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Discussion Starter #1
I am getting wiring done by an electrician for a ductless split AC system and for the condensor, this is the spec from the manufacturer.

It specs ~19A @ 208V with the voltage drop taken into account for all evaps operating, but only calls for a max breaker size of 20A.

I have two questions:

1. At startup, would the transients cause the breaker to trip even if its momentary?

2. Operating with all indoor evaps "ON", and drawing ~19A, there is only a 5% derating factor, I would be happy with at least 20%, are my assumptions incorrect? Should I have him run a 25A or 30A wire instead?

Any help is greatly appreciated and thanks in advance.
 

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Licensed Electrical Cont.
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May I ask WHY you are questioning this???

Also, this is NOT a continuous load.

If the specs call for a max breaker of 20A then that is all you can do. NOT 25 or 30.
#12 is FINE for this.
 

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Discussion Starter #3
Ofcourse I would keep the 20A breaker, that is not what I am questioning. I am questioning the 20A wire rating versus the ~19A continuous load on it. I like the idea of derating and would like to have at least a 20% margin, so thats why I was asking if I should have my electrician run a 25A or 30A rated wire.

How is this not a continuous (steady state) load? What is that number then? Peak or transient?
 

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It is not continuous since it is not intended to run at full capacity for three hours or more.
The 19A is the full running load amps. It is not 19A all the time, just when everything is running.
My question is WHAT good would it do to oversize the wire, unless of course the run was very long, like over 100'??? What is the purpose of this "20% margin", and who told you you need it?
 

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Discussion Starter #5
Its an industry practice to derate, isn't it? I was simply taking a 20% derating factor into account for the wiring, even if it would run for <3 hrs, I don't think the accumulated heat over that period of time is good for the wire.

The run isn't very long, only about 25'. Isnt it just good practice to upgrade to a 30A wire, for example, to be on the safe side for this application?
 

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No!

You are pulling 16+/- amps at 240 anyway.
The manufactorer has already figured the calculations and i s safe with the rating they have posted.
It is also a code violation to not follow the name plate of the unit!
 

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Licensed Electrical Cont.
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Its an industry practice to derate, isn't it?
Not in the way you are thinking. Not even close.

You are trying to out think the NEC. Personally I think you are over analyzing this whole thing. Let the electrician do his job.
 
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Discussion Starter #10
I didn't refer to any of the NEC tables. If NEC has already taken the derating into account, then that answers the question, thanks for all your help.

I was under the impression that the actual wire rating is its continuous load rating at 100%.
 

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Wire Chewer
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I would not see anything wrong in derating it, but as said, you don't really have to. It actually takes quite a lot to get a "wall grade" wire even warm, let alone hot to the point of melting.
 

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Discussion Starter #12
Actual vs maximum current rating
Nominal Rated Circuit Capacity Continuous Rated Circuit Capacity
5 amps 4 amps
10 amps 8 amps
15 amps 12 amps
20 amps 16 amps
30 amps 24 amps
50 amps 40 amps
100 amps 80 amps
200 amps 160 amps
Most commonly available circuit breakers are rated to carry no more than 80% of their nominal rating continuously (3 hours or more) (NEC Art. 100). 100%-rated circuit breakers are manufactured for and may carry 100% of their nominal rating continuously.

Source:
http://en.wikipedia.org/wiki/National_Electrical_Code

So my concern was, is the 20A 12AWG wire rating at 80% derated or at 100% maximum current rating? In this part of the city, they require arc-fault breakers too, so I dont think it will accept any current draw over 20A even for a brief moment, at startup transients for example. But at full capacity, it will be running at ~19A.
 

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Most commonly available circuit breakers are rated to carry no more than 80% of their nominal rating continuously (3 hours or more) (NEC Art. 100).
Correct. This is called a continuous load.
Like you have been told several times, yours is NOT a "continuous load".




So my concern was, is the 20A 12AWG wire rating at 80% derated or at 100% maximum current rating?
Again, this is not a continuous load so the 80% rule does not apply.




In this part of the city, they require arc-fault breakers too, so I dont think it will accept any current draw over 20A even for a brief moment, at startup transients for example. But at full capacity, it will be running at ~19A.
This circuit does not fall under arc-fault requirements.
Also, ANY circuit breaker IS designed to carry much more than it's rated capacity for a certain amount of time. A 20A breaker for instance will run at 22-24 amps for possibly hours before tripping.



I get the impression that you will not be satisfied until you are told just what you want to hear. In this case tell your guy to use #6cu wire for this 20A circuit, on an arc-fault breaker, fed through a GFI protected relay, just to be safe. :whistling2:
 
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Discussion Starter #14
I understand that "continuous load" is defined to have a load for >3 hours but if I have all evaps running at full capacity for >3 hours, isnt that a continuous load? plus what exactly does that ampacity mean then, ~19A?
 

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I understand that "continuous load" is defined to have a load for >3 hours but if I have all evaps running at full capacity for >3 hours, isnt that a continuous load? plus what exactly does that ampacity mean then, ~19A?
So have the #10 installed and be done with it. No one here should continue to try and convince you otherwise. Time is money. Stop wasting yours.
 

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Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.
The key word here is expected.

A continuous load is not one where it is possible to run for three hours or more. That would make any and every load a continuous load.
 

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Master Electrician
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[

I get the impression that you will not be satisfied until you are told just what you want to hear. In this case tell your guy to use #6cu wire for this 20A circuit, on an arc-fault breaker, fed through a GFI protected relay, just to be safe. :whistling2:[/quote]

You forgot the explosion proof boxes.
 

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See post #16.

An A/C system is NOT expected to run for three hours or more continuously. It might in some weird situation, but it is not expected to.
 

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Discussion Starter #20
Oh, so every AC is designed to run < 3 hours?

Also, why would this manufacturer print those values in the spec if its not a continuous load? I'm just trying to understand the terminology and reason for that spec.
 
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