Sizing Motor OCPD's

Status
Not open for further replies.
K

Karl H

Senior Member
Location
San Diego,CA
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! :)
 
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.
 
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.
 
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.
 
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..........
 
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.
 
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.
 
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.
 
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) :)
 
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. :)
 
Karl H said:
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) :)
Click to expand...

I understand what you want, but the devil is in the details. Are you feeding power to a single non-combination (has no OCPD in it) motor starter on the wall 50 feet away? If so, that might end up being considered a branch circuit and the rules for branch circuits apply. If you are building your own motor starter, then article 430 rules apply (let's leave UL out of this discussion for the moment).

So let's assume you are are building a motor starter and it will have a properly sized OLR as part of the starter assembly. To size a breaker then, let's start with 125% FLC because that's what you seem to fear someone else is going to do. So 27A x 1.25 = 33.75, next size up rule; 40A TM CB. Cross checking the specs on the breaker, I see however that it comes with the Instantaneous Trips factory set at 400% In, non adjustable. That means the mag trips will go off at 160A. Going backwards on that 27A motor then, that is slightly under 600% FLC. Seems as though it would work, but if it's a new energy efficient motor, as almost all new installations are now, the magnetic inrush can be a LOT higher than that and it would nuisance trip. I personally would start higher, or make sure I used a TM CB that has an adjustable mag trip setting, or at least something higher than 400% In.

Now, cross checking on my little slide chart from a starter mfr, they say the "recommended" inverse-time (thermal-mag) breaker is 70A, which means they just go immediately to the 250% point. To be honest, that's what I do too. You can spend more time on it and try to get more exact, but what's the difference in cost of a 40A breaker vs a 70A breaker? And how much is your time worth?
 
Sizing motors

Sizing motors

I use this hand out when teaching motors;

Motors

430.6(A) 1 states;
(1) Table Values. Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 shall be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, instead of the actual current rating marked on the motor nameplate. Where a motor is marked in amperes, but not horsepower, the horsepower rating shall be assumed to be that corresponding to the value given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250, interpolated if necessary. Motors built for low speeds (less than 1200 RPM) or high torques may have higher full-load currents, and multispeed motors will have full-load current varying with speed, in which case the nameplate current ratings shall be used.

Table 430.248 indicates 12 amps for a 230V 2HP single phase motor.

430.22(A) General states;

(A) General. Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor?s full-load current rating as determined by 430.6(A)(1).

12A x 125% = 15A

Table 310.16 indicates #14AWG is rate for 20A.

Now before you get your panties in a bunch the asterisk refers us to the bottom of Table 310.16 where it states: See 240.4(D).

240.4(D) states;

(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.

240.4(E) refers to tap conductors which is not applicable here. However, 240.4(G) is a table and tells us that Article 430 applies for motor branch circuit conductors. This means that #14 AWG is specifically permitted to be used for conductors supplying this 20A motor load.

Next we look at sizing the ground fault, short circuit, overcurrent protective device.

430.52 states;

430.52 Rating or Setting for Individual Motor Circuit.
(A) General. The motor branch-circuit short-circuit and ground-fault protective device shall comply with 430.52(B) and either 430.52(C) or (D), as applicable.

(C) Rating or Setting.
(1) In Accordance with Table 430.52. A protective device that has a rating or setting not exceeding the value calculated according to the values given in Table 430.52 shall be used. (Also read the exception.)

Table 430.52 states that for an inverse time circuit breaker (ITCB) protecting a single phase motor it is to be sized at 250% of the FLA as stated in Table 430.248.

12A x 250% = 30A

Based on the above calculation a 30A ITCB is correct.

If the motor fails to start and run then 430.52(C) (1) Exception No. 2(c) can be applied which states;

(c) The rating of an inverse time circuit breaker shall be permitted to be increased but shall in no case exceed 400 percent for full-load currents of 100 amperes or less or 300 percent for full-load currents greater than 100 amperes.

12A x 400% = 48A

Since we can?t exceed 400% of the motor FLA we round down to a 40A ITCB on a piece of #14 AWG conductor.

The real kicker is that if this was a pool filter motor it is code compliant.
 
Greeting all,
Dear friend, it is good information that you are teaching. I have also posted in the first page please consider in teaching those methods too.
 
Karl H said:
When sizing the OCPD for motors in the design process. Is there a "Rule of Thumb" for sizing the OCPD?

...do you wait and see if the motortrips 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,...
Click to expand...
Karl -
Let's see if I understand your post. Drawing on jaerf's post:

First, you are doing design work (engineering). Second you are considering basing your engineering on what someone else might bid.

So you are considering bidding the job using a smaller frame cb because it is cheaper and you are certain your competition will also bid the smaller frame. They are bidding the smaller frame because they don't know any better. You would bid the smaller frame because you are sure they are and you want to get the bid - but you know that it might not work.

If I have this right, then I'd ask:
Supose you put in the smaller frame CB and to increase the CB rating requires that you jump to the next frame size, and the motor will not start, but trips the CB. You are bid so tight that he job can not afford another CB. What is your plan?

cf
 
Last edited:
Jraef,
Right on!
As long as you are given the latitude use it otherwise one risks nuisance tripping.
Essentially the protection that either the TM of the MCP are providing is short circuit and "ground Fault" protection. A breaker with an adjustable magnetic setting may be adjusted low enough such that it doesn't nuisance trip upon the energizing of the motor. If the breaker did trip it must be reviewed for a cause as it occurred outside of the operating parameters of the motor. Also, knowing that motor failure often occurs when a winding shorts to ground, this failure would be considered a ground fault. I personally would like magnetic adjustments to be as low as practical just above the nuisance trip point such that any fault can be picked up as fast as possible which can reduce any damage done to the motor.
 
I have seen the time where the decision could be a high $$ one.
You may have a panel where there is no space left that will accept larger frame breakers and a 50 or 100 amp increase would mean a whole new panel or subpanles, etc.
That's when you hire jraef :)
 
Karl H said:
When sizing the OCPD for motors in the design process. Is there a "Rule of Thumb" for sizing the OCPD? ...
Click to expand...

Karl H said:
... 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. ...
Click to expand...

Following on with jaref's and electures' posts, my "rule of thumb" is that if I am designing (doing engineering) then that's what I do - engineer it, don't guess.

In electures' example there is not much difference in price between a QO240 and a QO230 so that one really does not matter.

So, lets say the motor in question is on the edge between a 100A frame and a 225A frame, and the difference in price a $200. (No, I do not have the frame sizes or costs immediately available - this is an example.) And the $200 makes a difference to the job.

If that is the case, then do the engineering. Get the motor LRA. Get a copy of the CB trip curve. Lay out a motor starting curve over the CB trip curve. If the motor starting curve is inside of the smaller frame trip curve, you are good to go. If not, go with the bigger frame.

Too much trouble? Maybe. Depends on your plan if the smaller frame trips on motor start.

cf
 
Last edited:
If my customer is going to give the other guy the job because he is $200 lower, and I don't take the time to make sure we are bidding apples to apples, I think i deserve to lose the bid.

There is ALWAYS someone who will do it cheaper. Not always better, but always cheaper. On more than one occasion I lost a project to the cheaper guy, only to get hired back after it turned into a nightmare.

And my rates go up for mulligans... :grin:
 
Thanks all for your responses, and I have to say, they have all
been good ones. I'd like to say this question isn't based on a
project I have right now or am I trying to build a starter for
any particular motor. Jref thanks for pointing my focus on the
actual breaker itself, something I haven't thought of before.

My question is about the science or "gut-feeling" when sizing a OCPD
for a motor we have never energized before. Since we have a FLC
and a NEC max % of protection. The safest bet would always be
250% of FLC. But, in some applications 250% would be an unwarranted
expense based on the fact that there isn't any calculation that would
say a 27 FLC motor would trip on start-up if only protected at 40I.
As most of us have seen motors start with OCPD's set at nameplate
FLA's and not trip and not trip in years of service protected at FLA.

Does anyone get where I'm going here? No one sized a OCPD for the 20HP
in my example by the way. I still say, take a motor's HP and the OCPD will be
different per designer based on gut and not math.

I did learn one very important thing in this post and that's to pay close
attention to trip-curves when protecting motors with breakers. :)
 
Status
Not open for further replies.
Top