Air Conditioning Breaker settings

[Humble2000]

Dear Friend ,
I have a 250AMp breaker which will go before a 187AMp Air conditioning unit, The breaker has an instantanious, long time delay and contionious setting. What settings do I have to set theses.
Obviously I have no idea in protection. any comments is apprecited.

Thank you

 

[rbulsara]

You need a coordination study, and at least compare it to its upstream protective device and the load characteritic.

 

[l3city]

Circuit breaker: 175% Compressor Ampacity plus the ampacity of any other motor(s) load in the unit. per NEC 440.22(B)(1)

Disconnect per NEC 440.12(B), 115% Compressor ampacity, plus any other motor(s) load in the unit.

 

[Senselessticker]

Ask the A/C vender for damage curve info.

 

[Humble2000]

The issue is that the cable is already ran. 250MCM which based on CEC is 265 AMps.
The breaker can not be bigger than cable size apmrage?
can I use 400AMP breaker?

 

[resqcapt19]

The breaker can not be bigger than cable size apmrage?

It can be for air conditioning equipment. You are permitted by the NEC to size the conductors to the minimum circuit ampacity as shown on the equipment nameplate and the breaker to the maximum OCPD size shown on the same nameplate.
Don

 

[Humble2000]

Quote:
"to the maximum OCPD size shown on the same nameplate."
This means that if the unit FLA is 200 then I can only go with 200A breaker?

 

[l3city]

Ok, what is CEC, a code?? 1st time I heard of this. Anyways, NEC says (90 Deg C) 290Amps, or 255 Amps, at 75 Deg C (Table 310.16). But again, anyways. You are right about the breaker needs to protect the conductor, but the whole issue here is protecting the unit also.

You said: "I have a 250AMp breaker which will go before a 187AMp Air conditioning unit"
Is this 187A, total load, usually units this big have some motors (fans) and their corresponding compressor.
Do you have motor data/specifications for this A/C unit. As I said previously (see above), using NEC 440 will help you determine your CB and disconnect sizing.

Your wire shall have ampacity at least: 125% highest motor amps, plus any other motor(s) load in the unit. If the wire is already run, and your calculation is under the existing wire ampacity, then you are safe.

I think using a 400A CB, will be waayyy high (and dangerous!!)
Hope this helped, regards

 

[wareagle]

Quote by H2000 "The issue is that the cable is already ran. 250MCM which based on CEC is 265 AMps.
The breaker can not be bigger than cable size apmrage?
can I use 400AMP breaker?"

The code allows you to goto 175% of the nameplate rating FLA. If the rating is 187 amps x 1.75 = 327 amps.
You can use a 300 or a 350 amp breaker.

Quote:
"to the maximum OCPD size shown on the same nameplate."
This means that if the unit FLA is 200 then I can only go with 200A breaker?"
No. As stated above you can goto 200 x 1.75 = 350 amps.
In this case the ampacity of the conductor does not have to
agree with the breaker rating.

 

[Senselessticker]

It's important to gain a general understand what each term means:

The continuous setting is the % of the plug rating at which you wish to trip. (100% continuous for a 250A breaker -> means you set the breaker to trip at 250A)

The instantanious setting is used to open the breaker with virtually no delay (like in a fault condition). This setting is ussually on the order of kAmps.

The long time delay is the amount of time (usually in seconds) that the breaker will wait before triping due to current greater than continuous setting (like when the motor is starting). Note that the long time delay/pickup may effect the short time delay/pickup of a OCPD upstream(a coordination study as rbulsara suggested is needed).

The settings you are trying to determine should be choosen by someone with experience doing so and who is familiar with the existing distribution system you are dealing with, especially if its hermetic.

 

[resqcapt19]

Humble2000

"to the maximum OCPD size shown on the same nameplate."
This means that if the unit FLA is 200 then I can only go with 200A breaker?

If it only shows the FLA, then you have to do the calculations that the other posters have explained. Most, it not all of the units that I have worked with have additional information on the nameplate. That would be the MCA (minimum circuit amps) and Maximum Fuse/Breaker Size. If the nameplate has this information you use it directly. The wire is sized no less than the MCA and the breaker no larger than the maximum fuse/breaker size.
Don

 

[Senselessticker]

Humble2000,

I agree with resqcapt19 in that the nameplate should include some more information for sizing the breaker and selecting the continuous setting.

However, the instanateous and long time settings are more subjective to the distribution system you are dealing with.