AC condenser Name Plate Rating Calculations

[socalclem]

Sorry if this question has already been asked :

Im having trouble getting an absolute answer about name plate ratings on motors, particularly on AC Condensers for residential dwelling units. Copper prices are so high these days, i am trying to save some money by calculating the smallest branch-circuit feeder needed to feed my AC condensers for track homes.

For Example today i checked out a unit set yesterday and the name plate read:

1/6hp 220v 60hz

Minimum supply circuit ampacity : 25A
Minimum Fuse Size : 30A
Maximum Fuse Size : 40A

I would like to figure out how these numbers came to be?

I check out table 430.148 for 1/6 HP and saw that the Full-Load Current in Amperes is 2.2

430.22 led me to 125% for a single motor Full-Load Current.

2.2x1.25= 2.75 Amps.

Now Where do i go?

How do i get the name plate values the manufacturer listed?

If the Nameplate is correct, assuming the unit is rated @ 75C, are you saying i could use a 12-2 NMC per (310.16 & 240.4(d)) and install a 30,35, or 40 Amp breaker?

 

[augie47]

I believe you will find the 1/6 hp is the fan motor only. There should also be a compressor listing, however, your primary concern will be the min ckt amps and max ocvercurrent. Your branch circuit must be rated for the MCA as listed by the nameplate. In your case, 25 amps which at 75o (unless the unit calls for a lower temp termination) would require a #12.

From there it gets a little stickey. The OCP could be 30, 35 or 40, but if the nameplate says only "fuse" and does not mention HACR breaker, then there must be a fuse for OCP (as opposed to a breaker).

As far as 12/2NM, you might find some interesting reading in the NEC Forum on "NM in Flex" for that portion of the circuit that is in FNMC.

 

[infinity]

If the Nameplate is correct, assuming the unit is rated @ 75C, are you saying i could use a 12-2 NMC per (310.16 & 240.4(d)) and install a 30,35, or 40 Amp breaker?

This is correct. When sizing conductors and OCPD devices for AC units you must use the nameplate ratings. The MCA is for sizing the conductors. A #12 Cu @ 60 degrees C is good for 25 amps. We must use the 60 degree ampacity since 334.80 requires type NM to be sized according to the 60 degree C rating.

 

[briselec]

The nameplate ratings are the maximum the unit could possibly draw. HVAC guys have told me that with a lot of units they rarely if ever draw anywhere near that much but as it's possible, you have to allow for it.

 

[infinity]

The nameplate ratings are the maximum the unit could possibly draw. HVAC guys have told me that with a lot of units they rarely if ever draw anywhere near that much but as it's possible, you have to allow for it.

Where did the AC guy get this information? The MCA is a calculation of the compressor motors' (FLA)(125%)+ the evaporator fan motor. This is like a typical calculation for a circuit with two or more motors. And as it's name implies it is a minimum. The minimum or maximum overcurrent protection is determined by the maufacturer so that the unit will start and that it's components will have ground fault and short circuit protection.

 

[briselec]

Where did the AC guy get this information?

Ok that was a dumb thing to say. I was thinking only of the installations we've being doing lately where the units rarely run at full load.
I'll shut up now

 

[tlaidman]

I believe that all compressor motors for AC are hermeticly sealed motors and cooled by the refrigerant therefore they cannot be calculated by normal HP calculations. You must use the nameplate rating and min ckt ampacity.

 

[tincan44]

When sizing conductors and OCPD devices for AC units you must use the nameplate ratings. The MCA is for sizing the conductors. A #12 Cu @ 60 degrees C is good for 25 amps. We must use the 60 degree ampacity since 334.80 requires type NM to be sized according to the 60 degree C rating.[/quote]

240.4D clearly says 12 AWG cannot be fused over 20A. I think you must wire with 10 AWG and then breaker to the MCA.

 

[suemarkp]

240.4D clearly says 12 AWG cannot be fused over 20A. I think you must wire with 10 AWG and then breaker to the MCA.

But 240.4(D) also says unless permitted otherwise in 240.4(G). And 240.4(G) says article 440 parts III and VI are exempted which should cover the mentioned air conditioner. I think I'd go with Infinity's answer.

 

[infinity]

240.4D clearly says 12 AWG cannot be fused over 20A. I think you must wire with 10 AWG and then breaker to the MCA

I agree with Mark. 240.4(D) does not apply to AC units.

 

[tincan44]

I must not be reading it right. I have just read 240.4G, 440 III & VI and I cannot seem to confirm your statement. Could you give the exact code article I need to get thru my thick head?

 

[tincan44]

Re: AC condenser Name Plate Rating Calculations

Minimum supply circuit ampacity : 25A
Minimum Fuse Size : 30A
Maximum Fuse Size : 40A

Wire the minimum supply circuit ampacity, fuse the minimum to maximum ampacity even tho 310.16 disagrees with 25 amps for 12 AWG?
I do want to get this right.

 

[suemarkp]

I must not be reading it right. I have just read 240.4G, 440 III & VI and I cannot seem to confirm your statement. Could you give the exact code article I need to get thru my thick head?

Article 440 is so confusing. I've only seen HVAC units with MCA specified and not "branch circuit selection current". With all the different methods of calculating the ampacity in 440, your head can spin. I basically punted and went back to the title of 440 section III. It says "Branch-Circuit Short-circuit and Ground-Fault Protection". You're wiring an HVAC branch circuit aren't you? While you may not be calculating the breaker at 175% of the full load amp total, the manufacturer has done this for you with the max breaker label. So you can put a larger than normal breaker on a wire of the specified MCA.

Or is your issue more fudamental -- you don't see how 440 III and VI are exempted because you're looking for some more specific language?

 

[tincan44]

Minimum Fuse Size : 30A
Maximum Fuse Size : 40A

What I am questioning is the selection of 12 AWG conductors for this AC unit when it was stated that the minimum fuse size is 30 amps. What I have always thought is you can wire for the minimum OCPD and select a breaker between that and the maximum OCPD inclusive.

 

[infinity]

Minimum Fuse Size : 30A
Maximum Fuse Size : 40A

What I am questioning is the selection of 12 AWG conductors for this AC unit when it was stated that the minimum fuse size is 30 amps. What I have always thought is you can wire for the minimum OCPD and select a breaker between that and the maximum OCPD inclusive.

The conductors are sized according to the minimum circuit ampacity or MCA on the nameplate. Forget about the min or max fuse or CB size. These values do not apply to the conductor size, only the MCA does. Now go to 310.16 and find a conductor that is greater than or equal to the MCA. In this case it's 25 amps. A #12 THHN is good for 25 amps. Next to the values in 310.16 for #14,12,and 10 conductors there is an asterisk that directs you to the bottom of the table. This references 240.4(D) which says this:

(D) Small Conductors. Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

As you can see this says Unless specifically permitted in 240.4(E) or 240.4(G), since this unit falls under 240.4(G) the restrictions of 240.4(D) do not apply.

 

[tincan44]

Mark is certainly correct, "With all the different methods of calculating the ampacity in 440, your head can spin."
Thank you Trevor and Mark for your patience on this issue.
240.4G exempts 240.4D and takes you to 440 III & IV and also to 440.6 and a lot of other articles. The AWG in 310.16 is the minimum size to be accepted for MCA. :idea:

 

[hmspe]

The reason that the breaker or fuse rating for an AC unit is allowed to be higher than the rating of the conductor is that the breaker or fuse is essentially providing short circuit protection only. Overload protection is provided in the motors and/or in the AC circuiting.

Out were I live the reduced conductor size is only allowed between the AC unit and the first fuse or breaker. If you use a fused switch at the unit, the conductors between the fused switch and the panelboard have to be rated based on the ampacity of the breaker in the panel, just like any other circuit.

FWIW, my office standard is to size the conductors based on the rating of the breaker in the panel in all cases. Just because you can use smaller conductors doesn't mean you have to, and out here it's easily 140 degrees F on the roof in the summer, so temperature derating comes into play. I also don't know whether the disconnect at the unit will be fused or not (even though I generally call out fuses, especially on commercial). As an end user I don't want anything to be "code minimums". As an engineer I don't want problems in the field. If I was a contractor the economics might be different, but the cost to me for one re-work would probably be a lot higher that upsizing the wire up front.

Martin

 

[iwire]

Martin I agree with the points you bring up.

I do not agree with the enforcment in your area.

Out were I live the reduced conductor size is only allowed between the AC unit and the first fuse or breaker. If you use a fused switch at the unit, the conductors between the fused switch and the panelboard have to be rated based on the ampacity of the breaker in the panel, just like any other circuit.

The branch circuit starts at the panel not the fused switch. Therefore the allowances of 440 also start at the panel

The fused switch is not 'the final overcurrent device protecting the circuit and the outlet'. It is just an overcurrent device protecting the equipment.

Don R. of this forum made this clear to me as I was not sure where I could start with reduced conductors sizes if I wanted to.

I do agree the fact that it is allowed does not automatically mean we should take advantage of it.

 

[Pierre C Belarge]

Martin I agree with the points you bring up.

The branch circuit starts at the panel not the fused switch. Therefore the allowances of 440 also start at the panel

The fused switch is not 'the final overcurrent device protecting the circuit and the outlet'. It is just an overcurrent device protecting the equipment.

Don R. of this forum made this clear to me as I was not sure where I could start with reduced conductors sizes if I wanted to.

Bob
how did you and Don come to the conclusion that the fuse in the disconnect is there to protect the equipment and not also the conductors? (what about the short circuit ground fault protection of the conductors)

I know you know the definition of a feeder, so I won't post it, but I need you to show me how this fused disconnect does not conform to the definitions of feeder/branch circuit.

 

[iwire]

Bob
how did you and Don come to the conclusion that the fuse in the disconnect is there to protect the equipment and not also the conductors?

I will not speak for Don but I ask you this.

Does the NEC require an OCPD at the unit for protection of the conductors?

In other words is it often code compliant to use a non-fused disconnecting means?

The only reason we might be required to install a fused disconect at the unit is because the unit (the equipment) requires fuses and the panel supplying the circuit is a breaker panel.

That being the case that fused switch is in place to protect the equipment...not the conductors or outlet. This means that the final OCPD protecting the branch circuit is at the panel.