Confused by hvac mca/mocp

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Red Forman

Red Forman

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Longview,Texas
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Electrical Designer
Hello all,
I am a bit confused when it comes to specifying for hvac equipment. Going by the spec sheets (for instance) a 3 ton mini-split shows 19.8 mca (compressor 18.25, outdoor fan .83, indoor fan .37 = 19.45) with a 20a mocp. How does this work out. It is my understanding that mocp is supposed to be 125% of the load. By the math this should have a 25a ocpd. Could someone please explain this. Thank you in advance.
 
E

Eddie702

Licensed Electrician
Location
Western Massachusetts
Occupation
Electrician
@Red Forman

In your example the mini split is a multi motor piece of equipment so the compressor at 18.25 amps is the largest load so 18.25 x 1.25=22.81 + the other loads= 22.81 +.83+.37=24.01

So something is amuk. With multi motor equipment you take 125% of the largest load plus the other loads that gives you your wire size MCA
 
C

Carultch

Senior Member
Location
Massachusetts
Red Forman said:
Hello all,
I am a bit confused when it comes to specifying for hvac equipment. Going by the spec sheets (for instance) a 3 ton mini-split shows 19.8 mca (compressor 18.25, outdoor fan .83, indoor fan .37 = 19.45) with a 20a mocp. How does this work out. It is my understanding that mocp is supposed to be 125% of the load. By the math this should have a 25a ocpd. Could someone please explain this. Thank you in advance.
Click to expand...

Motors change everything from your general rules that govern circuit and breaker sizing. It is often that the OCPD may be higher than you intuitively expect, because we don't want it to nuisance trip on inrush current. The MCA is based on the amps it will draw in the long term, that will impact the heating of the conductor. It is common for there to be an overload protection built in to the unit for protecting against marginal overloads, while the external OCPD's primary purpose is to protect against fault condition overcurrent.

HVAC equipment in particular, has the onus on the manufacturer to account for their own nuances that govern sizing, and provide you a straight forward specification on the nameplate for the circuit. You don't need to do any calculations from it, just provide a circuit from an OCPD that is consistent with the limits on the OCPD rating, and with a circuit amps (both terminal and derated conductor amps) that meets or exceeds the MCA value.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Eddie702 said:
@Red Forman

In your example the mini split is a multi motor piece of equipment so the compressor at 18.25 amps is the largest load so 18.25 x 1.25=22.81 + the other loads= 22.81 +.83+.37=24.01

So something is amuk. With multi motor equipment you take 125% of the largest load plus the other loads that gives you your wire size MCA
Click to expand...
I agree something is not right. Essentially if you take the largest load (the compressor, the largest compressor, or one of the compressors if there are multiple and the same), multiply that by 1.25, and add the other loads you should get the MCA. Sometimes they throw in a few amps for any controls and electronics and do not list that on the data sheet.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
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Retired
So if the manufacturer knows that it's an inverter driven motor and that the next breaker size up from the MCA will hold, they are free to set the MOCP on the nameplate at the next breaker size above MCA?

Cheers, Wayne
 
Dennis Alwon

Dennis Alwon

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Chapel Hill, NC
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I really don't like the idea of the mfg changing the rules from what the nec states. I have seen hot tubs require a #8 wire installed back to the main panel. They apparently have rules to protect their butts.
 
david luchini

david luchini

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Connecticut
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Engineer
Dennis Alwon said:
Not if the manufacturer nameplate states 20 as the mocp.
Click to expand...
Why would the manufacturer put a lower mocp than Code allow? They want the breaker to trip so their unit doesn't run?

Also, why would the manufacturer put an MCA lower than what Code requires, as in the cut sheet that Wayne posted?
I'm wondering if this unit isn't meant for installation in NEC jurisdictions.
 
Dennis Alwon

Dennis Alwon

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Location
Chapel Hill, NC
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Retired Electrical Contractor
As I stated earlier I have seen many units that used to have a max protection at 35 or 40 amps that are now limited to 25 or 30 amps.

Years ago you could bet the max overcurrent protective device was 175% of the fla. Now it just isn't true. I assumed the units are being built better....
 
david luchini

david luchini

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Connecticut
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Dennis Alwon said:
As I stated earlier I have seen many units that used to have a max protection at 35 or 40 amps that are now limited to 25 or 30 amps.

Years ago you could bet the max overcurrent protective device was 175% of the fla. Now it just isn't true. I assumed the units are being built better....
Click to expand...
Sure, but they can't put an MCA lower than what is Code required. Something is amiss with this unit.
 
Dennis Alwon

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
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Retired Electrical Contractor
david luchini said:
Sure, but they can't put an MCA lower than what is Code required. Something is amiss with this unit.
Click to expand...


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
 
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