HVAC feeder size

[Strathead]

I have one chiller on a feeder marked minimum circuit ampacity 360.5 and MOCP of 500. Are these assumptions/calculations correct?

• Disconnect size, 600A disconnect is required because 115% of 360.5 is 415A over the 400A frame size
• wire ampacity is 125% time 360.5 or 450.6 amps. I don't see the standard "next higher breaker..." clause in 440.6 (A) so I am assuming that you must use a conductor that is over 440.6 amps. Is this correct? So you couldn't use one 600 CU THWN at 75 degrees, correct?

 

[jumper]

You do not need to add 125% to the MCA, it is built in.

MOCP is 500A.

 

[Strathead]

You do not need to add 125% to the MCA, it is built in.

MOCP is 500A.

That is not the way I read 440.32. (past tense read) If what you are saying is correct then "branch circuit selection current" in the NEC and MCA on the motor are not the same thing. I assumed they are the same thing and 125% applies. To be safe I will continue to assume so until I am convinced otherwise. Which is not difficult with all of you experts here.

 

[ron]

I have one chiller on a feeder marked minimum circuit ampacity 360.5 and MOCP of 500. Are these assumptions/calculations correct?

• Disconnect size, 600A disconnect is required because 115% of 360.5 is 415A over the 400A frame size
• wire ampacity is 125% time 360.5 or 450.6 amps. I don't see the standard "next higher breaker..." clause in 440.6 (A) so I am assuming that you must use a conductor that is over 440.6 amps. Is this correct? So you couldn't use one 600 CU THWN at 75 degrees, correct?

What is the size of the upstream overcurrent protection serving this MCA of 360.5A? I hope it is not >500A. I use the upstream overcurrent protection to then size my wire.

 

[jumper]

That is not the way I read 440.32. (past tense read) If what you are saying is correct then "branch circuit selection current" in the NEC and MCA on the motor are not the same thing. I assumed they are the same thing and 125% applies. To be safe I will continue to assume so until I am convinced otherwise. Which is not difficult with all of you experts here.

The MCA is precalculated by the mfgr. and stamped on the name plate for the unit, it includes the compressor, fan, etc and has the 25% of largest motor .

You don't have to figure out anything, code requires the mfgr. to do it.

 

[david luchini]

That is not the way I read 440.32. (past tense read) If what you are saying is correct then "branch circuit selection current" in the NEC and MCA on the motor are not the same thing. I assumed they are the same thing and 125% applies. To be safe I will continue to assume so until I am convinced otherwise. Which is not difficult with all of you experts here.

"Branch circuit selection current" and MCA are NOT the same thing. See the definition in 440.2.

See also 440.32. "Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than (that is to say, MCA) 125% of either the motor compressor rated load current or branch-circuit selection current.

In other words, MCA = 1.25*RLA or 1.25*BCSC, whichever is greater. MCA does not equal BCSC.

 

[ActionDave]

That is not the way I read 440.32. (past tense read) If what you are saying is correct then "branch circuit selection current" in the NEC and MCA on the motor are not the same thing. I assumed they are the same thing and 125% applies. To be safe I will continue to assume so until I am convinced otherwise. Which is not difficult with all of you experts here.

You can do the math yourself using the individual ratings listed on the unit and you will come up with the MCA that the manufacture gives you.

 

[Strathead]

Thank you Jumper and Dave. I accept your interpretation, but a glitch in HVAC calculations that I see. If I am doing a calculation on a feeder with multiple loads, I have to get information that usually isn't available on the cut sheets, because you would need the rated current for the additional motors beyond the first one, or you would basically be using 125% of each and every motor.

 

[Strathead]

What is the size of the upstream overcurrent protection serving this MCA of 360.5A? I hope it is not >500A. I use the upstream overcurrent protection to then size my wire.

500A it is. You don't need to size your wire to the upstream overcurrent protection. That is the point of the code on motor and HVAC wire sizing.

 

[jumper]

Thank you Jumper and Dave. I accept your interpretation, but a glitch in HVAC calculations that I see. If I am doing a calculation on a feeder with multiple loads, I have to get information that usually isn't available on the cut sheets, because you would need the rated current for the additional motors beyond the first one, or you would basically be using 125% of each and every motor.

Are you asking how to size a feeder that serves multiple HVAC units(which have several loads per unit) or just how to size a feeder for multiple motors?

 

[Strathead]

Are you asking how to size a feeder that serves multiple HVAC units(which have several loads per unit) or just how to size a feeder for multiple motors?

Neither, I am stating that given the information you put forward, you would not use the MCA to calculate the feeder size of more than one HVAC compressor on the same feeder, because that figure is already bumped up the 125%. You would need to get the compressor and fan information separately and that is not something I am used to seeing on any paperwork.

 

[Dennis Alwon]

Yes and the FLA is what you need for the calculation. It should be written on the unit

 

[kingpb]

I have one chiller on a feeder marked minimum circuit ampacity 360.5 and MOCP of 500. Are these assumptions/calculations correct?

• Disconnect size, 600A disconnect is required because 115% of 360.5 is 415A over the 400A frame size
• wire ampacity is 125% time 360.5 or 450.6 amps. I don't see the standard "next higher breaker..." clause in 440.6 (A) so I am assuming that you must use a conductor that is over 440.6 amps. Is this correct? So you couldn't use one 600 CU THWN at 75 degrees, correct?

MCA = 360.5; therefore size conductor to be capable of 451A.

MOCP =500A, use your 600A fused disconnect with 500A fuses.

 

[david luchini]

MCA = 360.5; therefore size conductor to be capable of 451A.

The MCA=360.5, so the conductor need only be sized to be capable of 360.5A.

 

[Strathead]

MCA = 360.5; therefore size conductor to be capable of 451A.

MOCP =500A, use your 600A fused disconnect with 500A fuses.

That is not what two of the moderators say.

 

[kwired]

That is not what two of the moderators say.

I agree with them. MCA already has 125% of largest compressor factored into it, there is no need to increase by 125% again. MCA of 360.5 means conductor ampacity (after any necessary adjustments) needs to be 360.5 or higher for the branch circuit to that particular unit. Feeder and service calculations (especially if you have multiple units) require knowing compressor RLA and additional FLA of accessory items - if you want to use smallest conductor that is permitted, if you use MCA of multiple units for feeder or service calculations you end up with larger conductor then what is actual minimum required.

 

[Strathead]

I agree with them. MCA already has 125% of largest compressor factored into it, there is no need to increase by 125% again. MCA of 360.5 means conductor ampacity (after any necessary adjustments) needs to be 360.5 or higher for the branch circuit to that particular unit. Feeder and service calculations (especially if you have multiple units) require knowing compressor RLA and additional FLA of accessory items - if you want to use smallest conductor that is permitted, if you use MCA of multiple units for feeder or service calculations you end up with larger conductor then what is actual minimum required.

Cool, this has been a very informative thread for something that doesn't come up that often as a Contractor. I'd bet that very few people doing multifamily dwelling unit calculations, take this in to account

 

[kingpb]

That is not what two of the moderators say.

Well then I must be wrong, cause we all know only the moderators provide correct answers ; and I should probably surrender my engineering seal because the minimum safety requirements of the NEC certainly provide the best design basis. hmy:

 

[iwire]

That is not what two of the moderators say.

Well then I must be wrong, cause we all know only the moderators provide correct answers ; and I should probably surrender my engineering seal because the minimum safety requirements of the NEC certainly provide the best design basis. hmy:

Maybe you should turn in your seal if you can't bother to explain why you feel differently.

 

[ActionDave]

Well then I must be wrong, cause we all know only the moderators provide correct answers ; and I should probably surrender my engineering seal because the minimum safety requirements of the NEC certainly provide the best design basis. hmy:

So much of the NEC is so over the top, wire sizing especially. Why go oversize with conductors?