Refrigeration Compressor Min Wire Ampacity and MAX OCPD

fifty60

Senior Member
Location
USA
Is it really common to see the ampacity of the wire on refrigeration compressors not match the OCPD on the compressor branch circuit. I just can't get used to sizing the wire at 125% of the RLA, and then being able to size the OCPD up to 225%.

Do I have to raise the wire ampacity to match the final OCPD, or allowed to match a minimum 125% RLA with the 225% RLA OCPD?
 

iwire

Moderator
Staff member
Location
Massachusetts
Is it really common to see the ampacity of the wire on refrigeration compressors not match the OCPD on the compressor branch circuit. I just can't get used to sizing the wire at 125% of the RLA, and then being able to size the OCPD up to 225%.

Do I have to raise the wire ampacity to match the final OCPD, or allowed to match a minimum 125% RLA with the 225% RLA OCPD?
Forget the RLA

Look at the MCA Minimum circuit ampacity and the MOP Maximum overcurrent protection.

You can choose to size the wire for the minimum and the OCPD for the maximum. The units internal overloads protect the conductors from overloading.
 

ron

Senior Member
Look at the MCA Minimum circuit ampacity and the MOP Maximum overcurrent protection.

You can choose to size the wire for the minimum and the OCPD for the maximum. The units internal overloads protect the conductors from overloading.
Bob is right on regarding code minimums.

Almost always I size the wire to match the OCPD. It is way to hard to keep track of HVAC engineer changes transferred to Elec drawings to now size the wire different than the breaker. Almost always the equipment selected has a different MCA than what I was told during design, so it all works out in the end with some horse trading.
 

fifty60

Senior Member
Location
USA
I generally use NFPA 70 table 310.16 to size my wire (75 C column). For 10AWG, this says the ampacity is 35A. When I look at the manufacturers data sheets for the 10AWG MTW wire I am using, the ampacity is listed as 44A per NFPA 79.

Am I able to use the 44A as the ampacity of this 10AWG MTW that will be used on a condensing unit branch circuit (compressor and small condenser fan)?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I generally use NFPA 70 table 310.16 to size my wire (75 C column). For 10AWG, this says the ampacity is 35A. When I look at the manufacturers data sheets for the 10AWG MTW wire I am using, the ampacity is listed as 44A per NFPA 79.

Am I able to use the 44A as the ampacity of this 10AWG MTW that will be used on a condensing unit branch circuit (compressor and small condenser fan)?
Is the installation covered by the rules in NFPA 70 or NFPA 79? If the installation is under the rules in the NEC, you are restricted to the ampacities in the NEC.
 

fifty60

Senior Member
Location
USA
Interesting point...I don't think it is either actually....it is all internal wiring on "laboratory" equipment....it is close to NFPA 79...definitely not NFPA 70 inside the equipment...
 

fifty60

Senior Member
Location
USA
Does the much higher rating for NFPA 79 recognize that the MTW wiring will be used inside of a machine and not inside of a house like NFPA 70?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Keep in mind thermal dissipation in a wall is RADICALLY different then one inside equipment. What might only get warm inside equipment might melt inside a wall or ceiling cavity. For this reason always us NFPA70 for anything outside of immediate equipment.
 

fifty60

Senior Member
Location
USA
When determining the which column to use in the wire ampacity charts (310.15 NFPA 70 and 12.5.1 NFPA 79), do I only look at the insulation rating on the wiring itself? Or, am I looking at the temperature rating on the terminals the wiring is connecting to?

For example, I have 10AWG MTW. The MTW has an insulation rating of 90 C. Do I then go to the 90C column to get the ampacity, or is there more to selecting the temperature rating column than that?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
In the NEC you must also factor in the lowest terminal temperature rating. For example, 75*C terminals place you on the 75*C ampacity column.
 
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