Thread: cord and plug ac unit

a cord and plug air conditioner that draws 8 amps on the same circuit as two 120 volt 100 watt lights would require a 20 amp breaker?

my math was based on 3702.12.2 of the IRC says the ac unit shall not exceed 50% of the rating of the branch circuit where lighting or other appliances are installed.
8 x .5 = 4
4+8 = 12 amps for the ac unit PLUS
120/100= 1.2 x 2 + 2.4 amps for the lights SO
2.4 + 12 = 14.4 or a 20 amp breaker.......

OR

do i not need the calculation for the lights because 3702.12.2 does not say to add the load of the lights to the calculation which would make it a 15 amp breaker?

am i correct in this math? and if not could someone give me a bit of guidance on it?

thanks for any input!
marshall

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Your math makes no sense to me. What is the MCA on the AC unit?

Read this section....

Chapter 37 - Branch Circuit and Feeder Requirements

Does it work OK ? Your math is off a bit. To get the amperage of a 100 watt bulb you divide the wattage by the voltage, ( 100/120 = .83 amps ). 8 amps for the A/C unit + .83 + .83 = 9.66 amps. I was not aware of the 50% max for a mixed use circuit, but you're definitely hair splitting. Assuming your code references are correct, that A/C unit is .5 amps too much - - - pretty tight. The ancillary loads, ( like light bulbs/fixtures ), are already accounted for in the code reference, so no calcs needed for them. What happens if that C/B is HACR rated ?

dom,
I see the 3702.12 and was looking at it. i looked at 3702.12.2 because it is specific to the question being that it serves lighting loads also. So i guess what i am getting at is would the 20 amp breaker be the size needed for this branch given the 8 amp draw from the ac unit and the lighting loads.....

thanks for any help again,
marshall

also i know the amp for the lights was off that was a hurry mistake in trying to post this...so .83 x 2=1.66 +12 = 13.66 amp total calculated draw.

Why are you adding a 50% on to the A/C unit load?

The reference to 3702.12.2 states
"Where lighting units or other appliances are also supplied"
The total marked rating of a cord and attachment plug connected room air conditioner shall not exceed 50 percent of the rating of a branch circuit where lighting or other appliances are also supplied. Where the circuitry is interlocked to prevent simultaneous operation of the room air conditioner and energization of the other outlets on the same branch circuit, a cord and attachment plug connected room air conditionershall not exceed 80% of the branch circuit rating.

your right i misread the code there...... my question still stands as if a 20 amp breaker is the correct ocpd in this case.


i was thinking that since half of fifteen is 7.5 the 8 amp is too much draw for the ac unit and additional lighting load. half of twenty is ten so the amperage is ok for the air conditioning load here...maybe i thought too hard on the question when getting the math involved.
Again thanks for the help on this it is greatly appreciated.
marshall

Is the AC fastened in place? Otherwise the 50% provision in NEC 210.23 does not apply.

I read that article 210.23 and agree that in that case it does not work. The article i refer to is from the IRC from which my question came and is worded a couple posts up.

Originally Posted by Marshall Feely

can you explain how it does not apply? to me it seems a cord and plug fit that well....i'm not trying to be argumentitive at all i would just like to understand why.

I don't have access to my code book or I would post the relevant section but in 210.23 it states that a branch circuit can be loaded to it's full ampere rating (this isn't without other restrictions). For a branch circuit with, in this example lighting, and utilization equipment fastened in place the ampacity of the equipment shall not exceed 50% of the circuit rating. If the utilization equipment is not fastened in place then it may not exceed 80% of the circuit rating.

And don't be afraid to ask questions, we're all here to learn.

Originally Posted by Robert Meier

And don't be afraid to ask questions, we're all here to learn.

Hats off to you Robert. So much better that "WRONG", whether it may or may not be wrong. Although I admit that a bit of provocative seems to add some spice on occasion.

Marshall: You are going way beond the SOP of inspectors, list what you see and have an applicable HVAC contractor evaluate and advise on any repairs.

(my opinion)

the reason i asked is because it was a question on an exam i took recently and while that is the only question that i can remember it is probably because it tripped me up in reading the code articles both that are listed above.....

The question was a cord and plug air conditioning unit is installed on the same branch circuit as 2 - 100 watt 120 volt lighting outlets. what is the ocpd size needed 15, 20, 25, or 30......

it seemed to me that a twenty was correct because of the above code articles that i used from the IRC.

all i'm after is was my code cite correct and if so is the twenty correct. I'm getting veried answers from you fine folk on these forums which is nice but at the same time makes me wonder if being an inspector is really how that inspector interprets the code. if that makes any sense..... i know some things are cut and dry stated the same way in the nec and irc codebooks. but other things like this issue are not as cut and dry and say the IRC is the code used but the nec and irc have different or slightly different views on it which do i go by?

you guys have all been awesome in trying to help me figure this out and i think i am being forced to figure it out on my own with "nudges" from the forum posters.

Marshall