Roof Top Unit

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pete m.

pete m.

Senior Member
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Ohio
Just looking for opinions..... of course backed up with Code ;)

Installation:

Roof top unit
Minimum circuit ampacity (per nameplate) 64
Maximum OCPD - 70
4awg copper conductors
100 amp breaker at panel
100 amp fusible disconnect at RTU fused at 70 amps

Is there a problem with the 4awg conductors being fed from the 100 amp breaker at the panel?

Thanks, Pete
 
Re: Roof Top Unit

Code wise you could have used 6 AWG assuming it is not NM.

The only issue could be voltage drop at start up if it is a long circuit.

[ November 09, 2005, 06:16 PM: Message edited by: iwire ]
 
Re: Roof Top Unit

What code section allows the 4awg conductors to be fed from a 100 amp breaker? :confused:
 
Re: Roof Top Unit

240.4(G)

The breaker is really only providing ground fault or short circuit protection of the conductors. The HVAC unit it self provides the overload protection.

The unit will shut down if it draws more than the MCA for any length of time.
 
Re: Roof Top Unit

240.4(g) does send us to article 440 for this application but parts I and VI talk about branch circuit conductors.......

The wire between the 100 amp breaker and the 100 amp disconnect fused at 70 amps are feeder conductors aren't they?
 
Re: Roof Top Unit

if my memory serves me, this is one of those questions that leads to the 52 page threads.
the differences of opinions seems to center around whether the device at the RTU is a "branch ciruit protective device" or a "disconnect" thus defininbg the conductors supplying it as "branch circuit" or "feeder" conductors.
I'm not sure I've ever seen a conclusive point that everyone agreed on.
 
Re: Roof Top Unit

I believe the 70A fused disconnect is the ground fault and short circuit protection for the unit. The conductors from the 100A CB will have to have an ampacity of 100A. JMO
 
Re: Roof Top Unit

This is a situation where I'm not sure if I like this forum or not :). 1 a.m. and I wake up thinking about this stupid question. Add to that, here I am second guessing myself..and, that, after receiving a compliment from Bob, is aggravating. All that being said, when I posted, I was thinking of a similiar age old situation, like this, except the max OCP was 100 (same 64 amp mca). If the disconnect was fusible would the #4 wire be permissable?
In this case, regrettably after making my case earlier, I can see no justification for the 100 amp breaker. I believe it could not exceed the 85 amp rating of the #4.
Sorry, Bob.
 
Re: Roof Top Unit

I see no reason to breaker it more than 70 amps.Putting it on the 100 is safe for now but later when the AC man does a service call and only has a set of 100 amp fuses with him we might end up with a problem.
 
Re: Roof Top Unit

Originally posted by augie47:
This is a situation where I'm not sure if I like this forum or not :). 1 a.m. and I wake up thinking about this stupid question.
Click to expand...
That is it, you are hooked. :eek:

Originally posted by augie47:
I can see no justification for the 100 amp breaker. I believe it could not exceed the 85 amp rating of the #4.
Click to expand...
There is no safety reason to treat the feeders differently than the branch circuit. It is imposable to overload the 4 AWG between the disconnect and the 100 amp breaker.

That much I was certain of, once the question of a code section to back it up came up I did have to do some looking.

After reading the scope of 430

430.1 Scope.
This article covers motors, motor branch-circuit and feeder conductors and their protection, motor overload protection, motor control circuits, motor controllers, and motor control centers.
Click to expand...
and 440

440.1 Scope.
The provisions of this article apply to electric motor-driven air-conditioning and refrigerating equipment and to the branch circuits and controllers for such equipment. It provides for the special considerations necessary for circuits supplying hermetic refrigerant motor-compressors and for any air-conditioning or refrigerating equipment that is supplied from a branch circuit that supplies a hermetic refrigerant motor-compressor.
Click to expand...
this is what I think.

Article 440 applies only to the branch circuit portion of this circuit, Article 430 applies to the feeder from the 100 amp breaker to the 70 amp fuses.

That being said I stand by my original opinion that the 4 AWG is fine and that even 6 AWG would have been fine.

430.62 Rating or Setting ? Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.
Where the same rating or setting of the branch-circuit short-circuit and ground-fault protective device is used on two or more of the branch circuits supplied by the feeder, one of the protective devices shall be considered the largest for the above calculations.
Click to expand...

[ November 10, 2005, 06:15 AM: Message edited by: iwire ]
 
Re: Roof Top Unit

Originally posted by jimwalker:
Putting it on the 100 is safe for now but later when the AC man does a service call and only has a set of 100 amp fuses with him we might end up with a problem.
Click to expand...
Jim that is not relevant to the code question, that is a design decision. :roll:
 
Re: Roof Top Unit

iwire,

430.62 Rating or Setting ? Motor Load
(A) Specific Load A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.

Using the language in this code section I believe that the feeder OCPD could not be larger than 70 amps. (notice the italic and bold in the code language)

If there had been a 70 amp breaker installed at the panel 6awg copper could have been used up to the unit and a non-fused disconnect could have been utilized for servicing.

Just my opinion and the way that I addressed this particular installation when the question was asked of me.

Thanks for the replys.

Pete
 
Re: Roof Top Unit

Thanks for taking the time to reply. Some of these situations that you make a "call" on make you sit back and scratch your head.....

If we all agree on this either we are all right or all wet :D

Pete
 
Re: Roof Top Unit

This area of overcurrent protection used as ground-fault short-circuit protection is the most misunderstood and debated. First, skip article 240 for motors and hermetic refrigerant compressors. Go to Articles 430 and 440. The motor branch circuit conductors are protected by two devices, an overcurrent device and the motor overload device. Motor conductors must be sized at 125% of the Table values based on HP and voltage. Because of high starting current, 5 to 6 times full load amperes, the circuit breaker at the source of power for a motor is allowed to be rated up to 250% of table value and provides short-circuit ground-fault protection. For hermetic compressor, the nameplate amperes is used. The ground-fault short-circuit value can be 175% of the FLA and where this value is not sufficient to allow the compressor to start, it can be increased up to 225%. The installation instruction provided by the mfg must be followed, it is part of the listing of the product.

If not provided, you can use the following from Sections 440.6(A), 440.22(A), and 440.32.
1.25X64A=80Amps, No.4CU,THWN, on 75degree terminations.
1.75X64=112amps, 100A overcurrent device at supply is fine.
 
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