inverse time breaker

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stev11

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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 :-

LOADFLACALCULATONSELECTED BREAKER
motor 130A2.5X30=75A70A
MOTOR240A100A100A
MOTOR335A2.5X35=87.5A80A
Main Breaker2.5x40+30+35=165A150A



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.
 
stev11 said:
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 :-

LOADFLACALCULATONSELECTED BREAKER
motor 130A2.5X30=75A70A
MOTOR240A100A100A
MOTOR335A2.5X35=87.5A80A
Main Breaker2.5x40+30+35=165A150A



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.
Click to expand...
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.
 
topgone said:
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.
Click to expand...

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

stev11 said:
and I need to understand the main concept''how to select breaker and when I need to use the next size down or up"???
Click to expand...

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).
 
david luchini said:
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).
Click to expand...

Oppsss. A bad day for me! Thanks for correcting, david.
 
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.
 
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.
 
fifty60 said:
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.
Click to expand...

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???
 
stev11 said:
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???
Click to expand...
Check the name plate. If nothing is specified about it, it is your choice.
 
Sahib said:
Check the name plate. If nothing is specified about it, it is your choice.
Click to expand...

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??
 
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%.
 
Sahib said:
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%.
Click to expand...

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??
 
stev11 said:
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??
Click to expand...

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
 
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.
 
stev11 said:
and where can I find these pdf's that explains the NEC code???
Click to expand...

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.
 
david luchini said:
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).
Click to expand...

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.
 
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