ac breaker size

[peter d]

Hey guys if you have an ac unit that has a minimum ckt ampacity of 22a and a max fuse size of 40a what size breaker would you use?

#12 wire (NM is fine as the 60C ampacity of it is 25 amps) on a 40 amp breaker. That's how I would do it.

 

[CopperTone]

yup #12 's with a 40 amp breaker. the ac unit has internal overload protection so you can use the minimun size wire from the nameplate. #12 in 60 degree C column is good for 25 amps. then use max breaker size 40amps - if says fuse then use 40amp fuses inside your outdoor disconnect.

nameplate always trumps code book

 

[billdozier]

but arent breakers rated at 75 degrees thus you have to wire the ampacity of the wire at 75? Or since this is an ac we can use 60 deg column. Please cite code reference thanks

 

[220/221]

#12 wire (NM is fine as the 60C ampacity of it is 25 amps) on a 40 amp breaker. That's how I would do it.

I have heard that referenced before but explain it to me in laymans terms.

In my experience, AC loads are the largest single draw on the panel (here in AZ). Every summer is filled with burned up breakers, bus and wire terminations on the AC's.

I've also be taught tha basic 14 ga = 15 amps, 12 ga = 20 amps, 10 ga = 30 amps all my life.

Now, (because it's an AC I believe?) I can run a 40 amp AC on a #12 NM?

How does that make any sense.

I have also never ever seen #12 wire of any kind on a 40 amp breaker. If I saw it on a 30 I would have said "what dumbass did this?"

I shoulda stuck with plumbing.

 

[peter d]

220,

Follow along with me here. All the breaker does in a motor/compressor application (article 440) is protect the wire from short circuits and ground faults. Nothing more, nothing less. A SC or GF will be in the 1000's of amps - well enough to trip any breaker. So we are safe there.

The breaker does not protect the unit from overload. The overload protection is done with the thermal protection of the fan motor(s) and compressor(s). If the motor stalls and overheats, the thermal trips. Also, locked rotor current is more or less a short circuit anyhow, and that will trip a breaker rapidily enough before the conductors are damaged. So we are safe once again.

Does that help?

As for burned up stuff you see all the time, I'm guessing that has to do with high ambient temps, bad/poor bus/breaker stab contact and otherwise bad connections. I highly doubt it has anything to do with this code rule. If anything, because you have cables running through 150 degree attics, they should be sized for that temperature.

 

[220/221]

All the breaker does in a motor/compressor application (article 440) is protect the wire from short circuits and ground faults

So....because it is an inductive load (motor/compressor), it can somehow carry 40 amps safely but any other load is limited to 20??

I think I follow what you are saying but I can't grasp the logic. I thought an amp was an amp no matter what the load.

 

[suemarkp]

It can't safely carry 40A continually. The 40A breaker is large enough that it will carry you through the start up surge without tripping. A 30A would probably work fine for that too. But if the thing is drawing 30 to 40A continually because it is broken or seizing, then the wiring could melt. However, the built in thermal protection in a HVAC compressor prevents this from happening in 99% of the fault conditions.

Section 240.4(G) lists all the things where there are similar conditions and you can put larger than "normal" breakers on a given set of wire. The most common of these are motors, HVAC compressors, and welders. This same rule also lets you violate the 15A on #14, 20A on #12 rule since the true 60C ampacity of those wires is 20A for #14 and 25A for #12.

 

[220/221]

So they calculate the minimum amps based on the start up load and since the operating load (after a few seconds) will be well onder 20 amps the use of #12 is allowed.

Interesting.

 

[infinity]

So they calculate the minimum amps based on the start up load and since the operating load (after a few seconds) will be well onder 20 amps the use of #12 is allowed.

Interesting.

The MCA should be figured by the manufacturer using the 125% of the largest load, usually the compressor, and 100% of the smaller load(s), the fan. So by sizing the conductors according to the MCA you'll see that the 125% factor has already been added in to the minimum conductor size which will allow the unit to safely operate without damaging the condcutors.

Those conductors can be sized according to the values listed in 310.16 for ampacity. When using NM cable 334.80 limits you to the ampacity of NM at 60 degrees regardless of the terminations on the CB's and equipment.

 

[iwire]

So....because it is an inductive load (motor/compressor), it can somehow carry 40 amps safely but any other load is limited to 20??

It will never carry 40 amps continuously

The current on the conductor is limited by the thermal motor overloads in the unit.

 

[busman]

Don't really know. I hear stories about old equipment having higher starting current due to age/wear, etc. but I've yet to see any proof of that.

If you've ever had to put a hard-start kit on an A/C, you'd be a believer. These can eek a few more years out of a really whipped compressor.

Mark

 

[Dennis Alwon]

All the breaker does in a motor/compressor application (article 440) is protect the wire from short circuits and ground faults. Nothing more, nothing less. A SC or GF will be in the 1000's of amps - well enough to trip any breaker. So we are safe there.

Well stated Peter but I am not sure of the above statement only because there is a limit set by the nec. What is the purpose of the limit if a 200 amp breaker would do the same think as a 40 amp breaker?

 

[don_resqcapt19]

but arent breakers rated at 75 degrees thus you have to wire the ampacity of the wire at 75? Or since this is an ac we can use 60 deg column. Please cite code reference thanks

The breaker temperature marking is the maximum rating of the conductor that is connected to it. See 110.14(C).

 

[daleuger]

Is this a trick question?



Edit.....I don't believe I've ever even seen a 25 or 35 amp breaker. That may explain my logic

I agree with that though I've seen built in breakers on AC units of 45 or 55. Usually you see them ending in 0, no? 20, 30, 40, 50, etc????

 

[daleuger]

I can see the logic most all the arguments thus far. OTOH, I don't think I'd be the only one wondering what was going on if I opened up a panel and saw #12's on a 40A breaker.......

 

[Karl H]

Once you guys get why you can have large ocpd's on small conductors
for A/C's. I suggest you study motors next,and it will all become clear.

 

[CopperTone]

AC's are motors - they size the conductors and breakers the same way you do for a motor. It does initially seem to be out of whack with everything you ever learned about sizing wire and breakers but this is true.

if you have a starter for your motor then you size your overload protection (heaters) based on 430.32 - in an AC unit the overload protection is built into the condensor unit - that is why you only put a breaker at the panel and maybe a fused disconnect outside if it calls for one. the internal overload protection is protecting the wires not the breaker.
On a regular motor you would size your breakers based on 430.52 - on an AC unit the nameplate give you the max breaker size - they did the work for you essentially.

The nameplate also already did the work for you telling you the min. circuit ampacity - the 75 degree C column #12 is good for 25 amps - you should always use the 75 degree C column for sizing wire for motors.

this is good to know when bidding jobs because you will be quoting smalled gauge wire when the other guy is sizing wire to breakers - on small stuff it isn't that much of a big deal but on large wires it makes a big difference.

I have refused to believe it myself before but this is all true.

 

[daleuger]

I understand it just fine I'm just pointing out how a lot of people (sometimes myself included) can get their head wrapped around

.

I've also be taught tha basic 14 ga = 15 amps, 12 ga = 20 amps, 10 ga = 30 amps all my life.

Every single one of us has been guilty of that logic at one time or another if we'll admit it.

 

[CopperTone]

there are inpectors out there that don't understand this so if someone doesn't fully understand it - you should because some day some inspector will try to crush your stones over this. And they are wrong.

 

[peter d]

I've also be taught tha basic 14 ga = 15 amps, 12 ga = 20 amps, 10 ga = 30 amps all my life.

Yes, we were all taught that. And it's correct information. 240.4(D)

BUT, there are exceptions to that rule, and 240.4(G) provides those exceptions. An motor/compressor load happens to be one of those BIG exceptions that we can use to our advantage, as CopperTone said.