parallel secondary sizing

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misterbill1972

misterbill1972

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I have a 1200A 3PH 480V existing parallel circuit with the wrong size wire. I am running an additinal 3" conduit and I will be pulling (3) 750 kcmil in each. My secondary goes to a box on the motor. this has (2) pre-threaded 5" holes. I will reduce these down to 4". My questian is I cannot fit (9) 750 kcmil in (2) 4" conduits. I can fit (12) 700 kcmil. I have already done the derating and correction factors. It will work. Is this in any way a NEC violation. I cannot find anything to say it is, but still wonder. Any comments will be appreciated.
 
misterbill1972 said:
I have a 1200A 3PH 480V existing parallel circuit with the wrong size wire. I am running an additinal 3" conduit and I will be pulling (3) 750 kcmil in each. My secondary goes to a box on the motor. this has (2) pre-threaded 5" holes. I will reduce these down to 4". My questian is I cannot fit (9) 750 kcmil in (2) 4" conduits. I can fit (12) 700 kcmil. I have already done the derating and correction factors. It will work. Is this in any way a NEC violation. I cannot find anything to say it is, but still wonder. Any comments will be appreciated.
Click to expand...

I don't get it. You say 3'' conduit and then 4' conduit. For starters 12 700's don't fit in a 4". Looks like your using the 100% fill column.
 
I think I follow you logic by doing a little guess work on number of conductors and number of conduits.
IMHO, the short answer to your question is YES, Code compliant.
That said, I'm left with a bunch of questions as to your design.
I feel there may be a flaw. I might be worth your while to have forum menbers review the plan.
Would you care to give us more specifics ?
Motor size?, type wire (AL vs Cu)?, type conduit?, any Flex ???
 
Are you aware conductors supplying a motor will be rated for the motors FLA + 25% and have little relationship to the breaker size?

As long as the conductors are large enough for the FLA +25% your all set even if the breaker rating exceeds the conductor rating.

If you already knew this I apologize for covering it.

It just seems odd that an existing installation of this type would be undersized.
 
augie47 said:
I think I follow you logic by doing a little guess work on number of conductors and number of conduits.
IMHO, the short answer to your question is YES, Code compliant.
That said, I'm left with a bunch of questions as to your design.
I feel there may be a flaw. I might be worth your while to have forum menbers review the plan.
Would you care to give us more specifics ?
Motor size?, type wire (AL vs Cu)?, type conduit?, any Flex ???
Click to expand...
This is a 800HP motor, THHN Cu wire, EMT with vertical GRC drops (for strapping distance), 5' seal tite at motor
 
iwire said:
Are you aware conductors supplying a motor will be rated for the motors FLA + 25% and have little relationship to the breaker size?

As long as the conductors are large enough for the FLA +25% your all set even if the breaker rating exceeds the conductor rating.

If you already knew this I apologize for covering it.

It just seems odd that an existing installation of this type would be undersized.
Click to expand...
yes sir i am
 
augie47 said:
I think I follow you logic by doing a little guess work on number of conductors and number of conduits.
IMHO, the short answer to your question is YES, Code compliant.
That said, I'm left with a bunch of questions as to your design.
I feel there may be a flaw. I might be worth your while to have forum menbers review the plan.
Would you care to give us more specifics ?
Motor size?, type wire (AL vs Cu)?, type conduit?, any Flex ???
Click to expand...
The original elecrical contractor ran parallel 250's (4 wire) and wires are and conduits are hot.
 
misterbill1972 said:
this will be (6) in each
Click to expand...
Your still over even with THHN in EMT.

6?0.9887=5.9322

4' EMT at 40% fill = 5.901

I agree with Gus. Could you explain this design in greater detail?
 
chris kennedy said:
Your still over even with THHN in EMT.

6?0.9887=5.9322

4' EMT at 40% fill = 5.901

I agree with Gus. Could you explain this design in greater detail?
Click to expand...
this could be done in 5", but NEC only shows up to 4"
 
misterbill1972 said:
This is a 800HP motor, THHN Cu wire, EMT with vertical GRC drops (for strapping distance), 5' seal tite at motor
Click to expand...
So you need about 1200 amps of wire. It looks like two conduits each with six 400kcmil copper will do the job. You can fit seven 400kcmil THHNs in a 4".
 
I still need to be convinced he needs 1200 amps worth of wire.
but, I'm not good at pulling teeth.
 
misterbill1972 said:
This is a 800HP motor, THHN Cu wire, EMT with vertical GRC drops (for strapping distance), 5' seal tite at motor
Click to expand...
I looked for the FLA for an 800 hp motor. Could not find it. What does the name plate show as the FLA for this motor?
 
don_resqcapt19 said:
Bob,
For my calculations, I just doubled the current in Table 430.250 for a 400 hp motor.
Click to expand...
That would require the feeder to have close to 1200 amp ampacity.
400 hp FLA = 477 amps x 2 = 954 x 1.25 = 1193 amps.
Circuit ampacity = 1193/3 ckts = 398 amps per ckt or 600 kcm per phase.

don_resqcapt19 said:
So you need about 1200 amps of wire. It looks like two conduits each with six 400kcmil copper will do the job. You can fit seven 400kcmil THHNs in a 4".
Click to expand...
4- 400 kcm per phase = 4 x 380 amps x 0.80 = 1216 amps.
Which would be the least costly?
 
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