Roof Top Units : Calcs

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Adrian,
Sorry the code references are in Article 240.
I have never said that you use the rated load current to size the conductors. When the nameplate has a "minimun circuit ampacity" shown, that is not the rated load current, it is the branch circuit selection current.
However this has nothing to with the sizing of the branch circuit short circuit and ground fault protective device. 440.22 says that the protective device can be rated at 175% of the branch circuit selection current. In your example of a 30 amp branch circuit selection current the conductor could be a #10 and the OCPD could be 50 amps and if the unit would not start using the 50 amp breaker you could increase the breaker to 60 amps still using the #10 wire.
Don
 
Sorry Don, you're twisting the code to suit what you want instead of what it means. 23.9 amps on the name plate is the rated current of the equipment. Multiply that by 175% and you would be able to use 41.825 amp OCPD. However the name plate also says not to exceed 35 amp OCPD. NEC says to use the name plate if that info is provided. Protecting the branch circuit conductors has nothing to do with the equipment. The equipment is UL Listed and has its own built in protection. Whatever size breaker you use, the conductors must be rated for that breaker. You can't use 12 AWG on a 35 amp breaker and there is nowhere in the code that says you can. The article that you cited (440.6A Exception #1) agrees with me completely.
 
adrian33773 said:
Whatever size breaker you use, the conductors must be rated for that breaker. You can't use 12 AWG on a 35 amp breaker and there is nowhere in the code that says you can.
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Adrian, you are mistaken, Don is correct.

240.4(G) says we can.

adrian33773 said:
23.9 amps on the name plate is the rated current of the equipment.
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12 AWG would be fine for 23.9 amps
 
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adrian33773 said:
You can't use 12 AWG on a 35 amp breaker and there is nowhere in the code that says you can. The article that you cited (440.6A Exception #1) agrees with me completely.
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I agree with Don and Bob. Since the OCPD is only providing ground fault and short circuit protection it can be sized larger than the values listed in 310.16.


Sorry Don, you're twisting the code to suit what you want instead of what it means. 23.9 amps on the name plate is the rated current of the equipment.
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If 23.9 amps is the Minimum Circuit Ampacity then that number is used to size the conductors. For #12 THHN at 75 degrees C the ampacity is 25 amps.
 
adrian33773 said:
Protecting the branch circuit conductors has nothing to do with the equipment.
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When the conductors supply motor loads or combined HVAC equipment the overload devices protecting the equipment also protect the branch circuit conductors from overload.

In these circuits the breaker at the panel only protects the conductors from short circuits and ground faults.
 
Okay, whatever, my company would still use #8 AWG aluminum with a 30 amp circuit on the roof. We are concerned with protecting the conductors. 35A OCPD does not protect #12 wire in the case of a short circuit and fire could be the result. The equipment contains its own protection or it wouldn't be UL listed. My foreman has been a commercial electrician for over 20 years, so its kind of hard to argue with him since he is the boss. He said we use #8 AL because its cheaper than #10 CU. He also said the voltage drop would be less also with the larger wire. We work on tall buildings, so voltage drop is a big consideration and he would also be tempted to even use #6 depending on the height of the building. NEC does not specify the maximum size of conductors, only the minimum. Refer to NEC 90.1(B) and (C). We that said, I rest my case.
 
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adrian33773 said:
Okay, whatever, my company would still use #8 AWG aluminum with a 30 amp circuit on the roof.
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No one has said you can't do that.

We are concerned with protecting the conductors. 35A OCPD does not protect #12 wire in the case of a short circuit and fire could be the result.
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We are all concerned with fire, but the fact is a 35 A OCPD will protect 12 AWG against short circuits and ground faults it will not protect the conductor from overload. The motor overloads (heaters) will protect the 12 AWG from overload.



He said we use #8 AL because its cheaper than #10 CU.
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It could be.

He also said the voltage drop would be less also with the larger wire.
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He is correct

We work on tall buildings, so voltage drop is a big consideration and he would also be tempted to even use #6 depending on the height of the building.
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No doubt.

No one has said increasing the conductor size is a bad idea, what we have been discussing is what the NEC minimum is.

Many engineers will run conductors that match the breaker size because of voltage drop as you pointed out.


My foreman has been a commercial electrician for over 20 years
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I have about 25 years in, I bet don_resqcapt19 has more than 35 years in, infinity has quite a few years in and suemarkp is an experienced engineer.

All of us regularly exceed code minimums but we are also very aware of what the code minimums are. :smile:
 
Adrian,
We are concerned with protecting the conductors. 35A OCPD does not protect #12 wire in the case of a short circuit and fire could be the result.
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If you would take a look at the time trip curves for a 20 amp breaker and a 50 amp breaker, you will find that for short curcuits and ground fault events that the trip time will be amost identical. The conductors are protected. A #12 can withstand 2678 amps for 1 cycle or 1093 for 6 cycles. That is well above the instantaneous[FONT=&quot] trip point of most 50 amp breakers.
Don
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Thanks Bob for the PM. This has nothing to do with the current subject, but the other day we had a light fixture on the roof burn up. It was surrounded by concrete so no other damage occured other than paint. It was a 13 watt fluorescent fixture on #12 wire with 20A OCPD. It's a 7 story building with the electrical room on the third floor. Anyway, when the fixture was installed, the hot wire got pinched against the metal mounting bracket which was properly grounded. The breaker didn't trip. The fixture was weather resistant and made out of plastic. The entire thing was a black gooey mess. The project engineer says the reason it happened was because of the resistance on the #12 wire and the distance from the panel. He said by the time the current reached the panel it wasn't enough to trip the breaker. Just some food for thought.
 
I would add that if a #12 wire on a 35 amp OCPD is a fire hazard than the permitted #18 wire fixture tap in a 6' whip to a fixture is really a bigger fire hazard.

For the record Bob, I've been in this business almost as long as you.
This is my 24th year.
 
Anyway, when the fixture was installed, the hot wire got pinched against the metal mounting bracket which was properly grounded. The breaker didn't trip.The entire thing was a black gooey mess. The project engineer says the reason it happened was because of the resistance on the #12 wire and the distance from the panel. He said by the time the current reached the panel it wasn't enough to trip the breaker. Just some food for thought.
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Given the localized heat damage it is much more likely that the fault had enough resistance to limit the current to a level below the trip point of the breaker, just like a heating appliance.
Don
 
adrian33773 said:
Thanks Bob for the PM. This has nothing to do with the current subject, but the other day we had a light fixture on the roof burn up. It was surrounded by concrete so no other damage occured other than paint. It was a 13 watt fluorescent fixture on #12 wire with 20A OCPD. It's a 7 story building with the electrical room on the third floor. Anyway, when the fixture was installed, the hot wire got pinched against the metal mounting bracket which was properly grounded. The breaker didn't trip. The fixture was weather resistant and made out of plastic. The entire thing was a black gooey mess. The project engineer says the reason it happened was because of the resistance on the #12 wire and the distance from the panel. He said by the time the current reached the panel it wasn't enough to trip the breaker. Just some food for thought.
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I hope this project engineer is not an electrical engineer...if a "current" was substantially delayed between panel and load implies that those 7 stories must have really really high ceilings!
 
adrian33773 said:
It was a 13 watt fluorescent fixture on #12 wire with 20A OCPD. It's a 7 story building with the electrical room on the third floor. Anyway, when the fixture was installed, the hot wire got pinched against the metal mounting bracket which was properly grounded.
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If the problem was the entire length of the circuit, the power would have been dissipated as heat the whole way. just about all of the voltage drop occured at the fixture. This damage was more likely the electronic ballast going under.

don_resqcapt19 said:
Given the localized heat damage it is much more likely that the fault had enough resistance to limit the current to a level below the trip point of the breaker, just like a heating appliance.
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My thoughts exactly. See above.

watercop said:
I hope this project engineer is not an electrical engineer...if a "current" was substantially delayed between panel and load implies that those 7 stories must have really really high ceilings!
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:grin: :grin: :grin:
 
i was re-reading back through this thread and saw that it toally got away from my original question - and it's still something that I want answered.

I guess I was never assured on doing roof-top calcualtaions.

please provide help.

thanks.

~~~
Personal Note: is it possible for future posts to stay on the subject or question asked and not steer away? I understand if another person has a similar question, but if there post takes importance over the original post - then that post should have been a new thread, IMO.
 
starbright28 said:
i was re-reading back through this thread and saw that it toally got away from my original question - and it's still something that I want answered.
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I just read through the first page again, and you got the best answers right there.

Personal Note: is it possible for future posts to stay on the subject or question asked and not steer away? I understand if another person has a similar question, but if there post takes importance over the original post - then that post should have been a new thread, IMO.
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You're right, of course, but I recommend that you don't hold your breath.

This forum is the epitome of not staying on topic. (Not really, because the mods are ever so vigilant, but we try.)
 
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