Conduit Hot

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garyfitzgerald

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I am feeding a centrifical chiller motor with (3) 4/0 XHHW conductors. Supply is 3 phase 480v. The conduit thermal scan is 127 degrees F. Ambient temperature is 74 degrees F. Load on the conductors is A=198, B=194 and C=160 amps. The customer (federal government) is concerned about the temperature of the conduit. The conductors read at 116 degrees F. inside the VFD cabinet at the terminations. I know that the load is within the conductor capacity and the temperature is within the insulation capacity. I'm trying to contact Southwire Tech Support to see if they can provide some insight. Is there somewhere else that may provide info on conductor temperature at various loads or do I have a real concern here?
 
Why are the phases so unballanced on a motor load? Is the conduit being used as the grounding conductor? There might be around 35 Amps going thru that conduit which might be the cause of the heat.
 
Two Ideas

Two Ideas

Remember that 75C is 167F. 75C will cause 2nd degree burns in seconds. 127F is only 52C, rather low when we talk about 75C and 90C conductors. The wire is probably fine.

I agree, unbalance is an issue. Even if there is no current in the conduit itself, the net current in the 3-phases will lead to a magnetic field and cause heating of the conduit.

Run a clamp-on ampmeter around all three conductors at one time. Is there a net current? If so, where is it going? Some leakage to ground is expected but not what you report.

You can estimate the temperature. Temperature rise is a function of heating. Heating is based on I squared R. Your 127F in 74F is 53F rise, that is 29C rise. The rise will be 2x if current increases by square root 2.
 
Is the conduit in the sunlight? The air temp may be 74 but in direct summer sunlight metal pipes can get awefully hot.
 
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Mr. Bill said:
Why are the phases so unballanced on a motor load? Is the conduit being used as the grounding conductor? There might be around 35 Amps going thru that conduit which might be the cause of the heat.
Click to expand...

There is a seperate grounding conductor installed with the phase conductors.

Thanks,

Gary
 
beanland said:
Remember that 75C is 167F. 75C will cause 2nd degree burns in seconds. 127F is only 52C, rather low when we talk about 75C and 90C conductors. The wire is probably fine.

I agree, unbalance is an issue. Even if there is no current in the conduit itself, the net current in the 3-phases will lead to a magnetic field and cause heating of the conduit.

Run a clamp-on ampmeter around all three conductors at one time. Is there a net current? If so, where is it going? Some leakage to ground is expected but not what you report.

You can estimate the temperature. Temperature rise is a function of heating. Heating is based on I squared R. Your 127F in 74F is 53F rise, that is 29C rise. The rise will be 2x if current increases by square root 2.
Click to expand...

I just spoke with the person who did the load readings. They were 183, 180 and 169. Quite a bit less of a variance. Your formula is a little beyond my abilities to try to figure out what the conductor temperature might calc out at.

Thanks

Gary
 
beanland said:
Some leakage to ground is expected but not what you report.
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Leakage to ground is not expected! You don?t get leakage unless there is a breakdown in the insulation system. I don?t know anything about the age or condition of this particular motor, but conductors are supposed to be remain in service for decades without allowing any leakage current to ground, unless they are subjected to physical damage, or unless they experience severe and prolonged overheating.

beanland said:
Run a clamp-on ammeter around all three conductors at one time. Is there a net current? If so, where is it going?
Click to expand...
That is a good suggestion, and a good question. If there is a non-zero reading in a combined reading of all three currents, it could mean there is a breakdown in the motor?s insulation system, that current is leaking from the hot conductors to the frame of the motor, and that that current is traveling back to the source along the conduit. You could be seeing some I^2-R losses within the conduit itself, and that could be causing your overheating problem. It is a quick and simple troubleshooting step, and I echo the suggestion that you give it a try.
 
garyfitzgerald said:
I know that the load is within the conductor capacity . . . .
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You didn't say this explicitly, but are we to understand the conductors are copper?
 
Mr. Bill said:
Why are the phases so unballanced on a motor load?
Click to expand...
garyfitzgerald said:
I just spoke with the person who did the load readings. They were 183, 180 and 169. Quite a bit less of a variance.
Click to expand...
beanland said:
I agree, unbalance is an issue. Even if there is no current in the conduit itself, the net current in the 3-phases will lead to a magnetic field and cause heating of the conduit.
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ptonsparky said:
If I did it right that is still a 4.7% imbalance and to much.
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Pierre C Belarge said:
Have you checked to see what may be causing the imbalance?
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The approximately-12v or so lower current on one phase may simply be the single-phase load on the other two phases of ancillary equipment, such as fans, controls, etc. This is easy enough to determine by looking at the schematics of the equipment, and/or by testing currents at the equipment.
 
garyfitzgerald said:
Your formula is a little beyond my abilities to try to figure out what the conductor temperature might calc out at.
Click to expand...
A method of calculating a fair estimate of conductor temperature is:
(Iconductor)?/(Iampacity)?= T3/(T2-T1)​
where
Iampacity = appropriate table value and includes corrections for ambient temperature (and derating if appropriate)
T1 = Ambient temperature
T2 = conductor insulation temperature rating
T3 = conductor temperature​

I emphasize, this method only provides an estimated conductor temperature. Note the above does not account for external sources of raceway heating not accounted for with ambient temperature, such as direct sunlight.
 
The motor is new. The conductors are copper.
We have identified that there is 3.2 amps running on the ground conductor with the machine pulling about 160 amps. There are multiple conductive paths for current flow to ground; control conduits, water piping, feeder raceway. This motor is controlled with a VFD. Carrier is trying to tell us that current on the ground conductor of a motor controlled with a VFD is normal. I disagree. The code states that the gound fault path is a non-current carrying pathway except when clearing a ground fault. I believe that the existing conditions create a personal safety hazard.
We don't have an ammeter with a clamp large enough to go around all of the conductors as was suggested but we feel we have identified there is a problem with leakage to ground.
An applications engineer from Southwire confirmed the conductors are at the appropriate temperature for the load. The feeder raceway is running about 11 degrees F hotter.
 
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Ground Current

Ground Current

Can you check the frequency of the current on the ground conductor? There is always some leakage current from insulated windings to equipment. This is from the capacitance between them. The higher the frequency, the more leakage. This 3.2A might be high frequency from the VFD. The NEC recognizes that the ground conductor is not to be "intentionally used" as a circuit conductor. Leakage current is considered "unintentional".
 
beanland said:
Can you check the frequency of the current on the ground conductor? There is always some leakage current from insulated windings to equipment. This is from the capacitance between them. The higher the frequency, the more leakage. This 3.2A might be high frequency from the VFD. The NEC recognizes that the ground conductor is not to be "intentionally used" as a circuit conductor. Leakage current is considered "unintentional".
Click to expand...

I don't have the equipment for checking frequency. That will probably be up to Carrier to prove. My concern is that the 3.2A on the ground conductor is only one leg of multiple paths for the leakage current. The total of all pathways could be quite high. I guess the easiest way to figure the total leakage current is to do a net current reading on all the conductors as was suggested.
 
Current on Ground

Current on Ground

It could be high, or lower. With multiple ground paths you could have circulating currents. However, your concern is valid and needs investigation. Leakage current, if it travels through bearings, can ruin them.

Though the 3.2A is alot of current if flowing through a person, it may be driven by 2VAC or less. Since current is a function of voltage and resistance, and you have a low resistance path, it does not take much voltage to drive the current. Can you measure the voltage from one end of the ground conductor to the other?
 
Minamal voltage

Minamal voltage

I haven't measured the voltage drop from end to end of the ground conductor but based on the length of the run and the resistance of the conductor I calculate the voltage to be about .061v. This is a #4 copper conductor about 60' long.
 
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