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Old 02-06-2010, 07:35 PM   #1
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A/C Electrical


What is the Min Circuit Ampacity rating referring to on the data plate on the side of an A/C unit? I've always used the max fuse rating to size the breaker, but there is a huge difference between Mn Circuit Ampacity and the max fuse rating on my new unit.

35 Min Circuit Ampacity
60 Max Fuse

What type of breaker would an electrician put in? The panel is not overloaded.

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Old 02-06-2010, 07:44 PM   #2
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The Min Circuit Ampacity rating is the smallest wire you can use.

The Max Fuse Size is the largest fuse or breaker you can use.

Contrary to what a lot of people (including inspectors) think, it's completely code compliant to have a breaker rated higher than the wire for A/C units.

The same is true for motors and welders as well.

If you need code references, I or any one of about about a dozen other guys who post here can provide them.

Rob

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Old 02-06-2010, 08:06 PM   #3
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6 awg is the wire size
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Old 02-06-2010, 08:13 PM   #4
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Quote:
Originally Posted by pomelo View Post
6 awg is the wire size
You could use #8.
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Old 02-06-2010, 10:04 PM   #5
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I thought 60 amp breakers required 6 awg wire

45 amp 8 awg
30 amp 10 awg
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Old 02-06-2010, 10:20 PM   #6
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Quote:
Originally Posted by micromind View Post
The Min Circuit Ampacity rating is the smallest wire you can use.

The Max Fuse Size is the largest fuse or breaker you can use.

Contrary to what a lot of people (including inspectors) think, it's completely code compliant to have a breaker rated higher than the wire for A/C units.

The same is true for motors and welders as well.

If you need code references, I or any one of about about a dozen other guys who post here can provide them.

Rob
That is what I was taught. I always though that you couldn't have a breaker rated higher the wire. That is why I was always given ballpark wire to breaker ratings:

30 amp 10 gauge
40 to 45 amp 8 gauge
60 amp 6 gauge

I've just never seen a unit have such a large difference between min amp capacity and max fuse. What size breaker would you put on a unit that has 35 min cir ampacity and 60 amp max fuse? I normally would just put the maximum rated fuse/breaker to avoid nuisance trips.
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Old 02-06-2010, 10:28 PM   #7
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As micromind said it's a special exception for certain dedicated loads like A/C and motors and welders.
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Old 02-07-2010, 12:18 AM   #8
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My electrical guide book indicates the following copper wire sizes are allowed for HVAC equipment sighting Table 310.16

20 Amps = #14
25 Amps = #12
35 Amps = #10
50 Amps = #8
65 Amps = #6
85 Amps = #4

While my guide box does not sight references, it says to size wires based on minimum circuit ampacity, and size the circuit breaker to match the listed max fuse size.
[Edit to scratch that]

Ok, I found something in my guide box references Note 240.4 (G) and says "Motor loads and motor operated HVAC equipment are not subject to the same rules as non-motor operated equipment. Overcurrent protection will be sized based on nameplate data not necessarily on conductor size."

Last edited by HooKooDooKu; 02-07-2010 at 12:25 AM.
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Old 02-07-2010, 01:13 AM   #9
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One thing not considered here is what type of wire is being used. If using NM or UF, those numbers are not going to be acceptable but if using THHN or THWN, hooku's numbers are good.

NM or UF

20 amp #14
25 amp #12
30 amp #10
40 amp #8
55 amp #6
70 amp #4

and the reason you can use such a large breaker compared to the wire ampacity is start up current for a motor (a/c compressor and the fan motor) has a high in-rush current that drops quickly so the wire is not exposed to those high currents for long but the larger breaker is allowed so you do not trip the breaker while the equipment starts up.
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Old 02-07-2010, 01:49 AM   #10
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I have 2x30 amp breakers dedicated to my AC unit, but the wire is 10/2! Should I consider replacing it with 6/2? The walls are currently open to do so, so it would be pretty easy to do so at the present time.
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Old 02-07-2010, 03:23 AM   #11
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Quote:
Originally Posted by nap View Post
One thing not considered here is what type of wire is being used. If using NM or UF, those numbers are not going to be acceptable but if using THHN or THWN, hooku's numbers are good...
Interesting... I can't imaging what difference the wire TYPE would have on ampacity rating. Now I will say that I failed to mention that the table does assume conductors listed for 75C.

Other wise, my book, "Electrical Code Reference, The Ultimate Code Reference for Electrical Installations" (yet right), doesn't say much about the type of conductors. It does say that for outdoor equipement you can not use the NM cable (not suited for wet locations) and that THWN wire must be used. Otherwise it has a note saying "Most air handlers or furnaces installed indoors are generally supplied with NM or SEU cable..." and this on the same page as a reference to the table with the values I gave.

Assuming you're right, so much for "The Ultimate Code Reference..."
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Old 02-07-2010, 04:22 AM   #12
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If you are serious about the code number here it is.,

Art 430
Art 440

I know Art 440 do cover HVAC system while the Art 430 cover on hardwired motors { cord / plug motor are not included on this part }

As Micromind mention that so that something you have to watch it carefull otherwise double check the nameplate on the unit.

HooKooDooKu.,
Quote:
Interesting... I can't imaging what difference the wire TYPE would have on ampacity rating. Now I will say that I failed to mention that the table does assume conductors listed for 75C.
Hookoodoku keep in your mind all the NM and UF cable are use with 60C rating coloum even thru the NM-B are rated at 75C but the NEC code stated use the 60C so the larger conductor will affect it a bit but smaller one { 6.0mm or #10 AWG } that is unaffected at all due it allready lock in by default in the code.

Merci,Marc
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Old 02-07-2010, 09:03 AM   #13
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Also motors, not sure about A/C units, have overloads which will protect the wires. These overloads are set just a little higher then what the unit is suppose to draw, and once it starts drawing that the overloads will trip. The breaker is for startup and short circuit protection.
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Old 02-07-2010, 12:36 PM   #14
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Quote:
Originally Posted by brons2 View Post
i have 2x30 amp breakers dedicated to my ac unit, but the wire is 10/2! Should i consider replacing it with 6/2? The walls are currently open to do so, so it would be pretty easy to do so at the present time.
no!!!
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Old 02-07-2010, 01:04 PM   #15
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The type of wire matters only because 334.80 states that the ampacity of type NM cable (Romex) shall be based on a temperature rating of 60C.

Basically, conduit and THHN/THWN will use the 75C column in table 310.16, while NM and UF cable will use the 60C column.

Rob

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