HVAC Name Plate MCA

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FrancisDoody

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Durham, CT
I was installing a Tropical T-55 pool heater. A typical heat pump that has an MCA of 19.7 amps. The wire has to be able to carry the calculated load or the MCA rating. ( In this case 12 gauge/20 amp MC cable) But what about the MOP? The name plate states a maximum fuse size of 30 Amps. Can we wire the heater with 12 gauge MC and load it on a 30 amp breaker and be code compliant. I do believe you still have to think about the load on the breaker. Is there a provision that will let you up the size of the OCPD and still let you use the 12 gauge MC for the calculated load. Overload protection should either be built into the motor or at the disconnect. Thanks for your help in trying to clear this issue up for me.
Fran
 
You could wire it with 14 AWG and still use a 30 amp breaker.

Regardless of the wire size you use I would not use less than a 30 amp breaker.
 
dickricker said:
I understand the 30 Amp breaker (MOC); but please explain 19.7 Amps through 14 AWG wire. Thank You!
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Because art. 440 allows the values in table 310.16 to be used & in that case 14 AWG is 20A, 12 AWG is 25A, & 10 AWG is 35A. You get to use different rules then branch circuit wiring.:D
 
If the nameplate states "Max Fuse" and there is no mention of a breaker or general over-current device, it may well mean the device was NRTL listed for fuse protection only and your overcurrent device will need to be a fuse.
 
dickricker said:
I understand the 30 Amp breaker (MOC); but please explain 19.7 Amps through 14 AWG wire. Thank You!
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The OP mentioned MC cable, one other thing to remember is that the 20 amp rating of #14 is for MC cable or THHN in a raceway. If you were to use NM cable then you're limited to 15 amps.
 
don_resqcapt19 said:
Yes, that was a change in the 2011 code. Prior to the 2011 code #14 was a 20 amp conductor in the 60? and 75? columns. Now it is 15 amps in the 60? column.
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Any ideas why it changed? I doubt the conductors themselves with the same load and other conditions suddenly started producing more heat than they used to.
 
kwired said:
Any ideas why it changed? I doubt the conductors themselves with the same load and other conditions suddenly started producing more heat than they used to.
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Maybe they were trying to allow in the table for the difference in performance between free air and the usual insulation filled stud cavity for outside walls?
 
The original Table 310.16 was derived from the Canadian electrical code with a few differences. The changes in the 2011 NEC eliminated those differences. Not a really good reason for a change IMO. :roll:
 
It never made sense to me that #12 and 14 had the same ampacities in both the 60? and 75? columns. The physics of current flow and heat produced would have never supported the same ampacities as the different temperatures.
 
Anyone atempted the calcualtion in 310.15(C) for #14 looks like you are allowed to.
And if you have where do you get the values of Yc and Rca?

Really code harmonization was the reason for changing the table?
If we are going with the Canadians lets revert back that 1969 code change to T250.122 and go back to their table 16.
 
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