Confused by hvac mca/mocp

[david luchini]

David I have seen many units like this. They are all lower than what the nec allows.... I think the efficiency of the units has gotten so good that they don't think it needs a high overcurrent protective device.

Seriously take a look at the new units. Everyone I have seen has had a max just above the min cir ampacity

The efficiency of the units doesn't change the MCA requirements. The unit doesn't give and MCA value as per Code requirements.

If the compressor RLA is 18.25A, the unit MCA can't be 19.8A per NEC requirements.

 

[Dennis Alwon]

I see what you are saying. I just saw the total amps and not the breakdown. It doesn't make sense unless those number each had 125% added in... I have never seen that.

 

[david luchini]

I see what you are saying. I just saw the total amps and not the breakdown. It doesn't make sense unless those number each had 125% added in... I have never seen that.

That's why I was wondering whether its a unit meant for non-NEC applications.

 

[wwhitney]

Sure, but they can't put an MCA lower than what is Code required. Something is amiss with this unit.

Seems like they can, 440.4 is on equipment marking and 440.4(B) just says "The branch-circuit short-circuit and ground-fault protective device rating shall not exceed the value calculated by using Part III." So the manufacturer can put a lower MCA on its nameplate if it wants.

As to the RLA, the same number (18.25) is listed for 208V and for 230V. I assume that's unusual?

I'm a bit surprised that 440 Part IV doesn't have any language addressing equipment with inverter driven motors. Maybe the manufacturer has decided do something reasonable in this regard that isn't supported by any NEC language.

Cheers, Wayne

 

[david luchini]

Seems like they can, 440.4 is on equipment marking and 440.4(B) just says "The branch-circuit short-circuit and ground-fault protective device rating shall not exceed the value calculated by using Part III." So the manufacturer can put a lower MCA on its nameplate if it wants.

You are conflating MOCP and MCA.

See the sentence before "The branch-circuit short-circuit and ground-fault protective device...."

 

[wwhitney]

No, just a typo, I meant to say MOCP but typed MCA. My point still stands that the manufacturer can label the nameplate with a lower MOCP than Section IV allows. [Not true for MCA, as you point out.]

Cheers, Wayne

 

[electrofelon]

No, just a typo, I meant to say MOCP but typed MCA. My point still stands that the manufacturer can label the nameplate with a lower MOCP than Section IV allows. [Not true for MCA, as you point out.]

Cheers, Wayne

Sure but does the code ever require a certain size OCPD? I Would say all the "large" size OCPD's allowed for motors/hvac are allowed but not required.

 

[wwhitney]

Sure but does the code ever require a certain size OCPD? I Would say all the "large" size OCPD's allowed for motors/hvac are allowed but not required.

Sure, but the manufacturer is choosing to disallow a breaker size that would otherwise be NEC compliant and to require a smaller one. And they are allowed to impose that stricter standard, per 440.4. That's all.

Cheers, Wayne

 

[electrofelon]

Sure, but the manufacturer is choosing to disallow a breaker size that would otherwise be NEC compliant and to require a smaller one. And they are allowed to impose that stricter standard, per 440.4. That's all.

Cheers, Wayne

Oh ok I see what you are saying.

 

[Jraef]

The NEC is for when YOU have to determine MCA and MOCP in the field from basic nameplate information. For an EQUIPMENT MANUFACTURER, they need to satisfy UL standards, in this case probably UL1995, which has its own set of rules for determining the motor amp loading based on what the equipment mfr expects in terms of performance, then that is used in their MCA and MOCP data. So once THEY provide you with the MCA and MOCP, you need only follow those values, you do not calculate them again.

 

[david luchini]

The NEC is for when YOU have to determine MCA and MOCP in the field from basic nameplate information. For an EQUIPMENT MANUFACTURER, they need to satisfy UL standards, in this case probably UL1995, which has its own set of rules for determining the motor amp loading based on what the equipment mfr expects in terms of performance, then that is used in their MCA and MOCP data. So once THEY provide you with the MCA and MOCP, you need only follow those values, you do not calculate them again.

The NEC says the equipment shall be marked with the minimum supply circuit conductor ampacity, as determined by Part IV of Article 440 of the NEC.

 

[topgone]

When sizing for cables and OCPD size, manufacturer cut sheets should be followed as compressors do not follow usual HP or kW rules.

 

[Fred B]

When sizing for cables and OCPD size, manufacturer cut sheets should be followed as compressors do not follow usual HP or kW rules.

? Hp of motor for a large commercial woodworking equipment would indicate a #8 wire yet cut sheet says a #10 is adequate, am I wrong to put the #8 on it? Interestingly the internal wiring is only a #12.

 

[retirede]

? Hp of motor for a large commercial woodworking equipment would indicate a #8 wire yet cut sheet says a #10 is adequate, am I wrong to put the #8 on it? Interestingly the internal wiring is only a #12.

Nothing wrong with up-sizing the wires.
“Adequate” does not mean “absolute”.

Internal wire size is irrelevant. Other factors in play here. It’s not unusual for internal wiring to safely utilize wire with a much higher temperature insulation than would be allowed for premesis wiring, for example.

 

[Red Forman]

Can you post a link to this spec sheet?

This isnt the unit I mentioned but this shows an mca of 19.8 with a mocp of 20

 

[Red Forman]

Just googling the numbers in the OP suggests it's this:

https://www.daikinac.com/content/assets/DOC/SubmittalDataSheets/Single-Split/FTX36NVJU-RX36NMVJU%20Submittal%20Sheet.pdf

I'm surprised that worked. : - )

Cheers, Wayne

Wow... I'm surprised that worked too. But yes that is the unit in question.

 

[LarryFine]

This isnt the unit I mentioned but this shows an mca of 19.8 with a mocp of 20

Maybe it's the MCA that is high, and not the MOCP that is low.

 

[Red Forman]

Thank you all for your input and quick responses.
So what I'm getting out of this is, Just to go by whats in the cut sheet as the manufacturer SHOULD have it right.
I must admit I'm a little worried about putting a 19.8a load on a 20a breaker.

 

[LarryFine]

I must admit I'm a little worried about putting a 19.8a load on a 20a breaker.

I'm suggesting that the load is nowhere near 19.8a, they just want wire sized for it.

 

[david luchini]

I'm suggesting that the load is nowhere near 19.8a, they just want wire sized for it.

But they list the Compressor Rated Load Amps as 18.25.