BE CAREFUL) the voltage while the compressor is running at the breaker, the voltage should be very close to 220 or 240 -whatever the no load voltage is. If the voltage at the breaker is way low, then there is a problem with the feeder, or the current is very high out of spec. #10 wire is used up to 30 Amps, but V=IR; the further the run the more voltage loss you will see regardless of cable gauge.

 

[wyliesdiesels]

#10 NM-b is 30a. THHN is higher!

 

[alfredeneuman]

If the motor is 7.5 HP, this means about 5625 Watts (1 HP is about 746 watts). If you are running 220V, the steady state current will be about 5625/220=25.6 Amps

The motor nameplate says 32 Amps, it's a 230V motor running on 240V.

The NEC lists a 7.5 HP 230V motor at 40 Amps, and that's the thing you need to size the wire and breaker to by Code. (at 125% of the load, no less)

#8 minimum

 

[wyliesdiesels]

^^beat me to it. And efficiency and load wasnt factored in either!

 

[artieb]

Hi again, I'm using a contact in my little relay to break the 120 vac contactor circuit. Relay contacts are rated 10a and the contactor pulls .040 a.
Tomarrow I'll have the materials, but need to babysit the grandkids in the afternoon.
I visually inspected all connections today and will meter check Monday.
I have 200a service and a 100a load center in the garage directly from the service.
Installed in 1997.
L1 and L2 will be 6 thhn and a 10 thhn for the neutral in EMT conduit.

 

[alfredeneuman]

Hi again, I'm using a contact in my little relay to break the 120 vac contactor circuit.

Why not just use the pressure switch to run the contactor directly?

All you would need to do is connect the neutral to one side of the coil.
You may have to move some of the jumper wires on the starter.

Don't use the neutral as a ground no matter what you do

 

[justsam]

Why not just use the pressure switch to run the contactor directly?

All you would need to do is connect the neutral to one side of the coil.
You may have to move some of the jumper wires on the starter.

Don't use the neutral as a ground no matter what you do

I believe the OP wants to have a compressor ON/OFF function by opening the pressure switch to contactor connection with the NO relay contact inserted in the circuit. Compressor ON/OFF will then be done by energizing the relay.

 

[artieb]

That is correct Justsam, I will be using a Insteon controlled 120 vac relay. I will have remote and local control.

 

[justsam]

That is correct Justsam, I will be using a Insteon controlled 120 vac relay. I will have remote and local control.

Artieb,

I need to do something similar to what you are doing. Mine is not quite so easy in that I have a 5HP IR t30 compressor, that does not use a contactor. The pressure switch 2P contacts switch the full motor load.

My compressor ON/OFF, (and disconnect) is the breaker which is about 10 feet away, line of sight. I do not switch the breaker when compressor is running, and have had no issues.

I too will probably implement some type of remote control via Z-Wave or Insteon.

 

[artieb]

Insteon has a 240 vac 30a load controller, I have heard not to dependable and short lived. Should be some sort of contactor easily found, such as air conditioner, etc.