feeder to heeting and air package unit

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gogod

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I'm new to the field and still trying to find things out. i have a unit on a roof needing to be hooked up.it is 208/230 v has a name plate rating of min. circuit amps 28, max rated amps 23.4, and max over current protection 45 amps. i want to use a fusless disconnect on the roof and run a feeder form the main disconnect box in side. how big does my feeder wire have to be using thhn, and the length is within 30 ft. also what size of circuit breaker do i need. i thank you for your time, and would like it if you would explains how to calculate it.
thanks
god bless
daniel rich
 
Re: feeder to heeting and air package unit

The section you start with is Article 440. The disconnect shall be sized per 440.12(B). So in your case, 23.4A x 115% = 26.91A. So you will need a min 30A rated disconnect.

440.22 is used to size the branch circuit ground-fault and short-circuit device (circuit breaker). However, 440.22(C) states that you can use the manufacturers nameplate which is 45A.

440.32 is used to size the branch circuit conductors before any derating that may be necessary. However, this has also been done for you on the nameplate. You need a circuit that is good for at least 28A. 310.16 would allow the use of #10 AWG per the 75 degree column.

I would be concerned about the ambient temperature for the conductors exposed to the sunlight and outdoors up on the roof. You may need to do some derating that would require a larger sized conductor or two.
 
Re: feeder to heeting and air package unit

You might also want to check the nameplate to see if a HACR rated breaker or fuses is required for the OCPD.
 
Re: feeder to heeting and air package unit

so i was right but what gets me is #10 thhn will only 40 amps, but you are aloud to use a 45Amp breaker. will your wire get to hot before the breaker trips
 
Re: feeder to heeting and air package unit

Originally posted by gogod:
so i was right but what gets me is #10 thhn will only 40 amps, but you are aloud to use a 45Amp breaker. will your wire get to hot before the breaker trips
Click to expand...
How could that happen?

The load will never increase beyond about 25 amps.

If there is a short circuit the 10 AWG will trip a 45 amp breaker as fast it trips a 30 amp breaker.
 
Re: feeder to heeting and air package unit

Read section 430.52(C). The increased size of the device is typically for motor starting currents.

Part III of Artcile 430 covers conductor overload protection.
 
Re: feeder to heeting and air package unit

If i'm in doubt I always go one size larger. I would use #8 awg, with a 40amp breaker. I would also use THWN wire since it is outdoors.
 
Re: feeder to heeting and air package unit

Welcome to the forum sparky59. :)

Originally posted by sparky59:
I would use #8 awg, with a 40amp breaker. I would also use THWN wire since it is outdoors.
Click to expand...
Why use 8 AWG when the load is 28 amps?

10 AWG THWN is rated 35 amps, the size of the breaker has nothing to do with the wire size. There are thermal overloads in the unit itself that protect the conductors from overload.

Good point about the THWN. :)

Bob

[ November 26, 2004, 04:39 AM: Message edited by: iwire ]
 
Re: feeder to heeting and air package unit

Originally posted by bphgravity:
I would be concerned about the ambient temperature for the conductors exposed to the sunlight and outdoors up on the roof. You may need to do some derating that would require a larger sized conductor or two.
Click to expand...
Would you expect the ambient temp to be higher than 113 F?

I guess it depends on where you live. :)

Bob

[ November 26, 2004, 09:53 AM: Message edited by: iwire ]
 
Re: feeder to heeting and air package unit

One of the most difficult things I had to learn about motors is how the circuit is protected. The motor overloads protect the circuit from overload while the overcurrent device in the panelboard protects the circuit from short circuits and ground faults. This same concept is used to protect tap conductors under the various tap rules in 240.21 and air-conditioning circuits. It also applies to over 600 volt taps where they are engineered.

One of the bad things is that most people believe that it applies to service entrance conductors. The part about overload does apply, protection from short circuits and ground faults does not apply. At that point, you are depending on the electric utility to protect your conductors and they do not!
 
Re: feeder to heeting and air package unit

hey, thanks guy for all the help i really appreciate it. i still have a lot to learn about the field and will problem call on you all again.
thanks,
god bless
daniel rich
ps i hope you all had a happy turkey day.
 
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