voltage drop issue

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hilltop

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I have a voltage drop issue on a 115v single phase minus 70 freezer. The freezer draws 24 Amps when the compressor starts. We had past issues with the breaker tripping and increased the wire size to #10 Thhn on a 30 amp breaker. This seemed to solve the nuisance tripping for a while. We are now experiencing problems again. There are no joints in this run and the home run is approximately 160 feet from panel to source. When the compressor starts we have 115V at the breaker but only 89V at the unit with a 24 amp draw. I preformed a voltage drop calc and the calcs told me the max drop should be a little under 10V. Does anyone have any suggestions on why i would only have 89V at the unit when the compressor kicks in?
Also, my service Tech tells me the compressor is fine in this unit.
 
A 10V drop on a 115V circuit comes out to be an 8.25% drop. This is well above the 5% max that the NEC recommends. Your reading 89V at the compressor probably because of the additional voltage drop on the feeder powering the electircal panel. You have a 10V drop on the branch circuit conductors powering your compressor which leaves a voltage drop of 16V on feeder conductors. Check to see that panel feeders are size appropriatley. What is the length and gauge of the panel feeders?
 
090107-1343 EST

hilltop:

For #10 copper wire the resistance is close to 1 ohm per 1000 ft. Thus, your 160 ft run is about 0.32 ohms.

Measure the voltage at the main lugs in your main panel when the motor starts. You should see very little change here, maybe 1 to 3 V. Measure the voltage from the neutral bus to the breaker output terminal during startup.

I do not believe you need a 30 A breaker. I run a DeWalt saw at 120 V at the end of a run of about 100 ft or so of #12 copper on a 20 A QO breaker. During startup, several seconds, the voltage at the DeWalt drops to about 75 V at 70 to 80 A. If I cycle this on and off within maybe 15 seconds I may trip the breaker because of the residual heat in the breaker element.

At 24 A and 0.32 ohms the voltage drop is 7.68 V. Are you sure the starting current is 24 A?. What is the duration of the starting current?

What is the model and brand of breaker?

.
 
Oops, miss read the post, thought 24A was FLC not inrush. You should base the voltage drop on full load current, not inrush.
 
Where does the 24A number come from? Is that the rated starting current, or did you measure that value?

For how long is starting current drawn? What is the continuous load of the freezer?

A conventional 20A breaker should be able carry 24A without tripping for about 2 minutes or more. If the compressor is taking 2 minutes to start, then something else is wrong.

If the freezer came with a standard 15 or 20A cord and plug, then it is designed to be protected by no more than a 20A breaker. Using a 30A breaker is a violation, and if the freezer is tripping a 30A breaker then the freezer itself is probably being damaged.

It is possible that excessive voltage drop is what is causing the excessive current draw, and that fixing the voltage drop would solve your problems. Fixing voltage drop can require: finding bad connections in the service, feeders, or branch circuits, increasing the size of conductors in the service, feeder, or branch circuit, replacing the supply transformer, or some combination of these. You should not increase the size of the branch circuit breaker.

I suspect that something is wrong with the freezer and that the excessive voltage drop that you are seeing is caused by excessive starting current draw or excessive starting time.

-Jon
 
hilltop said:
I have a voltage drop issue on a 115v single phase minus 70 freezer. The freezer draws 24 Amps when the compressor starts. We had past issues with the breaker tripping and increased the wire size to #10 Thhn on a 30 amp breaker. This seemed to solve the nuisance tripping for a while..
Click to expand...

what does the nameplate say for this unit.? you upgraded the wire size from what ? #12 to 10. is that 24 amps running?


here a link to help. http://www.csgnetwork.com/voltagedropcalc.html
 
The 24A is inrush current only lasting a couple of seconds.
There are no breaks in the feed to the unit. It is a continuous run. I agree this unit should not be on a 30A breaker and that it is a code violation, this is part of the reason i am asking for advice with this issue. The Tech we have looking at the unit insists this is a voltage drop issue. I don't agree since the unit has worked fine for over 8 months without tripping the 30A breaker.
The upgrading of wire sixe to #10 and installing a 30A breaker was done previous to my hire. I am being told that the manufacturer of the freezer ok.d this install. I still don't agree with this scenario and have vioced this to the service tech. The freezer FLA is 15A.

I appreciate the tip on checking the voltage on the panel feeders, i will investigate that as well. This install is only around 6 years old and engineered by architects, so i assumed this would not be the problem.
 
winnie said:
Where does the 24A number come from? Is that the rated starting current, or did you measure that value?

For how long is starting current drawn? What is the continuous load of the freezer?

A conventional 20A breaker should be able carry 24A without tripping for about 2 minutes or more. If the compressor is taking 2 minutes to start, then something else is wrong.

If the freezer came with a standard 15 or 20A cord and plug, then it is designed to be protected by no more than a 20A breaker. Using a 30A breaker is a violation, and if the freezer is tripping a 30A breaker then the freezer itself is probably being damaged.

It is possible that excessive voltage drop is what is causing the excessive current draw, and that fixing the voltage drop would solve your problems. Fixing voltage drop can require: finding bad connections in the service, feeders, or branch circuits, increasing the size of conductors in the service, feeder, or branch circuit, replacing the supply transformer, or some combination of these. You should not increase the size of the branch circuit breaker.

I suspect that something is wrong with the freezer and that the excessive voltage drop that you are seeing is caused by excessive starting current draw or excessive starting time.

-Jon
Click to expand...
I meaured 17A inrush at 108V, 116V with no load. The freezer service tech says he measures 24A inrush at 89V. I tested it 3 times and only got 17A max. It has a running current of approx. 14A at 112V. The measurements i took were at the receptacle where the unit plugs in. i pulled out the receptacle aqnd put an ampmeter on the incoming hot and measured the voltage on the receptacle terminals.
I agree we should not need a 30A breaker and that it is not NEC compliant. That is why i am asking for advice.
 
090112-2152 EST

In an earlier post I estimated 0.32 ohms loop resistance for your 160 ft run of #10.

Using 24 A and 0.32 ohms you get a drop of 7.68 V. That tends to check your 116 to 108 V change. It is not unlikely that some of the voltage change preceeds the breaker, and then one moves closer to your 17 A.

The freezer person's 89 V does not correlate with what you describe.

If the running current is 14 A my guess is that the inrush current is higher than 24 A.

Neither of you may have the correct instrumentation to measure what is going on.

I have not reread the posts. Is there a problem with the freezer or is it that the freezer person thinks there is a problem?

.
 
Cow,

Given your assumptions, your calculations appear correct.

The 12.9 number is based upon a conductor temperature of 75C (if I recall correctly; the number is 10.4 at 20C).

The 3% value is a suggestion, not a requirement.

-Jon
 
hilltop said:
I meaured 17A inrush at 108V, 116V with no load. The freezer service tech says he measures 24A inrush at 89V. I tested it 3 times and only got 17A max. It has a running current of approx. 14A at 112V. The measurements i took were at the receptacle where the unit plugs in. i pulled out the receptacle aqnd put an ampmeter on the incoming hot and measured the voltage on the receptacle terminals.
I agree we should not need a 30A breaker and that it is not NEC compliant. That is why i am asking for advice.
Click to expand...

What kind of meters? True RMS? What kind of capture frequency? =)
Have the #10's been run on a 20a breaker? ( It sounds the 10's came w/ the 30a breaker? )

Basically your argument w/ the freezer tech is easy. It's got a 15a cord. It's made to run at 15a. Tell'm to fix his freezer or you put 14awg in! =}

But serously I think most of us have seen the kind of numbers before with and with out nuisance trips, my guess is that it is pulling more than 17a at start up. Are either of you using a True RMS meter? Is it possible that this thing might be flickering at start up? If the T-stat is a mechanical apparatus, I've seen that happen. The contactor chatters, hammering the circuit with arcing, and you get the picture. Even if it's in 1 second on / off cycles.

Good Luck, you'll figure it out.

Doug S.
 
I use a calibrated fluke meter. The service techs meter has not recently or probably ever been calibrated since he bought it. I find it hard to believe that the voltage drop would be on the panel feeder side as this is an addition to an existing building and the service is less than 10 years old with no major changes or remodels...but then again i did not check the feeders, only the branch circuit. The inrush current only lasts approx 3 to 4 secs. This happens when the freezer switches from low stage to high stage which is normal. I feel the readings i got were acccurate. The problem is that something in the freezer tripped the 30A breaker which is why we are both takng these readings to start with. Again it is bizarre that the techs readings are 24A at 89V. We have not taken these readings together, they were on separate dates.
The freezer has been running for about a week now with no further problems which still leads me to believe the issue is with the freezer and not a voltage drop issue which they insist is the problem.
 
090116-1418 EST

hilltop:

What is the brand of breaker?

Is it possible the freezer starts, stops, within a few seconds starts again, and maybe this repeats several times?

Under startup, 3 to 4 seconds, can you see your meter reading stablize? I really expect a larger starting current.

What is the motor HP rating?

.
 
hilltop - sounds like a whole bunch of people are trying to complicate your problem. Think about what you said: This install is 6 YEARS old, no trouble until some 8 months ago, when the tech convinced another electrician or management that it needed larger wire and a larger breaker - even though it ran for almost 5 1/2 years the way it was with no problem.
Check out your connections and the volts/amps. I'm not convinced there is not a problem with the unit.
I remember a repair manual for a brake system on a large hoist - it explained that if a certain problem occured, and if you did these certain steps, and the problem still existed.....then it was an electrical problem.....At that point the mechanics would give up, it never was an electrical proble, but we had to find the problem and show it to them before they would even begin repairs. I wanted to take that book and tear out the page.
I once had a problem with a pump tripping off - happened several times in a week period, the last time I was able to get there immediately after it tripped - one of the windings would open .....when the motor got hot.
 
Hilltop,

Different meters do provide different data.
It is difficult to get 'continuous data' from a digital meter,
even with a 'peak hold' function.

There is a characteristic I call 'taper down' time which a 'peak' reading cannot show.
If the 'taper down' time is very long (like 3 seconds), then a CB may trip.
The negative effect of 'taper down' current draw is different on each machine.

I use a clamp-on Ameter, with an analog scale.
Analog is one with a mechanical meter and a swinging needle display.
This old style meter will show in-rush and duration and taper down visually.
The needle movement responds under 1/10 second,
so the duration and taper down is very evident.
The Duration and Taper down are most important causes in CB tripping.
The initial 'tip' of the peak should be the same as the Lock Rotor Current.

I can visually follow the 'peak' in-rush and the 'taper down' currents.
The most accurate I've used is a Digital Recording Oscilloscope.

Just a thought for you.

You could have a very high in-rush peak, followed by a long taper down.
This scenario can trip breakers.
I've measured motor startups with 90V initial voltage, due to in-rush peak draw,
but if the taper down is quick, then no CB tripping.
 
Last edited:
Was wondering what the final outcome is on this. We are having the same issue at a clients facility with -80 freezers. 3 freezers all experiencing voltage drop.Running current of about 10 amps.Lowest voltage drop is down to 87 volts from 116 volts.One of our techs reported today that the freezers are fed from 2, 10/3 multi circuits. There is a 4th freezer on one of the circuits that i'm told is having no problems. He says there is no other items on the circuits.

Distance of the circuits is about 250 feet. Would the multiwire circuits be part of the issue as well as the distance?

Kelley
 
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