Voltage Drop Motor Space Heaters

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Shells

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I am currently reviewing an existing installation for 2.4kV motor. The motor is equipped with a 500kW, 120V motor space heater and is supplied from the motor starter by an 8AWG cable. The distance between the motor and the motor starter is approx. 1400m. If I apply a voltage drop calculation to this, the vd exceeds the 3% NEC requirement. Is there something I am overlooking? I consider this to be a branch circuit and normal vd applies.
 
Just to be clear, the 3% is only a recommendation, it is not a NEC requirement.

Roger
 
Shells said:
I am currently reviewing an existing installation for 2.4kV motor. The motor is equipped with a 500kW, 120V motor space heater and is supplied from the motor starter by an 8AWG cable. The distance between the motor and the motor starter is approx. 1400m. If I apply a voltage drop calculation to this, the vd exceeds the 3% NEC requirement. Is there something I am overlooking? I consider this to be a branch circuit and normal vd applies.
Click to expand...
Are you sure it is a 500kw heater? Thats pretty big for a 120 volt service?
Maybe a 500 or 5000 watt heater? Even a 5000 watt seems to large for 120 volts.
 
The space heater will run longer at the lower voltage to keep the temperature the same, but it should work OK. For utility substation work, we often specify 240 volt space heaters in equipment to be operated at 120 volts, so that they last longer.

The 500 kW must be a typo. This would overload the #8 AWG.
 
I apologise, that was a typo. It is 500W not 500kW. Actually, I believe the space heater was specified as you mentioned... rated at 240V but to operate at 120V. Thanks!
 
jghrist said:
The space heater will run longer at the lower voltage to keep the temperature the same, but it should work OK. For utility substation work, we often specify 240 volt space heaters in equipment to be operated at 120 volts, so that they last longer.

The 500 kW must be a typo. This would overload the #8 AWG.
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Hi, me again. Just wanted some further clarification on this issue. The motor and associated space heater being fed by the cable in question is being abandoned, but I would like to use these existing cables to power another new (smaller) motor and it's space heater which shall be in the same location. I hope this is making sense.. So my question is, seeing that it is a new installation, should I accept the voltage drop for the sake of cost savings.... based on what u said, the space heaters wont be damaged, just operate less efficiently I guess. I hope you understand my concern....Side note: The power cables dont have a voltage drop problem, its just the space heater cables im concerned about.
 
Jraef said:
Now it looks like maybe you went the other way...
500W and #8 cable? So 4A on a cable rated for 40A? I don't see a VD problem with that!
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I calculated VD as follows :

VD = 2 X L X R X I/1000

where L = length of cable run - 4500ft
R = Resistance of uncoated copper conductor in ohm/kft taken from Table 8 of the NEC, for an 8 AWG cable is 0.778
I = 4A
This gives me approx 28V or 23% at 120V.
Am I missing something here?
I really appreciate your help
 
Have you thought about the method of motor heating that puts a small amount of current through the windings and uses the motor leads to supply the power? If you used a 480 volt 500w heater the drop would be about 5%.
 
What is the rating of this heater? In post #5 you say that it is a 240V heater operated at 120V; you also say that this is a 500W heater. Is the 500W rating at the nominal 240V? Or is the 500W value what the heater will use at 120V?

For common resistance heaters, a 500W 240V heater nominally requiring a bit more than 2A of current, when operated at 120V will require a bit more than 1A of current and will only produce about 125W of heat.

-Jon

P.S. The resistance from table 8 is the DC resistance at 75C; at room temperature the resistance is perhaps 15-20% lower...but you may also need to consider the reactance of the cable, especially if it is in ferrous conduit.

(edited to add the PS)
 
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winnie said:
What is the rating of this heater? In post #5 you say that it is a 240V heater operated at 120V; you also say that this is a 500W heater. Is the 500W rating at the nominal 240V? Or is the 500W value what the heater will use at 120V?

For common resistance heaters, a 500W 240V heater nominally requiring a bit more than 2A of current, when operated at 120V will require a bit more than 1A of current and will only produce about 125W of heat.

-Jon

P.S. The resistance from table 8 is the DC resistance at 75C; at room temperature the resistance is perhaps 15-20% lower...but you may also need to consider the reactance of the cable, especially if it is in ferrous conduit.

(edited to add the PS)
Click to expand...



You are right. The existing motor's space heater was rated 500W @ 240V but is operated at 120V. However, the new motor's space heater that I would like to install in it's place using the existing cables is rated 350W @ 120V. This gives me a 17% VD or 20V @ 120V
 
1400m = 4593 ft. (0.87 miles)
350w @ 120vac = 2.92A
Cu = 12.8 ohms/ft

4593 (2) 2.92 (12.8) ? 120 (.03) = 95,371 CM
#1/0 = 105,600cm = 2.7%vd
#8 = 16,510CM = 17.3%vd

There is a very good possibility that the motor may burn up due to the insufficient voltage...:mad:

Even if you provided a 0.5kva, 1ph. (240V:120V) X'fmer to feed at that length, you'll still be shy of the 'desired' CM.... I appreciate the fact that Cu is expensive, but why take a chance on burning the motor up (this accomlishes nothing) or the building??:mad: :mad:
Notice: I didn't even apply the 125% factor!
 
Cajun,
Thanks for your response. Just to clarify I am referring to the VD on the motor's space heater... not the motor. I didn't think that the motor would burn if there is voltage drop on my supply to my heaters. Worst case, there would be insufficient heating of my motor windings leading to condensation etc causing damage to my motor. Maybe you can clarify this....
As one of the senior members suggested, for a constant resistance heater, if a reduce my voltage, i will reduce my current. Similarly, my power will be reduced, resulting in a longer time period for the required thermal energy to be generated. I am also trying to find out from the vendor if there is any tolerance on the space heater supply voltage..... usually there is a +/- 10% voltage tolerance for most equipment...
 
8 awg for 4 amps load at 120volts which is 1400 metres is definitely undersized...

Can you check the voltage at heater terminals when in use?

I reckon it would be around 85 volts...

Now, more importantly, the load is a space heater only.

If the load is sensitive to VD then i would recommend 2 Awg is for such distance..

It is economics again...are you ready to put a copper 2 Awg cable 1400 metres long for a space heater?....weigh the pros and cons and take your decision..

I would keep the VD below 3% for any load.

Cheers.
 
Shells said:
I am currently reviewing an existing installation for 2.4kV motor. The motor is equipped with a 500kW, 120V motor space heater and is supplied from the motor starter by an 8AWG cable. The distance between the motor and the motor starter is approx. 1400m. If I apply a voltage drop calculation to this, the vd exceeds the 3% NEC requirement. Is there something I am overlooking? I consider this to be a branch circuit and normal vd applies.
Click to expand...
a motor that is almost a mile away from its starter seems a little unusual.

if the voltage drop bothers you, add a small transformer to boost the heater voltage. I don't see that it is an option to run larger wires.

Or if you want to get really fancy add a 120X480 transformer at the source and a 480X120 transformer at the motor. Should reduce the voltage drop substantially.
 
petersonra said:
a motor that is almost a mile away from its starter seems a little unusual.

if the voltage drop bothers you, add a small transformer to boost the heater voltage. I don't see that it is an option to run larger wires.

Or if you want to get really fancy add a 120X480 transformer at the source and a 480X120 transformer at the motor. Should reduce the voltage drop substantially.
Click to expand...

Thanks for the response. The reason the motor is so far from the power supply is for reliability reasons. There are 4 of these motors used to drive pumps for an NGL system. We have two separate power grids here, one that is in closer proximity to the pumps location, and the other 1400m away. If one grid goes down for any reason, we want to maintain some reliability by having the other grid feed the remaining pumps. The thing is, the original motor in question is actually being fed by the 8AWG power supply, 1400m away as we speak for the last 2 yrs. The motor has actually not been running for this time so the space heaters should be on and there haven't been any problems to date. I will run a check on the voltage at the space heaters right now. I just dont want to introduce any unecessary expense into the project... what is the worse that can happen??
 
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