Circuit Breaker Rating / FLA Ratings

[Sarray]

Hi all,

I would like to clarify on the ratings of a Circuit breaker ratings given that the FLA ratings are known.

We are currently in the process of a having a vendor design 4 vacuum pumps but the schematics are showing Circuit breaker rating and FLA ratings of the motors to be the same

In my past experience, inspectors have asked to ensure the C.B was rated for 175% of the FLA but I believe it can be minimum 125% and maximum 175%

Please advise, if I advise the manufacturer to choose the CB such that it is between 125%-175%, will it be acceptable?

Thank you,

 

[kwired]

Per NEC you can go up to 175% for time delay fuses or 250% for thermal magnetic breakers. If those values won't allow motor to start you can increase those settings.

Most of the time I am seeing somewhere between 200 and 250% for thermal magnetic breakers being utilized with no problems.

 

[Jraef]

it depends also on the "circuit breaker" that is being used. For example of the design is using what are now called "Motor Protection Circuit Breakers (MPCB), those are different from the older "Motor Circuit Protector" (MPC) breakers, as well as different from traditional "Circuit Breakers" typically referencing Thermal-Magnetic" or "I²t" circuit breakers.

Motor Protection Circuit Breakers are thermal-magnetic devices, but are STRICTLY (in North America) relegated for use as motor protection devices, because the thermal trips are adjustable. So they act as the motor Thermal Overload protection device, i.e. they REPLACE the OL relay in a combination starter. So a complete combination motor starter can be just an MPCB + a Contactor. Since they replace the OL relay, you size them exactly the same, AT the motor FLA.

Side note:
The "125%" factor is often ALREADY factored into the pick-up settings of the OL protection element, be it the thermal device in an MPCB or an adjustable OL relay, meaning you do NOT necessarily ADD that to the FLA. RTFM though.

 

[Sarray]

Per NEC you can go up to 175% for time delay fuses or 250% for thermal magnetic breakers. If those values won't allow motor to start you can increase those settings.

Most of the time I am seeing somewhere between 200 and 250% for thermal magnetic breakers being utilized with no problems.

Noted & Thank you,

In my most recent experience with the inspector who was referring to Canadian Code, I had to remove all 44 supplementary protectors and replace with Circuit Breakers to 125% of the Load. In addition add over current protectors to further dial down the amperage


This time around I am advising the manufacturer to only use Circuit Breakers with the amperage rating of 125% of the load but since you mentioned there is a chance motors may not start...not too sure if should increase the rating but I guess there is no way to find out until its live

 

[Sarray]

it depends also on the "circuit breaker" that is being used. For example of the design is using what are now called "Motor Protection Circuit Breakers (MPCB), those are different from the older "Motor Circuit Protector" (MPC) breakers, as well as different from traditional "Circuit Breakers" typically referencing Thermal-Magnetic" or "I²t" circuit breakers.

Motor Protection Circuit Breakers are thermal-magnetic devices, but are STRICTLY (in North America) relegated for use as motor protection devices, because the thermal trips are adjustable. So they act as the motor Thermal Overload protection device, i.e. they REPLACE the OL relay in a combination starter. So a complete combination motor starter can be just an MPCB + a Contactor. Since they replace the OL relay, you size them exactly the same, AT the motor FLA.



Side note:
The "125%" factor is often ALREADY factored into the pick-up settings of the OL protection element, be it the thermal device in an MPCB or an adjustable OL relay, meaning you do NOT necessarily ADD that to the FLA. RTFM though.

Currently the manufacturer is using a traditional circuit breaker but i'll have a look at the MPCB's as it seems to be a better option but most probably not the cost effective option I presume...

 

[Sarray]

it depends also on the "circuit breaker" that is being used. For example of the design is using what are now called "Motor Protection Circuit Breakers (MPCB), those are different from the older "Motor Circuit Protector" (MPC) breakers, as well as different from traditional "Circuit Breakers" typically referencing Thermal-Magnetic" or "I²t" circuit breakers.

Motor Protection Circuit Breakers are thermal-magnetic devices, but are STRICTLY (in North America) relegated for use as motor protection devices, because the thermal trips are adjustable. So they act as the motor Thermal Overload protection device, i.e. they REPLACE the OL relay in a combination starter. So a complete combination motor starter can be just an MPCB + a Contactor. Since they replace the OL relay, you size them exactly the same, AT the motor FLA.

Side note:
The "125%" factor is often ALREADY factored into the pick-up settings of the OL protection element, be it the thermal device in an MPCB or an adjustable OL relay, meaning you do NOT necessarily ADD that to the FLA. RTFM though.

if I understand this correctly

I can either use a Motor Protection Circuit Breaker and a contactor and adjust the trip setting to the motor FLA

Or

Use a regular Circuit breaker in conjunction with an adjustable overload relay and dial down the current to the exact motor FLA settings

Which brings me to this...concern. The specs that I am being provided by the manufacturer who is building is machine are following:


Power: 3.2KW
Voltage 3 phase: 380V
Current: 6.4A

which doesn't make sense unless this following is the case:
Input power = 3.2KW
Input Voltage = 380V
I = 6.4A

Power Factor = 0.76
I = 3200/(1.73*380*0.76) = 6.4A

Since the FLA is not provided on the motor itself can I set up the Overload Protection settings to 125% of 6.4A i.e 8 Amps ?
&
use a fuse for Overcurrent protection to 175% of 6.4 i.e 11A

Thank you

 

[Jraef]

You have to understand that "kW" over there is used the same way we use "HP", in that there is a MECHANICAL kW rating, separate from the ELECTRICAL kW rating. Electrical kW is referred to as "absorbed power" and is not shown on a motor nameplate.

That value is the mechanical kW, the shaft power; Torque (in Newton-meters) x RPM / 9550.
Absorbed power (also expressed in kW) is A x V x PF x Eff x 1.732 (for 3 phase), you would need all of those values to be accurate.

Because the acronym "FLA" is based on an English phrase, many IEC motor mfrs don't use that, they just say "Amps" or "A" or "I". But it means the same thing, so if your motor says "I = 6.4A", that's 6.4 FLA to us.

Also, don't make the common mistake of automatically setting your OLs at 125%. First off, IEC motors are NEVER rated for any Service Factor, the concept is foreign to them. Or looked at it from our perspective, all IEC motors are 1.0SF. Secondly, you must read the instructions of your specific OL relay for the correct settings. MANY of them ALREADY factor in the pick-up point as a percentage of the dial setting. So if they ALREADY use 125% for example, as some do, and then YOU add another 25%, your OL is now not going to take action until after the current is above 156% (125% of 125%), the motor burns up, and people wonder why the OL relay didn't protect it. MOST modern OL relays will tell you to set the dial at 100% of the motor FLA. So break yourself of that bad habit; it's been keeping the motor business in the black for decades, but at the expense of a lot of user budgets.

 

[Sarray]

You have to understand that "kW" over there is used the same way we use "HP", in that there is a MECHANICAL kW rating, separate from the ELECTRICAL kW rating. Electrical kW is referred to as "absorbed power" and is not shown on a motor nameplate.

That value is the mechanical kW, the shaft power; Torque (in Newton-meters) x RPM / 9550.
Absorbed power (also expressed in kW) is A x V x PF x Eff x 1.732 (for 3 phase), you would need all of those values to be accurate.

Because the acronym "FLA" is based on an English phrase, many IEC motor mfrs don't use that, they just say "Amps" or "A" or "I". But it means the same thing, so if your motor says "I = 6.4A", that's 6.4 FLA to us.

Also, don't make the common mistake of automatically setting your OLs at 125%. First off, IEC motors are NEVER rated for any Service Factor, the concept is foreign to them. Or looked at it from our perspective, all IEC motors are 1.0SF. Secondly, you must read the instructions of your specific OL relay for the correct settings. MANY of them ALREADY factor in the pick-up point as a percentage of the dial setting. So if they ALREADY use 125% for example, as some do, and then YOU add another 25%, your OL is now not going to take action until after the current is above 156% (125% of 125%), the motor burns up, and people wonder why the OL relay didn't protect it. MOST modern OL relays will tell you to set the dial at 100% of the motor FLA. So break yourself of that bad habit; it's been keeping the motor business in the black for decades, but at the expense of a lot of user budgets.

Thank you so much!
As you mentioned, without knowing the power factor and efficiency I can't provide the protection circuit specs therefore I have based it on the motor FLA ratings.