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Thread: 150 HP Motor--Wire Size Discrepancies

  1. #1
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    150 HP Motor--Wire Size Discrepancies

    Hello!

    I am supposed to order all materials for a compressor install and have encountered 3 different specs for wire size. This is a Kaeser DSD 150--150 HP, 460 V, 3 PH Motor with and FLA of 225 Amps.

    Using the code calcs I found that I should use a 4/0 AWG wire for power. This was the same on the SquareD slide rule motor calculator I used.

    I also have an SPD Bussmann Electrical Protection Handbook that says I need 350 kcmil wire for power. This figure is closer to the manufacturers recommendation of 400 kcmil wire.

    I want to believe the manufacturer but with all the other calculations, 400 kcmil wire almost seems like overkill but I am really not sure. I was taught low voltage process control in school and this is slightly out of my educational capacities. I don't want to screw anything up and I want to save the company money at the same time. I know that bigger equals more expensive and if 400 kcmil wire is not necessary I would like to use something smaller. I just don't understand why there would be such a big difference in wire sizing.

    I would appreciate any help with this.

  2. #2
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    This is a Kaeser DSD 150--150 HP, 460 V, 3 PH Motor with and FLA of 225 Amps.
    Conductor should be 1.25 x FLA = 1.25 x 225 amps = 281.25 amps.
    4/0 at 75C = 230 amps. 300 at 75C = 285 amps. If the MFG equipment has 60C terminals then you must use the 60C col. 400 kcm = 280 amps.
    Check an see if the 75C terminals are being used. If you are strictly going by the NEC, then the 400 kcm would not be adequate.

  3. #3
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    I'm sorry. 225 A FLA is what I calculated. The nomenclature on the compressor says 182. And it is 75C terminals.

  4. #4
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    Quote Originally Posted by lowE4me
    I'm sorry. 225 A FLA is what I calculated. The nomenclature on the compressor says 182. And it is 75C terminals.
    4/0

  5. #5
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    What is the Locked Rotor Code? And the length of the branch circuit?

    If the nameplate rating is 182A, that already exceeds the Table 430.150 value for 150hp motors. That surprises me a bit since the Table is based on the absolute worst motor any NEMA manufacture makes.

    In any case, the manufacturer may be concerned about motor starting voltage drop as part of their recommendation.

    BTW, 75C terminals are standard for NEMA Design B,C and D motors. Design B is pretty much the "garden variety."
    "Bob"
    Robert B. Alexander, P.E.
    "I know that you believe you understand what you think the NEC says, but I am not sure you realize that what you read is not what it means." (Corollary to Charlie's Rule)

  6. #6
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    Thanks to all.

    I have decided to go ahead and use the manufacturer's recommendations because of warranty stipulations. When I talked to the manufacturer's rep on the phone, he said that the wire needs to be able to handle inrush current. I think it's safe to assume the manufacturer has done tests and made calculations that led them to 400 kcmil. Now, if I can just get the rest of the BOM correct all will be well.

    Thanks again.

  7. #7
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    We wired a 50HP compressor last week. Mfr instructions were 3/0 THHN CU wire, 200Amp circuit breaker, etc. Voltage was 208, 3ph.

    Wired everything, turned on fusible disconnect, turned on breaker, hit start on compressor--POW breaker tripped!

    Reconnected compressor to a 300A breaker in panel, to verify problem was not 200A breaker.

    Compressor started and ran. Starting current was approx 910 Amps.

    Compressor is a hard start. Could have been ordered with soft start &/or variable speed drive.
    I would be more concerned with the breaker. I-line breakers are not cheap off the shelf!

  8. #8
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    Breakers are certainly not cheap. This is the first time I ever priced any of this stuff and I just can't believe how expensive some of it is. The mfr told me to use 8 * FLAof 225A (which equals 1800) to size the breaker and I am looking now at those and wondering if the 1800 would refer to the magnetic trip settings. I could order a breaker the same size as the fuses in the disconnect and the hi-lo ac mag trip settings would handle the 1800.

  9. #9
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    That Mfg. may be taking Power Factor into consideration. The NEC makes similar adjustments to Motor FLA at the bottom of Tbl. 430-150.

    For a 150 HP 460vac 3ph at 0.80 Power Factor.
    180 * 1.25 (for PF)= 225 FLA
    225 * 1.25 (continuous use)= 281.25 FLA or 300 kcmil @75c
    Roger Ramjet NoFixNoPay

  10. #10
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    If the MFG equipment has 60C terminals then you must use the 60C col. 400 kcm = 280 amps.
    Check an see if the 75C terminals are being used.

    Terminals for this application would be required by 110.14(C)(1)(b) to be listed for 75 degrees since they are for #1/0 AWG or larger.
    Rob

    Chief Moderator

    All responses based on the 2011 NEC unless otherwise noted

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