# Wire Ampacity Exception/RTUs



## k_buz (Mar 12, 2012)

In the past, we've always run RTU circuits according to the max OCP specs. If it was speced that max OCP was 50A we ran 8's.

As times are tight and profit margins are becoming slimmer and slimmer, I am trying to cut costs to save my employer money. 

The current job I'm running has a RTU that speced a max OCP of 50A and a min circuit ampacity of 34A. I ended up running 8's because this isn't just a unit for AC, it also heats (gas fired). Did I interpret the code correctly that the wire ampacity exception does not apply because the unit is also for heating?


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## Dennis Alwon (May 9, 2009)

I would not interpret it that way. IMO, without regard to voltage drop a #10 copper conductor in conduit (75C) would be compliant. Obviously nm would not be compliant


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## RIVETER (Sep 26, 2009)

k_buz said:


> In the past, we've always run RTU circuits according to the max OCP specs. If it was speced that max OCP was 50A we ran 8's.
> 
> As times are tight and profit margins are becoming slimmer and slimmer, I am trying to cut costs to save my employer money.
> 
> The current job I'm running has a RTU that speced a max OCP of 50A and a min circuit ampacity of 34A. I ended up running 8's because this isn't just a unit for AC, it also heats (gas fired). Did I interpret the code correctly that the wire ampacity exception does not apply because the unit is also for heating?


I am getting away from answering your question just to point out that it is not your responsibility to "SAVE" your employer money in terms of possibly shortchanging electrical systems. If you violate the code without your boss's knowledge, you are held liable for anything that goes wrong. I am not saying that there is not a very large safety factor with using copper conductors; It is just that the accepted rules are the rules. Good luck.


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## blueheels2 (Apr 22, 2009)

Dont understand the part about the heat. The ones I have wired have a chart that shoes the heat strip accessories. The hvac man marks which one he used and it has a chart with the MOCP and MCA. We always size to MCA.

ETA i never did it this way unti my employer made me.


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## Cletis (Aug 20, 2010)

*seu*

just run a 4-4-6 seu into the back of the disconnect?


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## k_buz (Mar 12, 2012)

RIVETER said:


> I am getting away from answering your question just to point out that it is not your responsibility to "SAVE" your employer money in terms of possibly shortchanging electrical systems. If you violate the code without your boss's knowledge, you are held liable for anything that goes wrong. I am not saying that there is not a very large safety factor with using copper conductors; It is just that the accepted rules are the rules. Good luck.


Who said anything about me violating code without my boss's knowledge? In fact, I really have no clue how your post relates to my question at all.




blueheels2 said:


> Dont understand the part about the heat. The ones I have wired have a chart that shoes the heat strip accessories. The hvac man marks which one he used and it has a chart with the MOCP and MCA. We always size to MCA.
> 
> ETA i never did it this way unti my employer made me.


The RTU is both a gas fired furnace and an A/C. All one unit, that is why I didn't think I could use the exception from your "typical" ampacities.


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## Cl906um (Jul 21, 2012)

I remember this question from school, for any hvac equip you can use circuit ampacity. But if the nameplate says calls out a fuse for oc. You have to use a fuse. Not a breaker. I went to school in green bay under Kevin weigmann. Just thought I would throw that in there since you were from wi. Packers almost came back huh? How disappointing. Better luck when they play bears. :thumbup:


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## sbrn33 (Mar 15, 2007)

HVAC equipment is the easiest thing out there to wire. Most of the work is done for you. Just go off the name plate and life is good.
I don't understand Riveters post either. I think he was bashing you for trying to help your boss make some money. I am guessing he is one of the guys that thinks employers are the devil.


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## rlitland (Sep 7, 2012)

It is always better to go with the manufacturer's specs. I would go with the max over current protection and size the wire accordinglly in order to protect the device. There might be an issue down the road with the equipment and you don't want the customer blaming you for inefficient wiring. It could save your employer some heart ache in the future. Cover your a$$ from the begining.


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## beElectric (Sep 16, 2012)

rlitland said:


> It is always better to go with the manufacturer's specs. I would go with the max over current protection and size the wire accordinglly in order to protect the device. There might be an issue down the road with the equipment and you don't want the customer blaming you for inefficient wiring. It could save your employer some heart ache in the future. Cover your a$$ from the begining.


Sounds like this is what they call a package unit. Never heard of one with gas. With the gas heat I would go with the minimum breaker size in my option.
One way i save money on outside heat pumps is go by the full load current rating for the branch circuit size. Then the minimum breaker size for the breaker in the panel.
Example : fla says 14.8 amps. You would run #14 awg.
Minimum breaker size 25 amps.
I would put in a 25 amp breaker.
You can do this on a locked-rotor motor with a thermal protector.
Read 440.41 to 440.53 in the code book. You don't save money on the breaker size, just wire size.


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