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Thread: Sizing Motor OCPD's

  1. #1
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    Sizing Motor OCPD's

    When sizing the OCPD for motors in the design process. Is there a "Rule of Thumb" for sizing the OCPD? Such as, 100%,125%, of the FLC or do you
    go for the maximum of 250% for breakers.Or do you wait and see if the motor
    trips at it's minimum FLC and go from there? I personaly would like to go
    straight to 250% of FLC but that isn't always cost effective when you are
    bidding against someone that sizes the OCPD by the FLC, not realizing the motor has to start BEFORE reaching it's FLC. I'm just curious, and hoping
    to learn something. Thanks!

  2. #2
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    perhaps someone with a lot more motor knowledge like jraef can give you a number but from my experience it's not that simple. assuming you are speaking of motor short-circuit/ground fault protection, I have found the range is so dependent on variables that it's difficult to select. One thing that should help you is to check the LRA Code and Table 430.7(B), but the type of load and starting torque varies so much I could not guesstimate a "average". I always leaned toward the high end but as your point out that can change your breaker frame size and cost considerably.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  3. #3
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    This is the way I have considered this:
    Unless the motor has integral thermal protection a properly sized heater is used for motor overload protection. Thermal magnetic circuit breakers are not intended to provide overload protection for motors. If a TM breaker is used it is the magnetic element that provides short circuit protection in the event of motor failure should a winding short to ground for example. Since the motor should already be protected with a properly applied overload relay it will provide the overload protection and the thermal element of the breaker is essentially useless.
    A motor circuit protector is a much better device for short circuit protection in this application when used with a listed combination motor starter. If you can't use an MCP by code you have to default to a TM breaker as applied by art 430.
    But still look at them both as short circuit protect unless the intent of a TM breaker is sized to provide cable protection.

  4. #4
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    Greeting all,
    For choosing motor breaker size, branch cable size, feeder breaker size or when there are more than one motor working as a parallel for a feeder or Special breaker type. I am posting all step by step please follow and I hope it will you alot.

    breakers have a TD built in, it varies by type, some are inverse time...they make special breakers for motors, MCP or motor circuit protectors...so for a very small time, they let a large current thru, as the time grows longer, the smaller the trip point until it levels out at ~100% or so of rating...

    a 100A MCP or CB
    may let 700A for 1.5 sec
    350A for 2
    175 for 4
    125 for 8
    100 indefintely

    for an example

    the NEC has good stuff on this..

    the breaker really protects the wiring, the ol's protect the motor...they are usually set to trip at 115%

    when coordinating protection, motor type and load characteristics must considered...there is a code on the motor for inrush current and the load may be variable or constant torque...cetrifugal pump vs loaded conveyer...etc.
    "Imagination is more important than knowledge."

  5. #5
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    Motors and Branch-Circuit Conductors

    Overcurrent and short-circuit protection aren’t the same for motors

    The best method for providing overcurrent protection for most circuits is to use a circuit breaker that combines overcurrent protection with short-circuit and ground-fault protection. However, this isn't usually the best choice for motors. With rare exceptions, the best method for providing overcurrent protection in these cases is to separate the overload protection devices from the short-circuit and ground-fault protection devices

    Motor overload protection devices like heaters protect the motor, the motor control equipment, and the branch-circuit conductors from motor overload and the resultant excessive heating (430.31). They don't provide protection against short-circuits or ground-fault currents. That's the job of the branch and feeder breakers, which don't provide motor overload protection. This arrangement makes motor calculations different from those used for other types of loads. Let's look at how to apply Art. 430, starting at the motor.

    Overload protection. Motor overload devices are often integrated into the motor starter. But you can use a separate overload device like a dual-element fuse, which is usually located near the motor starter, not the supply breaker.


    Fig. 1. Overcurrent protection is generally accomplished by separating the overload protection from the short-circuit and ground-fault protection device.
    If you use fuses, you must provide one for each ungrounded conductor (430.36 and 430.55). Thus, a 3-phase motor requires three fuses. Keep in mind that these devices are at the load end of the branch circuit and that they don't provide short-circuit or ground-fault protection.

    Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started [430.32(C)] must have an overload device sized per the motor nameplate current rating [430.6(A)]. You must size the overload devices no larger than the requirements of 430.32. Motors with a nameplate service factor (SF) rating of 1.15 or more must have an overload protection device sized no more than 125% of the motor nameplate current rating.


    Fig. 2. When working with motors that have a service factor rating of 1.15 or higher, size overload protection devices no more than 125% of the motor nameplate rating.
    Let's look and work through a sample calculation.

    Example No. 1: Suppose you use a dual-element fuse for overload protection. What size fuse do you need for a 5-hp, 230V, single-phase motor with a service factor of 1.16 if the motor nameplate current rating is 28A?

    (a) 25A
    (c) 35A
    (b) 30A
    (d) 40A

    The overload protection shall be sized according to the motor nameplate current rating [430.6(A), 430.32(A)(1), and 430.55].

    You also have to consider another factor: nameplate temperature rise. For motors with a nameplate temperature rise rating not over 40°C, size the overload protection device no more than 125% of the motor nameplate current rating. Thus, 28A×1.25=35A [240.6(A)]


    Fig. 3. Size the overload protection device of a motor with a nameplate temperature rise rating of 40°C or less at no more than 125% of the motor nameplate current rating.
    Let's look at Fig. 3 and work through another example problem.

    Example No. 2: Again, suppose you're using a dual-element fuse for the overload protection. What size fuse do you need for a 50-hp, 460V, 3-phase motor that has a temperature rise of 39°C and motor nameplate current rating of 60A (FLA)?

    (a) 40A
    (c) 60A
    (b) 50A
    (d) 70A

    The overload protection is sized per the motor nameplate current rating, not the motor full load current (FLC) rating. Thus, 60A×1.25=75A. Overload protection shall not exceed 75A, so you need to use a 70A dual-element fuse [240.6(A) and 430.32(A)(1)].

    Motors that don't have a service factor rating of 1.15 or higher or a temperature rise rating of 40°C and less must have an overload protection device sized at not more than 115% of the motor nameplate ampere rating (430.37).


    Fig. 4. Refer to Table 310.16 when selecting the proper size conductor to serve a single motor.
    Sizing branch-circuit conductors. Branch-circuit conductors that serve a single motor must have an ampacity of not less than 125% of the motor's FLC as listed in Tables 430.147 through 430.150 [430.6(A)]. You must select the conductor size from Table 310.16 according to the terminal temperature rating (60°C or 75°C) of the equipment [110.14(C)]. Let's reinforce this concept by working through a sample calculation.

    Example No. 3: What size THHN conductor do you need for a 2-hp, 230V, single-phase motor?

    (a) 14 AWG
    (c) 10 AWG
    (b) 12 AWG
    (d) 8 AWG

    Let's walk through the solution:

    Step 1: Conductor sized no less than 125% of motor FLC

    Step 2: Table 430.148 shows the FLC of 2-hp, 230V, single-phase as 12A

    Step 3: 12A × 1.25 = 15A

    Step 4: Per Table 310.16, you need to use 14 AWG THHN rated 20A at 60°C

    The minimum size conductor the NEC permits for building wiring is 14 AWG [310.5]. However, local codes and many industrial facilities have requirements that 12 AWG be used as the smallest branch-circuit wire. So in this example you might need to use 12 AWG instead of 14 AWG.


    Short-circuit and ground-fault protection devices are designed for fast current rise, short-duration events. On the other hand, overload protection devices are designed for slow current rate, long-duration situations.
    Branch-circuit protection for short-circuits and ground-faults. Branch-circuit short-circuit and ground-fault protection devices protect the motor, motor control apparatus, and conductors against short circuits or ground faults. They don't protect against an overload (430.51)

    Continue..........
    "Imagination is more important than knowledge."

  6. #6
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    The short-circuit and ground-fault protection device required for motor circuits isn't the type required for personnel (210.8), feeders (215.9 and 240.13), services (230.95), or temporary wiring for receptacles (527.6).

    Per 430.52(C), you must size the short-circuit and ground-fault protection for the motor branch circuit — except those that serve torque motors — so they're no greater than the percentages listed in Table 430.52.

    When the short-circuit and ground-fault protection device value that you find in Table 430.52 doesn't correspond to the standard rating or setting of overcurrent protection devices as listed in 240.6(A), use the next higher protection device size [430.52(C)(1) Ex. 1].

    Did that statement stop you? Does it strike you as incorrect? That's a common response, but remember, motors are different than other system components. Motor overload protection devices, such as heaters and fuses, protect the motor and other items from overload. The short-circuit and ground-fault protection doesn't need to perform this function. Therefore, oversizing won't compromise protection. Undersizing will prevent the motor from starting.

    Use the following two-step process to determine what percentage from Table 430.52 you should use to size the motor branch-circuit short-circuit ground-fault protection device.

    Step 1: Locate the motor type on Table 430.52.

    Step 2: Select the percentage from Table 430.52 according to the type of protection device, such as non-time delay (one-time), dual-element fuse, or inverse-time circuit breaker. Don't forget to use the next higher protection device size when necessary.

    Let's see if you have this concept down with a short quiz. Of the following statements, which one is true? Use Table 430.52 to look up the numbers.


    1.The branch-circuit short-circuit protection (non-time delay fuse) for a 3-hp, 115V, single-phase motor shall not exceed 110A.

    2.The branch-circuit short-circuit protection (dual-element fuse) for a 5-hp, 230V, single-phase motor shall not exceed 50A.

    3.The branch-circuit short-circuit protection (inverse-time breaker) for a 25-hp, 460V, 3-phase synchronous motor shall not exceed 70A.


    Let's address each question individually. We'll be referring to 430.53(C)(1) Ex. 1 and Table 430.52.


    1.Per Table 430.148, 34A×3.00=102A. The next size up is 110A. So this is true.

    2.Per Table 430.148, 28A×1.75=49A. The next size up is 50A. So, this is also true.

    3.Per Table 430.150, 26A×2.50=65A. The next size up is 70A. This is also true.


    Remember the following important principles:


    •You must size the conductors at 125% of the motor FLC [430.22(A)].

    •You must size the overloads no more than 115% to 125% of the motor nameplate current rating, depending on the conditions [430.32(A)(1)].

    •You must size the short-circuit ground-fault protection device from 150% to 300% of the motor FLC [Table 430.52].


    If you put all three of these together, you can see the branch-circuit conductor ampacity (125%) and the short-circuit ground-fault protection device (150% to 300%) aren't related.

    This final example should help you see if you've been paying attention.


    Although this example may bother some people, the 14 AWG THHN conductors and motor are protected against overcurrent by the 16A overload protection device and the 40A short-circuit protection device.
    Example No. 4: Are any of the following statements true for a 1-hp, 120V motor, nameplate current rating of 14A?

    (a) The branch-circuit conductors can be 14 AWG THHN.

    (b) Overload protection is from 16.1A.

    (c) Short-circuit and ground-fault protection is permitted to be a 40A circuit breaker.

    (d) All of these are true.

    Walking through each of these, you can see:

    (a) The conductors are sized per 430.22(A): 16A×1.25=20A; Table 310.16 requires 14 AWG at 60°C.

    (b) Per 430.32(A)(1), overload protection is sized as follows: 14A (nameplate)×1.15=16.1A.

    (c) Short-circuit and ground-fault protection is determined based on 430.52(C)(1): 16A×2.50=40A circuit breaker.

    Therefore all three statements are true.

    The 16A overload protection device protects the 14 AWG conductors from overcurrent, while the 40A short-circuit protection device protects them from short circuits. This example illustrates the sometimes confusing fact that when you're doing motor calculations, you're actually calculating overcurrent and short-circuit protection separately.

    Motor calculations have long been a source of confusion and errors for many people. Understanding what makes these calculations different should help you do your motor calculations correctly every time. Next month we'll look at sizing motor feeders in Part 2.
    "Imagination is more important than knowledge."

  7. #7
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    Motor Calculations Part 2: Feeders



    What’s the correct way to size motor feeders and related overcurrent protection?

    Part 1 of this two-part series explained how to size overload protection devices and short-circuit and ground-fault protection for motor branch circuits. Understanding the key point of that article, which was that motor overload protection requires separate calculations from short-circuit and ground-fault protection, clears up a common source of confusion and a point of error. But another source of confusion arises when it comes to sizing short-circuit and ground-fault protection for a feeder that supplies more than one motor. Let's look again at branch-circuit calculations and then resolve the feeder issues so your calculations will always be correct.

    Branch-circuit conductors and protection devices. Per 430.6(A), branch-circuit conductors to a single motor must have an ampacity of not less than 125% of the motor full load current (FLC) as listed in Tables 430.147 through 430.150. To illustrate this, let's size the branch-circuit conductors (THHN) and short-circuit ground-fault protection device for a 3-hp, 115V, single-phase motor. The motor FLA is 31A, and dual-element fuses for short-circuit and ground-fault protection are in use


    •Per Table 430.148, the FLC current is 34A.

    •34A×125%=43A.

    •Per Table 310.16 (60°C terminals [110.14(C)(1)(a)]), the conductor must be a 6 AWG THHN rated 55A.



    Don’t make the mistake of using a motor’s FLA nameplate rating when using the short-circuit and ground-fault protection devices. You must use the FLC rating given in Table 430.148.
    Per the motor FLC listed in Table 430.52, size the branch-circuit short-circuit and ground-fault protection devices by using multiplication factors based on the type of motor and protection device. When the protection device values determined from Table 430.52 don't correspond with the standard rating of overcurrent protection devices listed in 240.6(A), you must use the next higher overcurrent protection device. To illustrate this, let's use the same motor as in the previous example.


    •Per 240.6(A), multiply 34A×175%
    •You need a 60A dual-element fuse.

    To explore this example further, see Example No. D8 in Annex D of the 2002 NEC. Once you've sized the motor overloads, branch-circuit conductors, and branch-circuit protective devices, you're ready to move on to the next step.

    Motor feeder conductor calculations. From 430.24, you can see that conductors that supply several motors must have an ampacity not less than:


    •125% of the highest-rated motor FLC [430.17], plus

    •The sum of the FLCs of the other motors (on the same phase), as determined by 430.6(A), plus

    •The ampacity required to supply the other loads on that feeder.



    Fig. 2. Motor feeder conductors shall be sized not less than 125% of the largest motor FLC plus the sum of the FLCs of the other motors on the same phase.
    and solve the following problem.

    Example No. 1. For what ampacity must you size the feeder conductor if it supplies the following two motors? The terminals are rated for 75°C.


    •One 7.5-hp, 230V (40A), single-phase motor

    •One 5-hp, 230V (28A), single-phase motor


    (a) 50A
    (b) 60A
    (c) 70A
    (d) 80A

    Let's walk through the solution.


    •The largest motor is 40A.

    •40A×1.25+28A=78A.

    •80A is the closest selection that's at least 78A.


    What size conductor would give us this ampacity?

    (a) 2 AWG
    (b) 4 AWG
    (c) 6 AWG
    (d) 8 AWG

    Per Table 310.16, a 6 AWG conductor rated at 75°C provides 65A of ampacity, so it's too small. However, a 4 AWG conductor provides 85A of ampacity, which will accommodate the necessary 78A. Therefore, you need to size this feeder conductor at 4 AWG.

    Next, we have to determine what size overcurrent protection device (OCPD) we must provide for a given feeder.


    Fig. 3. To size overcurrent protection devices for each feeder, start by determining the ampacities required for each motor and move on from there.
    Example No. 2. Using a slightly more complex example, try sizing the feeder conductor (THHN) and protection device (inverse-time breakers, 75°C terminal rating) for the following motors


    •Three 1-hp, 120V, single-phase motors
    •Three 5-hp, 208V, single-phase motors
    •One wound-rotor, 15-hp, 208V, 3-phase motor

    Refer to 240.6(A), 430.52(C)(1), Table 430.148, and Table 430.52. Start by determining the ampacities required for each size of motor, then walk through each step until you arrive at the correct OCPD size.


    •1-hp motor: FLC is 16A.
    16A×250%=40A

    •5-hp motor: FLC is 30.8A.
    30.8A×250%=77A (Next size up is 80A.)

    •15-hp motor: FLC is 46.2A.
    "Imagination is more important than knowledge."

  8. #8
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    Continuing Part-2

    Each motor’s FLC will come into play when sizing the conductor.
    46.2A×150% (wound-rotor) 569A (Next size up is 70A.)

    Now, let's look at the feeder conductor. Conductors that supply several motors must have an ampacity of not less than 125% of the highest-rated motor FLC (430.17), plus the sum of the other motor FLCs [430.6(A)] on the same phase

    Continuing with this example, add up all the ampacities, multiplying the highest rated motor by 125%. Thus:


    •(46.2A×1.25)+30.8A+30.8A+16A=136A.


    Table 310.16 shows you need 1/0 AWG THHN because at 150A it's the smallest conductor that accommodates the 136A of ampacity we're working with. When sizing the feeder conductor, be sure to include only the motors that are on the same phase. For that reason, these calculations only involve four motors.

    You must provide the feeder with a protective device with a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device (plus the sum of the full-load currents of the other motors of the group) [430.62(A)]. Remember, motor feeder conductors must be protected against the overcurrent that results from short circuits and ground faults but not those that result from motor overload.

    When sizing the feeder protection, be sure to include only the motors that are on the same phase.


    Fig. 5. In this example, the largest branch-circuit fuse or circuit breaker allowed for Motor 1 is 70A.
    Refer to Fig. 5 for this sample motor feeder protection calculation.

    Example No. 3. What size feeder protection (inverse-time breaker) do you need for the following two motors?


    •5-hp, 230V, single-phase motor
    •3-hp, 230V, single-phase motor

    (a) 30A breaker
    (b) 40A breaker
    (c) 50A breaker
    (d) 80A breaker

    Let's walk through the solution.

    Step 1: Get the motor FLC from Table 430.148.


    •A 5-hp motor FLC is 28A.
    •A 3-hp motor FLC is 17A.

    Step 2: Size the branch-circuit protection per the requirements of 430.52(C)(1), Table 430.52, and 240.6(A)


    •5-hp: 28A×2.5=70A

    •3-hp: 17A×2.5=42.5A (Next size up is 45A.)


    Step 3: Size the feeder conductor per 430.24(A).


    •The largest motor is 28A.

    •(28A×1.25)+17A=52A

    •Table 310.16 shows 6 AWG rated 55A at 60°C as the smallest conductor with sufficient ampacity.


    Step 4: Size the feeder protection per 430.62.


    •It must not be greater than the 70A protection of the branch circuit plus the 17A of the other motor, which is the total of all loads on that feeder.

    •70A+17A=87A


    Choose the next size down, which is 80A.

    How can you be safe if you're selecting the next size down instead of the next size up? Remember, you've already accounted for all the loads, and the NEC requires that you not exceed the protection of the branch circuit. Again, keep in mind that you aren't calculating for motor overload protection. Motor calculations are different from other calculations. With motor feeders, you're calculating for protection from short circuits and ground faults, only — not overload.

    Putting it all together. Motor calculations get confusing if you forget there's a division of responsibility in the protective devices. To get your calculations right, you must separately calculate the motor overload protection (typically near the motor), branch-circuit protection (from short circuits and ground faults), and feeder-circuit protection (from short circuits and ground faults). Remember that overload protection is only at the motor.

    Any time you find yourself confused, just refer to NEC Figure 430.1. It shows the division of responsibility between different forms of protection in motor circuits. Example D8 in Annex D of the 2002 NEC illustrates this with actual numbers. Keeping this division of responsibility in mind will allow you to make correct motor calculations every time.
    "Imagination is more important than knowledge."

  9. #9
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    OK, here's a test. 20HP motor, 3ph, 460 volts, 1.15 SF,nameplate FLA 24.1,
    NEC FLC 27. Size the OCPD. Not the OL, the OCPD. The OCPD would be a breaker. The Maximum size in regards to the NEC would be 250% of the
    FLC. Thanks to everyone for the responses!! I'm just trying to see if there
    is a "Rule of Thumb" when SPEC'ing the OCPD for a motor. I have a feeling
    there will be different sizes of OCPD's depending on the person designing it.
    Then again I may be wrong!(again)

  10. #10
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    This is not a test question. Just a curiosity I have regarding "Average".

    BTW, my answer would be 50 for the OCPD for the 20 HP motor I mentioned
    above.

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