Air Conditioning Wire Sizing

[mtnelect]

What does this have to do with conductor sizing to an A/C condenser ?

Determining the correct size of the HVAC system in the first place. Larger size HVAC system, larger size wire, smaller size HVAC system, smaller size wire. Do you want to guess with the "Rule of Thumb" or perform a "Heat Load Enginering Study" that gives you the exact heat load of the structure to save on wasted energy. This is what we are doing in California and 18 other states that have adopted California's Title 24.

 

[infinity]

That will help I think I’m the young guy in the room here this argument didn’t go so well for me in a class of 10 people none of them agreed lol thank you

Show them this:

 

[electrofelon]

Determining the correct size of the HVAC system in the first place. Larger size HVAC system, larger size wire, smaller size HVAC system, smaller size wire. Do you want to guess with the "Rule of Thumb" or perform a "Heat Load Enginering Study" that gives you the exact heat load of the structure to save on wasted energy. This is what we are doing in California and 18 other states that have adopted California's Title 24.

Still not seeing the relevance of this for conductor sizing. How did we get into heat load studies?

 

[LarryFine]

Still not seeing the relevance of this for conductor sizing. How did we get into heat load studies?

I believe he's saying that some people install unnecessarily-large conductors because they're installing unnecessarily-large HVAC systems. However, I feel that's the responsibility of the HVAC people, not us.

 

[electrofelon]

I believe he's saying that some people install unnecessarily-large conductors because they're installing unnecessarily-large HVAC systems. However, I feel that's the responsibility of the HVAC people, not us.

Right, the conductors are not unnecessarily large if that is what the unit requires, even if he unit is sized wrong.

 

[kwired]

HVAC is a specialty and doesn't follow common motor calculations. Of the most common questions asked on this site and the most misunderstood is HVAC.

It mostly does follow every requirements of 430 for multi load applications involving motors, difference is it is a single unit with a nameplate and they happen to require the MCA and MOCP to be on the name plate, so you don't really need to do the calculations even though you usually come up with same values or very close if you did them according to 430. MOCP is more of a manufacturer suggestion however, calculating this with art 430 would usually give you a higher maximum value being allowed.

 

[kwired]

Determining the correct size of the HVAC system in the first place. Larger size HVAC system, larger size wire, smaller size HVAC system, smaller size wire. Do you want to guess with the "Rule of Thumb" or perform a "Heat Load Enginering Study" that gives you the exact heat load of the structure to save on wasted energy. This is what we are doing in California and 18 other states that have adopted California's Title 24.

First step to the HVAC installer, if he happens to be qualified to install wiring to it then so be it.

For the electrical only installer, his first step is reading nameplate of something already selected by someone else and he really doesn't care if it was selected correctly per energy codes, all he sees is his wiring needs to be able to handle the unit ratings.

Based on what you said though, apparently it isn't allowed to size a unit to accommodate a future addition or some future condition that would increase heating or cooling load when it does occur?

 

[kwired]

Where I’m am from in Pittsburg I feel like nobody knows this I kinda see it as a problem that even tech support for the air conditioners we sell don’t know this I think the person training doesn’t know then the manufacturer should have to put it in manual list the code or something I have 2 electricians that said it’s wrong the electric supply store said it was wrong but trying to prove this that don’t believe it is impossible I tried 3 different tech support guys all told me I was wrong about sizing wire for ac to mca

It really messes with those that don't understand the rules when you are not using NM cable and can go with the 75C ampacity table. A unit with a MCA of say 23 amps can be supplied with 12 AWG 75C conductor, and said unit may possibly have MOCP on the nameplate of 45 amps.

Same minimum conductor size in an actual art 430 application could even have a max inverse time breaker of 60 amps being permitted.
But if you really want to make their head spin show them an application that has an instantaneous trip breaker that possibly could have a trip setting up to 200 amps

 

[mtnelect]

It mostly does follow every requirements of 430 for multi load applications involving motors, difference is it is a single unit with a nameplate and they happen to require the MCA and MOCP to be on the name plate, so you don't really need to do the calculations even though you usually come up with same values or very close if you did them according to 430. MOCP is more of a manufacturer suggestion however, calculating this with art 430 would usually give you a higher maximum value being allowed.

Except when an Electrician sees an "Oversized" breaker feeding a HVAC unit and replaces the breaker to meet to match the wire size.

 

[electrofelon]

Except when an Electrician sees an "Oversized" breaker feeding a HVAC unit and replaces the breaker to meet to match the wire size.

And that is a somewhat common misunderstanding for all motor loads, not just 440 ones.

 

[infinity]

Except when an Electrician sees an "Oversized" breaker feeding a HVAC unit and replaces the breaker to meet to match the wire size.

Not really a safety issue. The wire is sized to the MCA which had 25% added to it so the unit would probably still run if the breaker matches the conductor size.

 

[mtnelect]

Not really a safety issue. The wire is sized to the MCA which had 25% added to it so the unit would probably still run if the breaker matches the conductor size.

Except on a 100 + degree day, when the HVAC unit is called upon to provide cooling. In those conditions with high refrigerant pressure, it will trip with the lower sized breaker every time.

 

[electrofelon]

Except on a 100 + degree day, when the HVAC unit is called upon to provide cooling. In those conditions with high refrigerant pressure, it will trip with the lower sized breaker every time.

I find that quite unlikely. I measure HVAC running current quite frequently, and I have never seen one draw over 2/3 of the MCA. I think someone would have to downsize the breaker quite a bit to have it trip. Worst case I can immediately think of is an MCA of 25 which would be fine on #12 MC or pipe and wire. Say someone sees the number 12 and thinks breaker has to be a 20, even at 80% that is 20 amps. I guess maybe it could trip on mag instead of thermal.

 

[mtnelect]

I find that quite unlikely. I measure HVAC running current quite frequently, and I have never seen one draw over 2/3 of the MCA. I think someone would have to downsize the breaker quite a bit to have it trip. Worst case I can immediately think of is an MCA of 25 which would be fine on #12 MC or pipe and wire. Say someone sees the number 12 and thinks breaker has to be a 20, even at 80% that is 20 amps. I guess maybe it could trip on mag instead of thermal.

Rated-load current
is established by the hermetic motor-compressor
manufacturer under actual operation at
rated refrigerant pressure and temperature, rated
voltage, and rated frequency. RLC is marked on
the nameplate. In most instances, the marked
RLC is at least equal to 64.1% of the hermetic
refrigerant motor-compressor’s maximum continuous
current (MCC).

 

[Lucia1222]

In California, no more "Rule of Thumb", Heat Load Engineering Study" are now required. See Attachment

That’s pretty nuts wouldn’t wanna work there

Show them this:

haha this I what I’m going to use

 

[Lucia1222]

One more question im being told the 125% rule kicks in this is crazy on how big of a confusion this makes I had a inspector tell me he wouldn’t pass it if he seen it

 

[infinity]

One more question im being told the 125% rule kicks in this is crazy on how big of a confusion this makes I had a inspector tell me he wouldn’t pass it if he seen it

What 125% rule? And while your at it who is telling you this stuff?

 

[Lucia1222]

What 125% rule? And while your at it who is telling you this stuff?

So tech support for comfortmaker and the electric supply house was diving into for me and they asked a inspector that said not on my watch lol but I was told if I have 30amp mca and 40amp max I have take 30*1.25 witch would give me 37.5 the wire needs to handle that i think I’m saying that right

 

[david luchini]

So tech support for comfortmaker and the electric supply house was diving into for me and they asked a inspector that said not on my watch lol but I was told if I have 30amp mca and 40amp max I have take 30*1.25 witch would give me 37.5 the wire needs to handle that i think I’m saying that right

30 amp mca means you need a wire that handles 30 amps (not 37.5 amps)

 

[Lucia1222]

30 amp mca means you need a wire that handles 30 amps (not 37.5 amps)

It didn’t make sense to me I’m telling where I’m from in McKeesport pa knows this it’s just extra money and work to change wires electricians heating and cooling which are supposed to know this