Article 440 - Air Conditioning Equipment

[payneeco]

Question 1 of Article 440 in the Electrical Exam Prep Book I'm using to study is below. The correct answer indicated on the exam answer key is C) 10 AWG. Intuitively, this is the answer I would expect and agree with the answer key. However, I'm having trouble understanding Table 310.16. It shows 12 AWG as I'm reading it (I'm looking at column that contains THWN-2 for copper and 30 amps requires 12 AWG). The column for TW, UF shows 10 AWG for 30 amps. Can someone help me understand what I'm looking at incorrectly?

An air-conditioning multi motor equipment has a nameplate that indicates a minimum circuit ampacity of 30A and a maximum overcurrent protective device of 60A. The smallest THWN-2 conductors permitted are _______________.

A) 14 AWG
B) 12 AWG
C) 10 AWG
D) 8 AWG

 

[don_resqcapt19]

In general, you can't use the 90°C ampacities for anything thing other than ampacity adjustment and/or correction.

 

[mtnelect]

This might help to explain.

 

[payneeco]

Thanks for the help. I appreciate the feedback. I was having trouble understanding Table 310.16. Even thought the question gave me THWN-2 and that is rated for 90 degrees Celsius, I'm not allowed to size it to that (I think you have to reference the "Equipment Provisions" in Article 110). I was perplexed that the chart didn't line up so I started searching through the code. Is 110.14 (C)(1)(a)(2) where it tells you that you have to use 60 C column, why you can't use the table directly? "Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on 60 C ampacity of the conductor size used."

 

[infinity]

Thanks for the help. I appreciate the feedback. I was having trouble understanding Table 310.16. Even thought the question gave me THWN-2 and that is rated for 90 degrees Celsius, I'm not allowed to size it to that (I think you have to reference the "Equipment Provisions" in Article 110). I was perplexed that the chart didn't line up so I started searching through the code. Is 110.14 (C)(1)(a)(2) where it tells you that you have to use 60 C column, why you can't use the table directly? "Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on 60 C ampacity of the conductor size used."
View attachment 2573411

If your terminations are rated for 75° C you can use the 75° C conductor ampacity from Table 310.16.

 

[mtnelect]

Thanks for the help. I appreciate the feedback. I was having trouble understanding Table 310.16. Even thought the question gave me THWN-2 and that is rated for 90 degrees Celsius, I'm not allowed to size it to that (I think you have to reference the "Equipment Provisions" in Article 110). I was perplexed that the chart didn't line up so I started searching through the code. Is 110.14 (C)(1)(a)(2) where it tells you that you have to use 60 C column, why you can't use the table directly? "Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on 60 C ampacity of the conductor size used."
View attachment 2573411

The manufacturers have done all the calculations for you, as explained in the above article. The manufactures know that HVAC equipment are often worked very hard and don't want the size of the conductors reduced. In fact, I usually increase the wire size on runs over 100 feet for "Good Measure".

 

[payneeco]

Ok. Thanks. I appreciate all the help.

 

[don_resqcapt19]

it is rare now, when you have to use the 60°C ampacities, most equipment is marked for 75°C. The product standard for air conditioning equipment has required 75°C ratings for a couple of decades.

 

[mtnelect]

it is rare now, when you have to use the 60°C ampacities, most equipment is marked for 75°C. The product standard for air conditioning equipment has required 75°C ratings for a couple of decades.

The HVAC manufacturers require 60-degree wire rating, no derating. Section 110.21 requires following the name plate requirements from the manufacturer. The termination could be rated for 75-degree rating but applying that reduction would result in a lower the wire size to the unit. HVAC is a "Different Animal" from standard electrical loads, and that would be a mistake to reduce the wire size and against the manufacturer's requirements.

 

[mtnelect]

Please check "Foot Note (a)

 

[don_resqcapt19]

The HVAC manufacturers require 60-degree wire rating, no derating. Section 110.21 requires following the name plate requirements from the manufacturer. The termination could be rated for 75-degree rating but applying that reduction would result in a lower the wire size to the unit. HVAC is a "Different Animal" from standard electrical loads, and that would be a mistake to reduce the wire size and against the manufacturer's requirements.

If they do, the product is not a listed product as the product standard requires the AC equipment to accept conductors at the 75°C ampacity.
From the UL Guide Information for "Heating and Cooling Equipment (LZFE):

Wiring Termination Provisions
For permanently connected equipment, the wiring termination provisions are based on tests during product investigation, and Table 310.15(B)(16) of NFPA 70, "National Electrical Code" (NEC) as follows:

1. 75°C insulated conductors at the 75°C ampacities.
2. 90°C insulated conductors at the 75°C ampacities in which case the equipment is marked for 90°C conductors.
3. Insulation temperature rating of 75 or 90°C and wire size as marked on the unit.

Can you post a picture of a nameplate that specifies a conductor size? I have never seen one. The ones I see specify a minimum circuit ampacity and a maximum overcurrent protective device.

 

[mtnelect]

If they do, the product is not a listed product as the product standard requires the AC equipment to accept conductors at the 75°C ampacity.
From the UL Guide Information for "Heating and Cooling Equipment (LZFE):

Can you post a picture of a nameplate that specifies a conductor size? I have never seen one. The ones I see specify a minimum circuit ampacity and a maximum overcurrent protective device.

Likewise ... you won't see "Suitable for 75-Degree Wiring" on a name plate. The manufacturer's want's the full "Minimum Circuit Ampacity" delivered, no derating. Also, in some situation you will have to increase the wire size. Since most power to HVAC units is delivered with expose conduits on the roofs.

 

[kwired]

Please check "Foot Note (a)

nothing in there says what termination rating is.

If you are using 60C conductor then you must use 60C ampacity even if you have 75C terminals.

HVAC is a "Different Animal" from standard electrical loads

I disagree, the compressors are similar to art 430 motors in many ways.

You may have a temporary surge of current during starting but after that it is somewhat a fixed load level with a refrigerant compressor where other motor loads may vary from lightly loaded to heavily loaded some maybe changes load level rather rapidly depending on what they are driving.

You still generally size conductors at 125% of rated load but have short circuit/ground fault protection of 200 to 250% of rated load for inverse time type breakers. This can allow for motors that only require a minimum of 14 AWG conductors to be permitted to have up to 35 amp circuit breakers for protection. HVAC labels tend to have a MOCP closer to 200% instead of 250% though, but for art 430 motor applications that 250% is permissible

 

[mtnelect]

nothing in there says what termination rating is.

If you are using 60C conductor then you must use 60C ampacity even if you have 75C terminals.

I disagree, the compressors are similar to art 430 motors in many ways.

You may have a temporary surge of current during starting but after that it is somewhat a fixed load level with a refrigerant compressor where other motor loads may vary from lightly loaded to heavily loaded some maybe changes load level rather rapidly depending on what they are driving.

You still generally size conductors at 125% of rated load but have short circuit/ground fault protection of 200 to 250% of rated load for inverse time type breakers. This can allow for motors that only require a minimum of 14 AWG conductors to be permitted to have up to 35 amp circuit breakers for protection. HVAC labels tend to have a MOCP closer to 200% instead of 250% though, but for art 430 motor applications that 250% is permissible

The HVAC systems are unique, in that the hermetic compressors run in refrigerant and have to start under a high head pressure, especially when the outside temperature is high.

 

[shortcircuit2]

The HVAC manufacturers require 60-degree wire rating, no derating. Section 110.21 requires following the name plate requirements from the manufacturer. The termination could be rated for 75-degree rating but applying that reduction would result in a lower the wire size to the unit. HVAC is a "Different Animal" from standard electrical loads, and that would be a mistake to reduce the wire size and against the manufacturer's requirements.

I do not see anything in 110.21 that requires following the nameplate requirements? 110.21 is about equipment Marking.

 

[kwired]

The HVAC systems are unique, in that the hermetic compressors run in refrigerant and have to start under a high head pressure, especially when the outside temperature is high.

Non VFD type residential grade units typically do not have enough torque to start under high head pressure and will simply trip motor overload after a few seconds of trying to start. Will automatically reset once overload cools and try again, eventually the head pressure will drop enough that it will be able to start. Many do have delay relays that won't let the contactor pull in right after a shutdown to give the pressure a chance to go down. But if shutdown were not via the normal control circuit (maybe not typical but not impossible) this delay time is not part of the equation.

 

[don_resqcapt19]

Please check "Foot Note (a)

I see nothing stating a wire size. It appears to me that the footnote is related to a table that actually specifies a conductor size.
EDIT: I found the rest of the page.
There is a table of wire sizes at the bottom of the page you showed us. The note applies to that table, but there in no language limiting you to the use of conductors at the 60°C ampacity.

COPPER WIRE SIZE — AWG (1% Voltage Drop)
Supply Wire Length-Feet​	Supply Circuit Ampacity​
200	150	100	50​
6	8	10	14​	15
4	6	8	12​	20
4	6	8	10​	25
4	4	6	10​	30
3	4	6	8​	35
3	4	6	8​	40
2	3	4	6​	45
2	3	4	6​	50

Wire Size based on N.E.C. for 60° type copper conductors.

 

[mtnelect]

I think the key word here is minimum on the equipment name plate.

"BRANCH-CIRCUIT CONDUCTORS: • THE CONDUCTOR AMPACITY RATING REQUIRED FOR THE AIR-CONDITIONING UNIT IS FOUND ON THE LABEL. THIS HAS BEEN DETERMINED BY THE MANUFACTURER TAKING INTO CONSIDERATION THE MOTOR COMPRESSOR CURRENT, FAN MOTOR CURRENT, AND HEATER CURRENT. THIS IS GENERALLY 125% OF THE LARGEST MOTOR PLUS THE FULL-LOAD RATING OF THE REST OF THE EQUIPMENT’S LOADS, SUCH AS FANS AND HEATERS".

"Branch-Circuit Selection Current (BCSC): The manufacturer of the HVAC end-use equipment might design better cooling and better heat dissipation into the equipment, in which case the hermetic refrigerant motor-compressor might be capable of being continuously “worked” harder than other equipment not so designed. “Working” the hermetic refrigerant motor-compressor harder will result in a higher current draw. This is safe insofar as the motor-compressor is concerned, but the conductors, disconnect switch, and branch-circuit overcurrent devices must also be capable of safely carrying this higher current draw. This higher value of current is marked on the nameplate using the term Branch-Circuit Selection Current (BCSC). Because the BCSC ampere value is always equal to or greater than the RLC ampere value, this BCSC value must be used instead of the RLC value when selecting conductors, disconnects, overcurrent devices, and other associated electrical equipment. In our example, the BCSC is 19.9 amperes. See Sections 440-2 and 440-4(c)"

 

[electrofelon]

Likewise ... you won't see "Suitable for 75-Degree Wiring" on a name plate. The manufacturer's want's the full "Minimum Circuit Ampacity" delivered, no derating. .

Using the 75 degree column vs 60 is not "derating"

 

[electrofelon]

The HVAC systems are unique, in that the hermetic compressors run in refrigerant and have to start under a high head pressure, especially when the outside temperature is high.

Ok but what does that change? If u calculate the ampacity required for a package unit, it's the same as a "regular" motor load.