Confused by hvac mca/mocp

[david luchini]

It is used by the manufacturer to calculate MOCP and MCA as required by the applicable standard not the NEC.

See 440.4

 

[Mgraw]

See 440.4

I've seen it. I guess the nec needs to update their code section to comply with the current ashrea standards for manufacturers.

 

[david luchini]

I've seen it. I guess the nec needs to update their code section to comply with the current ashrea standards for manufacturers.

ASHRAE is irrelevant to the electrical circuiting and protection of the unit. The MCA is required to be per the NEC. The MOCP cannot be larger than allowed by the NEC.

 

[Mgraw]

ASHRAE is irrelevant to the electrical circuiting and protection of the unit. The MCA is required to be per the NEC. The MOCP cannot be larger than allowed by the NEC.

The MCA has not been calculated according to the NEC since 1976. That is when ashrea changed to RLA on compressors. RLA is a percentage of MCA so how do you use RLA to calculate MCA? MCA divided by either 1.56 or 1.44 (depending on compressor manufacturer) equals RLA.

 

[david luchini]

The MCA has not been calculated according to the NEC since 1976. That is when ashrea changed to RLA on compressors. RLA is a percentage of MCA so how do you use RLA to calculate MCA? MCA divided by either 1.56 or 1.44 (depending on compressor manufacturer) equals RLA.

See 440.33

 

[Mgraw]

See 440.33

Seen it, doesn't answer my question. And I should add inverter compressors don't come anywhere close to NEC calculations.

 

[david luchini]

Minimum circuit conductor ampacity equals the sum of the rla of the compressor(s) and the fla of the other motor(s) plus 25% of the largest motor.

The rla is not a specific ratio of the minimum circuit ampacity. It can't be if the mca involves other motors in addition to the compressors, or if it involves multiple compressors.

 

[Mgraw]

I should also add the NEC has zero authority over manufacturers

Minimum circuit conductor ampacity equals the sum of the rla of the compressor(s) and the fla of the other motor(s) plus 25% of the largest motor.

The rla is not a specific ratio of the minimum circuit ampacity. It can't be if the mca involves other motors in addition to the compressors, or if it involves multiple compressors.

My bad there are two mca's. Minimum Circuit Amps and Maximum Compressor Amps. If you use RLA to calculate Minimum Circuit Amps it may be close on ac volt compressors but it is almost never exact. It is not close on inverter compressors.

 

[david luchini]

I should also add the NEC has zero authority over manufacturers

My bad there are two mca's. Minimum Circuit Amps and Maximum Compressor Amps. If you use RLA to calculate Minimum Circuit Amps it may be close on ac volt compressors but it is almost never exact. It is not close on inverter compressors.

Its exact with either type of compressor.

 

[Mgraw]

Its exact with either type of compressor.

According to the NEC but not to the manufacturers. I haven't come across any in a long time.

The NEC cannot dictate anything to manufacturers. Ashrea has standards that manufacturers follow. Units are tested and listed to ashrea standards, not NEC code. Electrical connections to those units are governed by the local AHJ. If the NEC is the code they follow then electrical connections to the units should use the MCA and MOCP given by the manufacturer.

 

[david luchini]

According to the NEC but not to the manufacturers. I haven't come across any in a long time.

The NEC cannot dictate anything to manufacturers. Ashrea has standards that manufacturers follow. Units are tested and listed to ashrea standards, not NEC code. Electrical connections to those units are governed by the local AHJ. If the NEC is the code they follow then electrical connections to the units should use the MCA and MOCP given

The NEC does dictate to the manufacturers. The MCA and MOCP from the manufacturer cannot be less than (mca) nor greater than (mocp) the NEC requirements.

 

[Mgraw]

The NEC does dictate to the manufacturers. The MCA and MOCP from the manufacturer cannot be less than (mca) nor greater than (mocp) the NEC requirements.

That is false. The NEC can not dictate to anyone, they are nothing more than a think tank that tries to sell their ideas to AHJ's. Only AHJ's have any authority, the NEC does not.

 

[david luchini]

That is false. The NEC can not dictate to anyone, they are nothing more than a think tank that tries to sell their ideas to AHJ's. Only AHJ's have any authority, the NEC does not.

The is false. The NEC makes the dictates, specifically mandatory rules. The AHJ has the responsibility for enforcement of these rules.

 

[Mgraw]

The is false. The NEC makes the dictates, specifically mandatory rules. The AHJ has the responsibility for enforcement of these rules.

That is absolutely false. The NEC writes code. The AHJ can adopt that code, change that code, completely ignore that code, or adopt another code. The NEC is purely an idea or suggestion until an AHJ adopts it. There is no national electrical code in the USA. Ashrea is a national standard that is followed by all USA HVAC manufacturers.

 

[wwhitney]

The NEC doesn't dictate how equipment is made, it just dictates how it needs to be made if you want to install it in a jurisdiction that follows the NEC.

ASHREA (or UL, say) can't dictate what equipment is allowed to be installed, it can just dictate how the equipment needs to be made to comply with its standards.

Hopefully the two requirements are in accordance. Sometimes they are not, like what happened with some switch-type devices using the EGC as circuit conductor for up to 0.5 ma, as allowed by UL, until the NEC made some changes to increase the availability of a neutral conductor at switching locations. [If I recall correctly.]

Cheers, Wayne

 

[david luchini]

Ashrea is a national standard that is followed by all USA HVAC manufacturers.

This is false. ASHRAE is a society of engineers, not a national standard.

 

[electrofelon]

That is absolutely false. The NEC writes code. The AHJ can adopt that code, change that code, completely ignore that code, or adopt another code. The NEC is purely an idea or suggestion until an AHJ adopts it. There is no national electrical code in the USA. Ashrea is a national standard that is followed by all USA HVAC manufacturers.

True, but most jurisdictions adopt the NEC wholesale or make small modifications. The track record of jurisdictions adopting all/most of the NEC is such that pretty much whatever the NEC says the manufacturer will be forced to follow (if they want to sell their equipment there which they will). Do you know of any jurisdictions that add "HVAC equipment" to 90.2(B)?

 

[Mgraw]

This is false. ASHRAE is a society of engineers, not a national standard.

ASHREA is a society of engineers that develop national standards for manufacturers that all us HVAC manufacturers follow. Their standards are adopted by ANSI and many are used worldwide.

 

[Mgraw]

True, but most jurisdictions adopt the NEC wholesale or make small modifications. The track record of jurisdictions adopting all/most of the NEC is such that pretty much whatever the NEC says the manufacturer will be forced to follow (if they want to sell their equipment there which they will). Do you know of any jurisdictions that add "HVAC equipment" to 90.2(B)?

Since all US manufacturers us the same standards do you know any jurisdiction that is going to tell their people they can not have AC or heat because the equipment does not meet the NEC's antiquated code?

 

[electrofelon]

Since all US manufacturers us the same standards do you know any jurisdiction that is going to tell their people they can not have AC or heat because the equipment does not meet the NEC's antiquated code?

Yes pretty much all of them. Just look at all the dumb things they put in the NEC and how many jurisdictions just blindly adopt it.

In actuality, that wouldn't happen because the manufacturers would comply.