inverse time breaker

[stev11]

hi

I have a unit with 3 motors and I need to select C.B for each motor then to add main breaker ,so I need to check the if the below selection is correct :-

LOAD	FLA	CALCULATON	SELECTED BREAKER
motor 1	30A	2.5X30=75A	70A
MOTOR2	40A	100A	100A
MOTOR3	35A	2.5X35=87.5A	80A
Main Breaker	2.5x40+30+35=165A	150A

and I need to understand the main concept''how to select breaker and when I need to use the next size down or up"???

thank alot.

 

[topgone]

hi

I have a unit with 3 motors and I need to select C.B for each motor then to add main breaker ,so I need to check the if the below selection is correct :-

LOAD	FLA	CALCULATON	SELECTED BREAKER
motor 1	30A	2.5X30=75A	70A
MOTOR2	40A	100A	100A
MOTOR3	35A	2.5X35=87.5A	80A
Main Breaker	2.5x40+30+35=165A	150A

and I need to understand the main concept''how to select breaker and when I need to use the next size down or up"???

thank alot.

Your main breaker should be the sum of a) the largest protective device (100A breaker) plus b) the sum of the full-load amps of the rest of the motors (30 + 35A = 75). So, in your case, it should be a 175A breaker. But if any of the motor protective device you have used trips on start, you are allowed to up your rating but not exceed 400% of its FLA.

 

[david luchini]

Your main breaker should be the sum of a) the largest protective device (100A breaker) plus b) the sum of the full-load amps of the rest of the motors (30 + 35A = 75). So, in your case, it should be a 175A breaker. But if any of the motor protective device you have used trips on start, you are allowed to up your rating but not exceed 400% of its FLA.

As 30 + 35 = 65, not 75, the 150A c/b would be correct.

and I need to understand the main concept''how to select breaker and when I need to use the next size down or up"???

You can round up for the branch breakers, per 430.52(C)(1) ex 1. So your branch c/b's can be 80, 90 & 100.

You have to round down for the feeder breaker per 430.61(A).

 

[topgone]

As 30 + 35 = 65, not 75, the 150A c/b would be correct.



You can round up for the branch breakers, per 430.52(C)(1) ex 1. So your branch c/b's can be 80, 90 & 100.

You have to round down for the feeder breaker per 430.61(A).

Oppsss. A bad day for me! Thanks for correcting, david.

 

[stev11]

david and topgone thank you for your support.

 

[stev11]

hi

how can select branch breaker and fuse for motor comp and heater???

thank you.

 

[fifty60]

You can use the above, but the feeder cannot be less than 125% of all of the continuous motor and non motor loads. The aforementioned method of largest breaker plus remaining FLA's gives you the maximum breaker size when using the minimum conductor size. The NEC dictates a minimum and a maximum. I believe you can size the feeder breaker as large as you want, however, as long as you size the conductors accordingly.

 

[fifty60]

For the heater 125% of the rated heater amps, then round up. For the motor, it depends on the OCPD you are using. You can start at 175% of the FLA found in the NEC tables, and can even go up to 300% if you are using class CC fuses.

 

[stev11]

You can use the above, but the feeder cannot be less than 125% of all of the continuous motor and non motor loads. The aforementioned method of largest breaker plus remaining FLA's gives you the maximum breaker size when using the minimum conductor size. The NEC dictates a minimum and a maximum. I believe you can size the feeder breaker as large as you want, however, as long as you size the conductors accordingly.

ok, according to 440.22.B, In case I have one comp then the protective device must not exceeding 175% of compressor current.
but what is the type of this protection device??? is it non time delay fuse,time delay fuse,inverse time...or what???

 

[Sahib]

ok, according to 440.22.B, In case I have one comp then the protective device must not exceeding 175% of compressor current.
but what is the type of this protection device??? is it non time delay fuse,time delay fuse,inverse time...or what???

Check the name plate. If nothing is specified about it, it is your choice.

 

[stev11]

Check the name plate. If nothing is specified about it, it is your choice.

ok, but do you mean that the 175% for time delay fuse as a protection device OR this value(175%) for all types of OCPD??
Because I think the 175% is not enough for non time fuse or instantaneous breaker??

 

[Sahib]

I have not gone through 440.22.B. (time too short.) But my guess about what it means from your quoting it is that you should set over load setting of compressor motor at not more than 175%. The over current protective device setting may go up to 400%.

 

[stev11]

I have not gone through 440.22.B. (time too short.) But my guess about what it means from your quoting it is that you should set over load setting of compressor motor at not more than 175%. The over current protective device setting may go up to 400%.

thank you , but I am asking bout single compressor and the selected OCPD

according to 440.22.B, In case I have one comp then the protective device must not exceeding 175% of compressor current.
but what is the type of this protection device??? is it non time delay fuse,time delay fuse,inverse time...or what???
Because I think the 175% is not enough for non time fuse or instantaneous breaker??

 

[Sahib]

thank you , but I am asking bout single compressor and the selected OCPD

according to 440.22.B, In case I have one comp then the protective device must not exceeding 175% of compressor current.
but what is the type of this protection device??? is it non time delay fuse,time delay fuse,inverse time...or what???
Because I think the 175% is not enough for non time fuse or instantaneous breaker??

Yes. You are correct. The article 440.22A refers to over current protection and not over load protection. If non-time delay fuse or instantaneous breaker trips at 175%, the setting may be increased up to 225% only. See
Part III in http://www.dantespeakheatingcooling.com/upload/Mike_Holt_NEC.pdf

 

[stev11]

Yes. You are correct. The article 440.22A refers to over current protection and not over load protection. If non-time delay fuse or instantaneous breaker trips at 175%, the setting may be increased up to 225% only. See
Part III in http://www.dantespeakheatingcooling.com/upload/Mike_Holt_NEC.pdf

thank you, also very good examples.

and where can I find these pdf's that explains the NEC code???

 

[fifty60]

You will need to bite the bullet and read from the very beginning. Start at the definitions and go all the way through 440, it will make things a lot clearer.

 

[Sahib]

and where can I find these pdf's that explains the NEC code???

If you search deep into Google, you may find as many pdf's as you like.

One more thing.

The maximum setting for overload relay i.e overload protection for air-conditioning compressor motor is not mentioned in 440. It is 175% per Electricians' bible.

 

[kingpb]

As 30 + 35 = 65, not 75, the 150A c/b would be correct.



You can round up for the branch breakers, per 430.52(C)(1) ex 1. So your branch c/b's can be 80, 90 & 100.

You have to round down for the feeder breaker per 430.61(A).

Round down only if conductors were sized for minimum requirements. If conductors are sized larger, than protection device is permitted to be based on the ampacity of conductors.