MCA vs MOCP

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dvcraven0522

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If a rooftop unit has a MCA of 48 amperes and a MOCP of 60 as listed on the nameplate. I sized the conductors based on the 48 amps and not the 60 MCOP. It is my understanding the MCA is already calculated for the 125% of the load. Is that correct?

I have a plan checker who wants me the size the conductors for the 60 amp CB.

Dan Craven
 
dvcraven0522 said:
If a rooftop unit has a MCA of 48 amperes and a MOCP of 60 as listed on the nameplate. I sized the conductors based on the 48 amps and not the 60 MCOP. It is my understanding the MCA is already calculated for the 125% of the load. Is that correct?

I have a plan checker who wants me the size the conductors for the 60 amp CB.

Dan Craven
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You are correct. The plan checker needs to go back to school.
 
There isn't really one simple sentence in the NEC that says ", MCA on nameplate shall include 25% of the largest motor", it's rather convoluted, but the name plate usually lists all the loads so you can do the math and show him that it includes the 25%.
 
electrofelon said:
There isn't really one simple sentence in the NEC that says ", MCA on nameplate shall include 25% of the largest motor", it's rather convoluted, but the name plate usually lists all the loads so you can do the math and show him that it includes the 25%.
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However the listing standard for the equipment requires that the MCA and MOCP on the nameplate be calculated exactly like what is required in the NEC.
 
The important point to remember is that the code makes a distinction between short-circuit and ground fault protection and overload protection. The MOCP is only responsible for the SC/GF protection and the motor-starter overloads are for overload. Article 240.4 is where the code says you have to protect the wires at their ampacity, but table 240.3 diverts you to 430, Parts III (overload) and Part IV, (SC/GF) before you get to 240.4.

I once worked with a mechanical contractor who refused to believe me because his Square D slide rule said that you have to use #2 wire with a 100A breaker.
 
JoeStillman said:
The important point to remember is that the code makes a distinction between short-circuit and ground fault protection and overload protection. The MOCP is only responsible for the SC/GF protection and the motor-starter overloads are for overload. Article 240.4 is where the code says you have to protect the wires at their ampacity, but table 240.3 diverts you to 430, Parts III (overload) and Part IV, (SC/GF) before you get to 240.4.

I once worked with a mechanical contractor who refused to believe me because his Square D slide rule said that you have to use #2 wire with a 100A breaker.
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My understanding of providing a safe electrical installation is to always be on the safe side of things. Your mechanical contractor cannot be at fault for using their rules.
On the other hand, HVAC manufacturers stamp their units the RLA - running load amperes! This figure is lower than the rated equipment current draw. RLA is derived thru test runs of the equipment until the protections trip.
They'll calculate the RLA by getting the maximum continuous amps MCA before overload trips and then divide the same by a fudge factor of 1.56 (25% for sizing requirement x 25% overload protection). Copeland sets their overload protection rule at 15%, resulting in them using a factor of 1.25 x 1.15 = 1.44.
It must be noted that the 25% fudge factor for conductor sizing is already included in the RLA figure that the manufacturers publish.
 
topgone said:
On the other hand, HVAC manufacturers stamp their units the RLA - running load amperes! This figure is lower than the rated equipment current draw. RLA is derived thru test runs of the equipment until the protections trip.
They'll calculate the RLA by getting the maximum continuous amps MCA before overload trips and then divide the same by a fudge factor of 1.56 (25% for sizing requirement x 25% overload protection). Copeland sets their overload protection rule at 15%, resulting in them using a factor of 1.25 x 1.15 = 1.44.
It must be noted that the 25% fudge factor for conductor sizing is already included in the RLA figure that the manufacturers publish.
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Do you have any documentation to back this up?
 
infinity said:
Do you have any documentation to back this up?
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I second that.

I don't know what they do but would seem more logical for a rating to first be dependent on however they determine mechanical rating of the compressor, then would have to presume they would design the motor portion to be able to deliver at least that mechanical rating at any voltage within intended voltage design ratings without overheating the windings. Chances are the actual output rating is a percentage of actual mechanical rating, this gives you more lifetime than if run right at the edge, plus you need to at least have it run through a certain warranty period as well or you will lose a lot on warranties.
 
topgone said:
My understanding of providing a safe electrical installation is to always be on the safe side of things. Your mechanical contractor cannot be at fault for using their rules.
On the other hand, HVAC manufacturers stamp their units the RLA - running load amperes! This figure is lower than the rated equipment current draw. RLA is derived thru test runs of the equipment until the protections trip.
They'll calculate the RLA by getting the maximum continuous amps MCA before overload trips and then divide the same by a fudge factor of 1.56 (25% for sizing requirement x 25% overload protection). Copeland sets their overload protection rule at 15%, resulting in them using a factor of 1.25 x 1.15 = 1.44.
It must be noted that the 25% fudge factor for conductor sizing is already included in the RLA figure that the manufacturers publish.
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Article you posted link to says not to confuse RLA to be running vs rated load amperes, it is rate load amperes;)

I believe the process you described is their method for telling you how you may be able to determine in the field what the RLA is on a unit that is not marked, and not how the manufacturer rates them.
 
In the NEC there are "motor loads" and then there is "everything that's not a motor"

With "motor loads" the circuit protection can generally exceed the wire size. With "everything else" it usually can't
 
Eddie702 said:
In the NEC there are "motor loads" and then there is "everything that's not a motor"

With "motor loads" the circuit protection can generally exceed the wire size. With "everything else" it usually can't
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But the motor load usually has overload protection as well, which inherently still protects the conductor at or even below it's ampacity. The fuse or breaker is normally just for short circuit and ground fault protection.
 
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