75hp compressor on high leg

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elvis_931

Senior Member
Location
Tennessee
I did a job running power to a 75hp compressor. The company provided the spec FLA @ 178.5amps @240volts. I sized my pipe and wire accordingly. 2" EMT; 2/0 THHN cu, good for 195amps per NEC 310.16. I fused the bus plug and disconnect @ 200amps dual element time delay. Any guesses as to what happened?!:)
 

crossman

Senior Member
Location
Southeast Texas
Well, it either started and ran just fine, or it blew the fuses. The title leads one to the assumption that there is an issue with the high leg, but there should not have been.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
my first two guesses would be that the motor blew the fuses and the job was rejected by the inspector.
430.6 would require you to base you coductors on 430.250 which would put you more in line with a 250kcmil conductor and your fuses would probably need to be more like 300 amp.
 

elvis_931

Senior Member
Location
Tennessee
crossman said:
Well, it either started and ran just fine, or it blew the fuses. The title leads one to the assumption that there is an issue with the high leg, but there should not have been.

Okay, here is what happened: It started and ran just fine... then blew the fuse in the bus plug and the disco on the high leg. The current on A phase was 190a; B(high leg)phase was 211a; C phase was 180a. I have no idea why the high leg is pulling a much higher current. There is no neutral load on this compressor to make the high leg an issue. It reads 240v between all three phases. What is going on here?:confused:
 

RUWired

Senior Member
Location
Pa.
elvis_931 said:
I did a job running power to a 75hp compressor. The company provided the spec FLA @ 178.5amps @240volts. I sized my pipe and wire accordingly. 2" EMT; 2/0 THHN cu, good for 195amps per NEC 310.16. I fused the bus plug and disconnect @ 200amps dual element time delay. Any guesses as to what happened?!:)

With a 75 hp motors conductors should be rated for 125% (4/0). Time delay fuses 175%(336amp).Conduit (2-1/2") Then you hooked up the control circuit on the high leg.Everything went boom.
 

elvis_931

Senior Member
Location
Tennessee
augie47 said:
430.6 would require you to base you coductors on 430.250 which would put you more in line with a 250kcmil conductor.

Actually a 3/0 conductor would do fine.


and your fuses would probably need to be more like 300 amp.[/QUOTE]

That is what the code says I agree, but why fuse a "180amp" load @ 300amps? Especially when using a 3/0 conductor, which then becomes your "fuse".

Of course this is a seperate discussion than what was originally posted. Please try to stay on topic.
 

elvis_931

Senior Member
Location
Tennessee
RUWired said:
Then you hooked up the control circuit on the high leg.Everything went boom.

The fan motor was 3 phase also. The controls for the system only pull minimal current, not enough to make this much difference.
 

crossman

Senior Member
Location
Southeast Texas
elvis_931 said:
and your fuses would probably need to be more like 300 amp.

But we must remember that the 175% is a maximum. It isn't a code violation to put somehting smaller.

elvis_931 said:
That is what the code says I agree, but why fuse a "180amp" load @ 300amps? Especially when using a 3/0 conductor, which then becomes your "fuse".

If I had to make a guess, the 3/0 would not become a fuse until the current reached, oh, I don't know, maybe around 2,000 to 2,500 amps.
 

RUWired

Senior Member
Location
Pa.
augie47 said:
430.6 would require you to base you coductors on 430.250 which would put you more in line with a 250kcmil conductor
Hey Augie. if we're not fused at the conductor amps, why could't we use the 90 column.(4/0)
Rick
 

elvis_931

Senior Member
Location
Tennessee
crossman said:
If I had to make a guess, the 3/0 would not become a fuse until the current reached, oh, I don't know, maybe around 2,000 to 2,500 amps.

If I had to make a guess the 3/0(NEC 310.16 rated 225amps) would start to become HOT at, oh I dont know, maybe around 240 to 300 amps. Not necessarily a problem as long as the current does not go above the FLA but, then if that never happened we would not need fuses at all I guess. Fuses are not only for short circuit protection.
Back to my original question; why did the high leg draw so much more current? Has anyone ever seen this happen before?
 

RUWired

Senior Member
Location
Pa.
elvis_931 said:
The fan motor was 3 phase also. The controls for the system only pull minimal current, not enough to make this much difference.

I said that because everything else was going wrong for you.
icon3.gif
 

RUWired

Senior Member
Location
Pa.
Augie you were right with the 250kcmil...

(4) For motors marked with design letters B, C, or D, conductors having an insulation rating of 75?C (167?F) or higher shall be permitted to be used, provided the ampacity of such conductors does not exceed the 75?C (167?F) ampacity.
 

nakulak

Senior Member
430.52, time delay fuse = 1.75 * 178 = 311 (next size up 430.52 C 1 ex 1)=350


conductors
430.6 A 1 - table 430.250 75hp, flc = 192

430.22 A 125% * 192 = 240

310.16 @ 75 -> 250kcmil conductors


(think that's right)
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
nakulak said:
430.52, time delay fuse = 1.75 * 178 = 311 (next size up 430.52 C 1 ex 1)=350


conductors
430.6 A 1 - table 430.250 75hp, flc = 192

430.22 A 125% * 192 = 240

310.16 @ 75 -> 250kcmil conductors


(think that's right)


same numbers I came up with basically... hopefully we are both right
 

RUWired

Senior Member
Location
Pa.
elvis_931 said:
The current on A phase was 190a; B(high leg)phase was 211a; C phase was 180a.

Did you check all your connections at the equipment disconnects and motor leads. Was this a 9 or 12 lead motor and did you check they were right.
 
Did you check the voltage when the large air compressor is off and check again when the unit kicked on ?

Did you check the type of POCO transfomer size ?? open delta system can wreck hovac on the system.

If a true Delta system the transfomer cans willbe same size allthe way thru.

The current should be the same all the way unless the POCO haveing issue with their supply system.

Merci,Marc
 

ducorse

Member
elvis_931 said:
If I had to make a guess the 3/0(NEC 310.16 rated 225amps) would start to become HOT at, oh I dont know, maybe around 240 to 300 amps. Not necessarily a problem as long as the current does not go above the FLA but, then if that never happened we would not need fuses at all I guess. Fuses are not only for short circuit protection.
Back to my original question; why did the high leg draw so much more current? Has anyone ever seen this happen before?


the fuses are basicly for short circuit protection. that is why you have motor overload protecion provided in the starter or vfd. if you are pulling 240 or 300 amps, the motor itself is going to fail before the feeder conductors will. that is why you have motor overload protection installed in the first place.

as far as the first question, i agree that the motor feeders are too small and the overcurrent protection is too small. however there seems to be either a tap miswire problem or someting wrong with the motor. if the voltage is the same from all phases, the the current should be the same (nearly).

either way, the motor overload protection (heaters) should have tripped.
 

ducorse

Member
elvis_931 said:
If I had to make a guess the 3/0(NEC 310.16 rated 225amps) would start to become HOT at, oh I dont know, maybe around 240 to 300 amps. Not necessarily a problem as long as the current does not go above the FLA but, then if that never happened we would not need fuses at all I guess. Fuses are not only for short circuit protection.
Back to my original question; why did the high leg draw so much more current? Has anyone ever seen this happen before?


in this case the fuses are essentially short circuit protection only. the motor overload should trip in the case of overcurrent. at 240-300 amps, the motor will fail before the conductors burn up.

in response to the original question, my guess is that there is a problem with the motor in addtion to the improper wirimg method.

i have seen examples of motors using as much as 10% difference per phase. however the motor should use much less than fla under no load. if it is possible, try to disconnect the motor from the load and see if it runs with much less amps. if so, you may have a situation where the load is too high. still you need to verify that there is proper overload protection or you are going to burn that baby up! not cheap for a 75 hp motor.
 
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