Thread: any votes as to what size is run off this 40 Amp breaker?

P6270142.jpg

40 amp central air circuit. Looks smaller than #8 to me but I couldnt find any writing on the cable sheath. black one below is #14 AWG.

thoughts??

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Originally Posted by Lanny Freng

P6270142.jpg

40 amp central air circuit. Looks smaller than #8 to me but I couldnt find any writing on the cable sheath. black one below is #14 AWG.

thoughts??

Hello Larry,

It looks like a #10.

When you say it is is a "40 amp central air circuit" are you referring to the breaker size, the max breaker permitted according to the a/c ID tag or the circuit ampacity shown on the ID tag?

Based on your question I'm guessing that it is a 40 amp breaker. A central A/C does not necessarily need a #8. More info needed in order to determine what the correct wire size should be.

Sincerely,

Corey

yeah 40 amp breaker to the C/A. #8 is the minimum that can be run on a 40 am breaker isnt it?

Lanny (not Larry) - That definitely doesn't look like #8. I assume it's a 40 amp breaker on account of your title: "any votes as to what size is run off this 40 Amp breaker?".

Originally Posted by Lanny Freng

yeah 40 amp breaker to the C/A. #8 is the minimum that can be run on a 40 am breaker isnt it?

Hi Lanny,

Nope.

Corey

Without a doubt, too small for a 40amp breaker....

However, is that breaker necessary? The unit may only warrant a 20Amp or 30Amp breaker...then a 10AWG would be fine. They could change out the breaker easily.

It is allowable to undersize a wire for a breaker run to an AC unit.

Originally Posted by Todd Stevens

Without a doubt, too small for a 40amp breaker....

Todd,

Nope.

The a/c condenser unit nameplate may state 'Maximum HACR breaker 40 amps' and 'Minimum circuit ampacity 30 amps' and you can use a #10 AWG (30 amp rated conductor size) on the specified 40 amp breaker size.

All depends on the nameplate ratings.

By the way, those are 40 amp breakers as the label states THQL2140.

40 amp breaker is usually used for 3.5 to 4 ton units. Wire should normally be #8 for 40 amp but unit rating and draw would dictate final determination.
30 amp usually up to 3 ton.

Originally Posted by Markus Keller

40 amp breaker is usually used for 3.5 to 4 ton units. Wire should normally be #8 for 40 amp but unit rating and draw would dictate final determination.
30 amp usually up to 3 ton.

My 14 SEER 4 ton unit only requires a 25 amp breaker. The newer units are using less power and require less amps.

Seeing that old breaker/panel set-up I was guessing the unit is also an old clunker.
I find it odd that your 14 SEER would use a 25 amp breaker.

Jerry,

Interesting response. Not sure how you can have a 40 amp breaker and 10AWG wire, I can t find any documentation that would support that conclusion. A 10AWG wire should not support an overcurrent situation where the wire would carry more than 30A regardless of the appliance attached to the other end. Now...if the service disconnect had a 30A fuse, then it may not be an overfused situation at the panel.

BTW, I'm more interested in the dialogue than being right...so, would love to hear the basis and reference for your comment.

Regards,

Todd

Go to this PDF and look at page three at the bottom. You'll see that 23.8 amp load (#10 wire) can be on a 40 amp breaker per the manufactures instructions.


http://hvacpartsshop.net/HS29_Series_1_Ton.pdf

James,

While I see the chart could lead one down that path, I don't interpret that chart the same way you are. I read it as two separate requirements that have a gated relationship, and in elecrical theory would not treatment as mutally exclusive requirements. It is a bit like the relationship between a square and rectangle....square is always a rectangle, but a rectangle isn't a square. Stretching it, i know, but bear with me. :-)

Opt 1: I do see how a min circuit ampacity of 23.8 warrants a 10AWG (80% x 30=24amp) for the min wire size. I would match that with a 30A breaker. This config would support RLA and startup draw (more than RLA)and keep it safe. GOOD OPTION.

Opt 2: I also see that the recommended max fuse is 40A. If I choose to install a 40A breaker, then I would pull an 8AWG wire. GOOD OPTION.

Opt 3: If I striclty stick to MIN and MAX according to the chart, I would install a 40A and pull a 10AWG. This configuration allows for a problem in an overcurrent situation. If the motor starts straining (or any one of several other possible scenarios), and drawing more than 30A but less than 40A, the wire could overheat, melt, catch on fire, but the breaker would not trip. UNSAFE OPTION.

I wouldn't abandon safety because of a chart, but stick to best practices that keep the occupants safe. The chart even stipulates at the footnote

"*Refer to National or Canadian Electrical Code manual to determine wire, fuse and disconnect size requirements."

That's how I would interpret it and gladly welcome a dissenting opinion. This is how we all learn, and we run into this particular issue regularly.

Originally Posted by Todd Stevens

Not sure how you can have a 40 amp breaker and 10AWG wire, I can t find any documentation that would support that conclusion.

Todd,

Have you looked at the nameplate on each of the a/c units you inspect?

That nameplate rating is what rules (underlining is mine): NEC 110.3(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling. (Jerry's note: I.e., the nameplate.)

That means that when the nameplate states a minimum circuit ampacity of 5 amps and a maximum breaker size of 15 amps (see photo of nameplate from another thread at http://www.inspectionnews.net/home_i...lt-240v208.jpg ), then you have, and are allowed to have, the overcurrent device larger than the circuit ampacity.

It is not uncommon to find that the minimum required circuit ampacity is frequently 5-10 amp less than the maximum allowed overcurrent device.

This allows for compressor startup current to not trip the breaker (the breaker is sized large for this) and the conductor is still within its ampere rating.

If you go to Table 310.16, you will notice that #10 THW has a current rating of 35 amps and THHN has a rating of 40 amps. Thus, the conductor itself can handle the current.

You will also notice the "*" next to 10, the "*" states "See 240.4(D)"

240.4(D) states (underlining is mine):
"(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied."

Thus we need to go to 240.4(E) and (G).

(E) Tap Conductors. (we can forget this as this does not apply)

(G) Overcurrent Protection for Specific Conductor Applications. Overcurrent protection for the specific conductors shall be permitted to be provided as referenced in Table 240.4(G).

Table 240.4(G) lists conductors used for and what Article/Section to see, for a/c that is Article 440, Parts III and IV

The information in 440, Parts III and IV basically tells you how to figure it out, or, you could just rely on the manufacturer's information stated on the nameplate, which gets us back to 110.3(B) Installation and Use.

Based on the nameplate photo shown, if you sized the conductors properly according to minimum circuit size, they would (be default) match the maximum breaker size.

The 5 amp minimum circuit ampacity would be a #14, and a #14 is rated for 15 amps, which just happens to be the maximum breaker size shown.

Todd..

Your Opt 3 does meet code and therefore is not a repair item. To change the wire to match the overcurrent device is an improvement and can not be on the summary page in NC.

Originally Posted by Lanny Freng

P6270142.jpg

40 amp central air circuit. Looks smaller than #8 to me but I couldnt find any writing on the cable sheath. black one below is #14 AWG.

thoughts??

Originally Posted by Todd Stevens

Jerry,

Interesting response. Not sure how you can have a 40 amp breaker and 10AWG wire, I can t find any documentation that would support that conclusion. A 10AWG wire should not support an overcurrent situation where the wire would carry more than 30A regardless of the appliance attached to the other end. Now...if the service disconnect had a 30A fuse, then it may not be an overfused situation at the panel.

BTW, I'm more interested in the dialogue than being right...so, would love to hear the basis and reference for your comment.

Regards,

Todd

Lanny and Todd,

The label on the condenser unit gives you the information that you need. An example label is attached.

On the label that I provided, note the lines:

• MIN CKT AMPACITY 17.83 AMP
• MAX HACR CKT BKR UL 30 AMP
• MAX FUSE SIZE UL 30 AMP

(similar lines with CSA are for installations under the Canadian electrical code)

The code (IRC and NEC) says you have to follow what is on the label. Here's the applicable section from the IRC:

§E3602.11 Branch circuits for air-conditioning and heat pump equipment. The ampacity of the conductors supplying multimotor and combination load equipment shall not be less than the minimum circuit ampacity marked on the equipment. The branch-circuit overcurrent device rating shall be the size and type marked on the appliance and shall be listed for the specific purpose.

For the unit in my example, the label is telling us that the conductors on the circuit for the air conditioner should not be rated for anything less than 17.83 amps. The overcurrent device protecting that circuit cannot be any larger than 30 amps. For this unit, you could use either a fuse or a breaker. If you use a breaker, it must be of the HACR type. If the label had only an entry for a breaker, and no entry for a fuse, you could not use a fuse on that unit.

Say what? Brandon, are you saying that you could wire that a/c unit with 12 AWG copper and protect it with a 30 amp breaker, and it's ok? Yep, that's what I'm saying! How can that be?

There are two separate functions of an overcurrent protection device (OCPD). It protects against overloads, currents that can damage the circuit if allowed to continue for too long, and it protects against high currents, such as short circuits and ground faults, that can do immediate damage to the circuit. On most home branch circuits, the OCPD in the panel serves both functions. On dedicated circuits with large motors, the two functions are often separated.

An air conditioner compressor is a large motor. Motors draw a certain amount of current when they are running (called RLA or Run Load Amps), a higher current for a short duration while they are starting up, and an even higher current if the motor seizes up (called LRA or Locked-Rotor Amps). If you go back to my sample label you'll find a line that reads:

• COMPRESSOR 13.46 RLA 76.0 LRA

That RLA can creep up over time as the unit ages, or if the refrigerant charge on the system is not within specifications. There's also a fan motor inside the unit, and based on the label it draws about 1.0 amps under full load (FLA):

• FAN MOTOR 208/230 VOLTS 1.0 FLA

Now, putting all this info together. The manufacturer is telling you that the compressor draws about 13.5 amps when running under normal conditions and the fan about 1.0 amp. You shouldn't size the conductors for the circuit that is dedicated to the unit for anything less than 17.83 amps. You can put up to a 30 amp OCPD in the panel to protect that circuit.

If you used 12 AWG copper and put a 20 amp breaker on it, there is a high likelihood that the breaker would trip during the startup phase of the compressor motor. This is why they allow the OCPD in the distribution panel to be up-sized as high as 30 amps. The OCPD in the panel will trip and protect the circuit in the event the compressor motor seizes (suddenly drawing 76 amps) or if there is a short circuit or ground fault. Overload protection of the branch circuit conductors, resulting from drawing more than about 20 amps for an extended period of time, is provided inside the a/c unit, usually by devices that monitor some combination of motor temperature and current draw.

Since the overload protection on the branch circuit conductors is being provided by the a/c unit and not by the OCPD in the distribution panel, when you find this allowable situation of conductor size and OCPD mis-match, it is important to check that there are no additional loads tapped into the a/c circuit.

Originally Posted by Brandon Whitmore

It is allowable to undersize a wire for a breaker run to an AC unit.

Brandon W. - I would say that a little differently. It is allowable to oversize the breaker on the wire running to the a/c unit. (per specs on tha a/c label).

Originally Posted by Jerry Peck

Todd,

Have you looked at the nameplate on each of the a/c units you inspect?

Absolutely, it is a critical step in the inspection process in our opinion.

Also, for clarity:
MIN Circuit ampacity is not my issue. I think that is clear.
MAX breaker is not my issue. I think that is clear.

Matching is. I'll read the references cited and comment later.

Originally Posted by Todd Stevens

Absolutely, it is a critical step in the inspection process in our opinion.

Very good.

Also, for clarity:
MIN Circuit ampacity is not my issue.

Yes it is ... it is 'part of' your issue.

MAX breaker is not my issue.

Yes, it is 'part of' your issue.

Matching is.

Ah ... but ... "matching" what?

Being as you read the nameplate on the a/c in question (yours), post the following information from that nameplate"

- 1) Maximum HACR breaker / fuse size:

- 2) Minimum Circuit Ampacity:

Now go back to the "Matching is" your issue part ... did your installation "match" those specified requirements?

I.e., was the breaker size within the maximum on the nameplate? Yes is good.

I.e, was the circuit size ampacity at least the minimum on the nameplate? Yes is good.

Do *NOT* try to "match" those two as there is no "match" between them, only a "match to" the nameplate specifications.

Brandon W. - I would say that a little differently. It is allowable to oversize the breaker on the wire running to the a/c unit. (per specs on tha a/c label).

Agreed.


I looked up the pertinent info. last night.... spent about 20 minutes on it. I wrote up the explanation as to why it was allowed, but by the time I was done, I couldn't even understand it. Brandon and Jerry---- Thanks for writing it in a way that actually made sense.