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Thread: Machine Amperage Question

  1. #1

    Machine Amperage Question

    Ok with the new machines coming in very soon I'm putting in a new subpanel and need to know what Amp breakers to get for the following:

    1. 5hp General Cabinet Saw
    2. Yorkcraft 5 hp 20" planer
    3. Oliver 3hp 10" Jointer

    So does anyone have any of these machines to tell me what breaker size to use.

    Thanks

    Corey

  2. #2
    as a rule of thumb for 220 1PH you can count on 5 amps/hp.

    So 5HP=25 amp
    3Hp=15 amps

    This is a rule of thumb and depends on the motor manufacturers effencies.

  3. #3
    Join Date
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    Quote Originally Posted by Corey Pionk
    Ok with the new machines coming in very soon I'm putting in a new subpanel and need to know what Amp breakers to get for the following:

    1. 5hp General Cabinet Saw
    2. Yorkcraft 5 hp 20" planer
    3. Oliver 3hp 10" Jointer

    So does anyone have any of these machines to tell me what breaker size to use.

    Thanks

    Corey
    Can you download any of the spec sheets from the place you bought them?
    "And remember, this fix is only temporary, unless it works." - Red Green

    THIS THREAD IS USELESS WITHOUT PICTURES


  4. #4
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    All of the tools should have their electrical requirements posted on them. You then want to go at leasT 25% over for the circuit. 50% is better.

  5. #5
    Corey,

    I don't have any of those machines, but mathmatically the current draw should be approximately 16 amps for the 5 hp machines and 10 amps for the 3 hp machine. The manufacturers nameplate ratings may vary somewhat from this.

    You should be OK with a 30 amp breaker and 10 guage wire for the 5 hp applications and with a 20 amp breaker and 12 guage wire for the 3 hp application.

  6. You don't need the spec sheets. A 5 hp motor will need a 30 amp circuit with #10 wire. The 3 hp motors will need a 20 amp circuit with #12 wire.

    Oops. Didn't see Jerry's post at the same time as mine. Although the circuit information is correct, his amperage values are too low.
    Last edited by Rick Christopherson; 04-12-2006 at 7:10 PM.

  7. #7
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    Here is what the NEC says,

    5HP = 28 amps full load current, x 125% for wiring and devices, and 250% for the circuit breaker.

    3 HP = 17 amps, X 125% = 21.25 wire or 12 gauge, with a 45 amp breaker

    5 HP motors that's 8 ga. wire with 40 Amp plugs and receptacles, with a 70 amp breaker.

    BTW by code, you must use the NECs numbers above, not the FLA listed on the motors.

    I know, some of you out there will wince at recommending 8ga wire on a 70 amp breaker, but that is what the code allows. Specifically it is the job of the thermal overloads to provide overcurrent protection, the branch circuit breaker is for ground faults and short circuit protection.

    Particularly when you get to 5 HP motors you have to look at the whole circuit for the wiring.

    The above is what the code allows, what I would do, is use a 10ga. 30 amp circuit, with a 30 amp circuit breaker for the 3 HP motor, and 8ga. 40 amp circuits, with 40 amp breakers for the 5 HP motors, but I would not be afraid to install a 50amp breaker for the 5 HP motors if I had trouble starting one or both of the motors.

    Steve
    Last edited by Stephen Dixon; 04-13-2006 at 2:10 AM.

  8. #8
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    Quote Originally Posted by Stephen Dixon
    .....5 HP motors that's 8 ga. wire with 40 Amp plugs and receptacles, with a 70 amp breaker......
    what I would do, is use .....40 amp circuits, with 40 amp breakers for the 5 HP motors.....
    This whole message puzzles me; but I am particularly confused by your recommendation which seems to conflict with what you say the NEC calls for??

  9. #9
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    Should have said "up to a 70 Amp breaker"

    I know I'm not the best at explaining this, but the hard part is to get past the breaker being larger than the wire size would indicate. The breaker in a motor circuit is to protect against ground faults and short circuits, the wire is protected by the motor or starter thermal elements.

  10. #10
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    Do not forget to take in inrush current into account

    Current requirements for machines can have 2 different values. One for steady state and one for initial turnon (Inrush). The initial turnon can be 25% higher than steady state for a few millisconds. These few milliseconds could trip the breaker or stress it to fail later on. So plan appropriately. Also consider the distance that the power has to travel from the fuse box. When in doubt size for the inrush, and that should take care of any voltage drop due to long cable runs.

  11. #11
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    Quote Originally Posted by Corey Pionk
    ...I'm putting in a new subpanel and need to know what Amp breakers to get for the following:

    1. 5hp General Cabinet Saw
    2. Yorkcraft 5 hp 20" planer
    3. Oliver 3hp 10" Jointer
    How far is the sub panel from the motors? Are you creating a separate circuit for each one?

    5hp motors generally have a constant load of around 20-25 amps, but the startup load is higher - need to check the motors specification plate to be sure. 3hp motors are usually 15-20 amps load while running and again the startup load is more.

    Realistically, you should be safe with a 30 amp breaker for the 5hp motors and a 20 amp for the 3hp motor but...

    I'd seek advice from a licensed electrician to be sure everything is kosher. Is the sub panel sized properly, proper wire size for the wiring between the main panel and sub panel, and proper wire size from the sub panel to the motors?

    There's a lot of variables you're not providing information for with your post. I'd hate for you to take anyones suggestions (my self included) for gosphel since there are some critical details missing.

    Larry

  12. #12
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    Quote Originally Posted by Stephen Dixon
    I know I'm not the best at explaining this, but the hard part is to get past the breaker being larger than the wire size would indicate. The breaker in a motor circuit is to protect against ground faults and short circuits, the wire is protected by the motor or starter thermal elements.
    hi steve
    can you please provide chapter and verse in the NEC for your recommendation ?

    thanks
    lou

  13. #13
    Stephen, What I remember a few years back from my inspector was that I could go with breaker recommendations like yours if I was really wiring a motor circuit. That is, I was hardwiring to the saw (with appropriate disconnect) and it was only used for motors. He chose to treat my circuits as 220V branch circuits (i.e. I use twist lock plugs and can plug what ever I want into them, including motors) and so my 3HP motors would work nicely on a 20A/12ga circuit and my 4.8HP motors would play nicely on a 30A/10ga circuit. He suggested making a motor circuit if I experience problems starting the motors on a branch circuit. I haven't ever had a breaker problem on startup so the branch circuit is fine. The wire sizing is the same for both just the breaker values vary so wouldn't the branch circuit be more conservative?

  14. #14
    Quote Originally Posted by lou sansone
    hi steve
    can you please provide chapter and verse in the NEC for your recommendation ?

    thanks
    lou
    This is all from Article 430 of the NEC. I think some explanation is in order:

    Article 430 specifices requirements for motors and motor circuits.

    circuit conductor ampacity - The Article says to ignore the nameplate FLA on the motor, and instead go by the HP rating. Take that horsepower number and look up the ampacity on the table in the Article, which will most certainly be higher than the nameplate FLA. The intent here is to cover the circumstance where, sometime in the future, the motor is replaced with a new one of the same HP, but perhaps a higher FLA. Thus, the conductors are sized for a "worst case" situation with the new motor.

    short circuit and GF protection - The article says this may be provided by a breaker that is rated a maximum of 250% of the current found in the aforementioned Table. It doesn't have to be this big, but if you need to increase the rating in order to prevent it from tripping at motor startup, this is the biggest you can make it.

    But what about overcurrent protection for the circuit conductors? A 70A breaker can't protect 8 AWG conductors from an overcurrent situation.

    Overcurrent requirements are defined in 240-4. In most cases, it refers the reader to the tables in Chapter 3 most people are familar with. But for motor circuits, it refers the reader to Article 430 instead. And in '430, it says that the motor overcurrent protection may serve as the circuit conductor overcurrent protection.

    One thing I've always been concerned about, though, is that in 240-21 itsays that the overcurrent protection for a conductor must be on the line side of the conductor, except in certain circumstances. And a single motor on a branch circuit isn't one of the listed circumstances. But if one reads the NEC Handbook (a publication of the NFPA that explains the NEC section-by-section), it specifically says it is OK, so I guess it is. I'd still like it if Article 430 explicitly said the overcurrent protection could be on the load side, though.

    I agree with Steve Wilson's inspector that this method really shouldn't be relied on in plug-and-cord connected motors, however. There's no guarantee that a motor with an integral OCPD will always be plugged into the receptacle, so there's no way to ensure that the conductors will have overcurrent protection even using this method.

    Which brings up the "interesting" question of the plug and receptacle. According to '430, these together constitute a motor Disconnecting Means. As such, they must meet the horsepower rating of the motor. The intent here is to ensure that in an emergency, you can cut all power to the motor by pulling the plug, without arcing or the plug contacts welding themselves to the receptacle. But the fact of the matter is that the horsepower ratings for NEMA connectors are surprisingly low. As an example, single-phase connectors with a 5 HP rating don't exist; even the biggest one, a NEMA 14-60, is only rated at 3 HP.

    So supposedly, it would be a Code violation to put a 5 HP device on a plug, or to use anything smaller than a 50A plug on a 3 HP machine. But we know this is not common practice. Either the Disconnecting Means provision is the most-ignored provision in the NEC, as some electricians just shrug it off as, or cord-and-plug connected woodworking tools aren't considered "motors" under the NEC at all. Some electricians and inspectors regard them a "motor driven appliances". In these case, most of '430 doesn't apply, including the requirement that plugs and receptacles be horsepower rated. And in this case, the branch circuit breaker must be sized to protect the circuit conductors.

    'sorry if this is more than what some wanted to know about this.

  15. #15
    Thanks for the explanation Barry. You can never have too much information and what you passed along is relevant for most of us. I just figure that if I'm friendly and honest with the Electrical Inspector and do some research before asking him questions that I'll get a fairly good response; so far, so good. NEC discussions are always good.

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