Electrical Question

Sandwalker

We just had a new A/C and heat pump installed with an air handler in our attic that controls the temperature for the second floor of our house. Previously there were only heat strips in for this portion of our house, and they required a 60 amp breaker. The new system has a max 45 amp breaker, and the installer recommended a 30 amp breaker (I'm not sure why the difference). The electrician came out today to change out the breaker and said that the wire was too large for the new smaller breaker. His plan is to leave in the 60 amp breaker, but to change the disconnect in the attic instead. Is this a viable alternative? I have little to no working knowledge of how these things work and want to make sure that I'm not risking a house fire. Any information would be appreciated!

Jamie Hall

There is no such thing as a wire "too large" for a breaker -- unless it is physically too large; that is, it won't fit into the terminals properly. That does happen, and it may be the case in this situation although I would rather doubt it.

As to the 45 amp vs. the 30 amp breaker to protect the equipment... generally is best to go with the manufacturer's recommendation, but if the installer is willing to state that the 30 amp breaker is correct for the equipment -- in writing -- there would be no real harm to trying it.

Is there a separate fused or breakered disconnect in the attic? It is readily accessible? If it's not readily accessible, I'd not be really happy with it; I've never been keen on having fuses or breakers scattered around the building here and there...

JUGHNE

The old 60 amp breaker might have been fed with #4 aluminum wire which could be physically too large for the lugs of 30 amp CB.

I would do the simpler thing of changing the CB in the main breaker panel that feeds the disconnect in the attic. If any problem the breaker in the main panel would trip first. The attic disconnect would be only that....a service disconnect, regardless of the fuse or CB size in it.

Now in the main panel there may be the problem of big conductor in small CB lugs, I have spliced on smaller copper pigtails with compression connectors inside the main panel in this case. Main panel box has usually enough volume for such splices. Air handler and attic disconnect might not.

You must now have less back up strip heaters than previously.
60 amp would usually feed 10 KW ( 2 X 5) heaters.
30 amp will give you 5 KW (1 strip typically)
The logic is that the heat pump will give you the extra heat, maybe, depends where you are located.

Koan

For my education, could this also be because the new contractor is basing wire size on MCO (Maximum Current Overprotection) rather than MCA (Maximum Circuit Ampacity)? I was looking at a Lennox 4 ton AC unit, the MCO is 50 amps but the MCA is 29.2 amps. So I assume one would base the wire on 30 amps (or maybe 40 amps) but use a 50 amp breaker bc of startup current. Is that how it is done?
I appreciate anyone shedding light on this,

Thanks.

Jamie Hall

It's really not rocket science, but it can get confusing. For powered equipment, such as your AC unit, the manufacturer will specify a particular overcurrent protection. Sometimes this is built into the unit; sometimes it will be a separate starter and overcurrent protection system. Either way, the only safe and appropriate way to protect the unit is to use exactly the equipment the manufacturer specifies. End of story for the equipment.

Now there may also be overcurrent protection for the wiring from a main distribution panel to the unit's own overcurrent protection, assuming that that is separate. In that case, that overcurrent protection must trip at less than the current rating of the wiring it is protecting -- or to put it another way, the wiring must be rated at at least the rating of the overcurrent device.

If the only overcurrent protection is at the main distribution panel -- which will be the case for most smaller machniery -- then that protection should be what the manufacturer recommends for the unit, and the wiring must be big enough to handle the maximum current allowed by the overcurrent device. It can be bigger, but not smaller.

One of the reasons it can get confusing is that some units require fast trip protection -- most electronic equipment, for instance -- while some units require slow blow protection, to allow for starting currents -- units with larger motors, for instance. In either case, though, the overcurrent device will have a definite carry rating (such as 45 amps, or 60 amps, or whatever) and that is the rating which determines the minimum wire size.

JUGHNE

The OP above had a simple air handler with strip heater and a fractional HP blower motor.

Your 4 ton AC with the compressor allows the MCO to be 50 amps, you could put smaller but may have nuisance tripping on some hard start days.

The MCA is Minimum Circuit Ampacity of 29.2amps which would require a minimum of #10 AWG if within a reasonable distance from CB. If there was much length involved I might consider #8 AWG.

Voltage drop must be considered, AC I&O manuals have a chart concerning length of conductors and load.

Hope this helps.

Koan

@JUGHNE Great - so I had it correct. The breaker has a higher current rating that the normal circuit wiring it protects. Minimum size for the wire would be for 29.2 amps and 5% (NEC max) voltage drop even though the breaker is 50 amps. It is a fairly short run - assume 40 feet.

A #10 wire over 40 feet at 30 amps 230v is only 1.1% voltage drop. I can go to 73 feet at 2% voltage drop with #10 wire that is not buried. even 100 feet is less than 3% drop. I have to use #10 anyway because of the 30 amps.

Just seems crazy using a #10 wire with a 50 amp breaker. I guess that is to stop nuisance tripping on start up.

If I went to #8 the drop would be 0.7% at 40 feet. I figure anything less than 2% drop is good, and don't see the need to go from #10 to #8 to get from 1.1% drop to 0.7% drop.

Koan

@Jamie Hall normally I would agree, however it seems the wire of circuit for residential AC condensers is designed to handle a certain level of capacity (30 amps) and the breaker can then be well over that level (50 amps) to handle start up current.

ratio

You size your conductors per the mfgr's nameplate MCA and your overcurrent protection device (circuit breaker/fuses) per the mfgr's nameplate MOCP (note that occasionally they specify fuses or circuit breakers). You can if course go larger on the wire size, in particular pay attention to voltage drop on longer runs, and you can go smaller on overcurrent protection, but by code you are permitted to "oversize" the breaker in relation to the conductor ampacity. It's not an uncommon argument I have with electricians around here.

ChrisJ

@Fred Would have a heart attack if he knew it's often 100% acceptable to run a 50A breaker with #10 wire under certain conditions. I just learned about this last night in a big argument on FB.

JUGHNE

If you go to the "big book of exceptions", AKA know as the NEC, my translation of article of 440.22 allows your CB/fuses to be 175% of the rated load current of the nameplate on the compressor itself. Then if that does not allow starting you can go to 225% of rated load current.

The MOCP and MCA info is thankfully placed on the outside of the unit to avoid standing on your head to read the compressor NP, doing the math and having lengthy discussions with other trades and especially AHJ of NEC. (also keeps your head from exploding reading the NEC )

440.31-32 says the branch circuit conductors have to carry at least 125% of comp rated-load. There is more to the can of worms than this simple statement.
Depending on other factors #10 THHN can be considered able to handle 40amps. (see exceptions )

Sorry Fred.

ChrisJ

https://youtu.be/9lPDYEAgWD4

Jamie Hall

"Just seems crazy using a #10 wire with a 50 amp breaker. I guess that is to stop nuisance tripping on start up. "

Maybe times have changed; I haven't had to enforce the NEC in quite some years. However, I can tell you this: when I was a code enforcement official had I caught you using #10 wire and a 50 amp breaker, you would have been red tagged at the meter until you had gotten it fixed.

No ifs, ands, or buts. There is no excuse for doing that. You can make the breaker size anything you want, to handle nuisance trips or whatever -- but so far as I was concerned, the wire itself had to be sized to the breaker, not the load. Period.

ChrisJ

Jamie Hall said:

"Just seems crazy using a #10 wire with a 50 amp breaker. I guess that is to stop nuisance tripping on start up. "

Maybe times have changed; I haven't had to enforce the NEC in quite some years. However, I can tell you this: when I was a code enforcement official had I caught you using #10 wire and a 50 amp breaker, you would have been red tagged at the meter until you had gotten it fixed.

No ifs, ands, or buts. There is no excuse for doing that.

See the video I posted above. It explains it and even references where and why NEC allows this.

You enforce code (in your case, used to), you don't get to create your own.

Fred

Don't apologize to me @JUGHNE . I wouldn't use #10 on a 50 amp circuit under any circumstance. Apparrently @ChrisJ would/will??. For me, (and I have a 50 amp circuit on my 5 ton AC) I use #8 wire. I use #10 on 30 and 40 Amp circuits. Call me crazy but I believe better safe than sorry. Also, I always feel like it's an investment in the future, if I need or want to upgrade some equipment, in the future. The wiring is there, just a breaker change out. If you have to pull wire once, for the incremental cost of the next gauge wire, it just makes sense for me.

JUGHNE

Chris, perhaps you have never had to deal with AHJ.
I don't argue with the Trooper who says I was 10 mph over limit.

Jamie if you were the AHJ on this job and we had a discussion looking at the AC label, I would concede and put a 40 amp CB in and you may possibly meet the middle and allow the 40.

Quite often the smaller CB will work, the manf goes by the NEC and wants to avoid nuisance calls. I do some 2 ton units that allow a 25, I get by with a 20 with 12 wire.

ChrisJ

JUGHNE said:

Chris, perhaps you have never had to deal with AHJ.
I don't argue with the Trooper who says I was 10 mph over limit.

Jamie if you were the AHJ on this job and we had a discussion looking at the AC label, I would concede and put a 40 amp CB in and you may possibly meet the middle and allow the 40.

Quite often the smaller CB will work, the manf goes by the NEC and wants to avoid nuisance calls. I do some 2 ton units that allow a 25, I get by with a 20 with 12 wire.

If I get pulled over when I was doing the speed limit or under and am accused of doing 10 over, I will respectfully disagree with the officer.

Which, is the case here as well.

Jamie said he would red tag someone for following NEC. How is that appropriate? I am always respectful towards @Jamie Hall as well. Though it may not seem like it via text, I'm not sure.

The man maintains a museum, how can you not be respectful to him?

@Fred Actually, I ran 10-2 wire to my outdoor unit and fused it at 30A. But, I could've used 12-2 with a 30A breaker and been completely legal.

Fred

Good for you @ChrisJ !

Jamie Hall

@ChrisJ -- I imagine it depends -- a lot -- on how one interprets the code, and I would add that -- as I said -- it would appear that the code has changed in the four decades since I was in that business. Which doesn't surprise me. And I certainly would have done what @JUGHNE said, and quite possibly allowed the 40 -- particularly if the device in question was already installed and hard wired, and had its own overcurrent protection. My interpretation came as follows, though, and you are welcome to respectfully disagree: If I see a breaker, I expected the wire -- all of it -- to be of the size required by the size of the breaker. 50 amp? Number 10. I did not enquire as to what was attached to the other end of the wire; that made no difference (well, not quite true -- long runs I might have had a word to say about voltage drop, but that was rare). The reasoning was that, in general, I had no control over what future changes might be made in the equipment (or whatever) was attached to the other end.

Consider, as an extreme but simple example, a lighting circuit in a home. It is now, as I understand it, required to be #12 or #14, protected, respectively, by 20 amp or 15 amp breakers. But it only powers 3 150 watt flood lights in the kitchen. About 5 amperes. Should I be able to wire that with 20 gauge? I hope not!

For what little it's worth, I can only recall red tagging three or four jobs in the years I inspected -- and they were all out of State contractors who were sure they had the right answers and I was just a country hick who didn't know anything!

JUGHNE

Chris, I was probably 7 MPH over, so that is what I mean.

Mike Holt is the man for training and layman interpretation of the NEC. We have used his training aids for continuing education for as long as I can recall. He also has a Handbook with practical applications including pictures. (Sort of like the Rosetta Stone for foreign languages). My inspector will use that handbook for examples but the NEC is the legal documented source that has the final word....subject to the AHJ. FWIW

Jamie, looking at the oldest NEC I have from 1965. Hermetic compressors were allowed 125% for branch circuit wire and 140% for CB oversizing. Maybe.....very hard to decipher....there was no article 440 at that time.

ChrisJ

@Jamie Hall This was specifically for AC units with internal protection.

I actually agree, I sleep better at night knowing the breakers in my panel protect all of my wires from over-current situations. Even if the compressor has an overload to protect it, and the wiring what happens if I get a partial short to ground? Or, if some neighbor decides to steal power? I bring this up, because I saw two instances of it on FB last night as well. Some neighbor tapped his RV into the guy's disconnect without telling him. Nice eh? Now you've got a #10 wire with a 40A or 50A CB and no external overload protection because of something a neighbor did while you weren't looking.

But, we were discussing what is and isn't allowed by NEC.

Like I said, I only learned this last night in a huge argument on Facebook. I never realized it was allowed and I'm kind of amazed. But, I am glad I learned it, it's interesting.

I also knew it would get @Fred going.

Jamie Hall

@JUGHNE -- That's probably the code edition I was playing with -- it was around that time. And I agree wholeheartedly that the blessed thing was a nightmare to understand and apply; probably still is. The National Plumbing Code of the day was no better -- we used to say that it was impossible to plumb a building more complicated than a privy and not have at least one code violation!

JUGHNE

Many early AC's had hard start components which would allow a smaller CB or fuse. Apparently fine tuning motor windings and the run cap have helped. The internal overload is quick to open if compressor does not take off. If you look inside any AC you have fed with #10, after that point you may see 14 going to the compressor. The rules really change after you are inside the machine.

ChrisJ

JUGHNE said:

Many early AC's had hard start components which would allow a smaller CB or fuse. Apparently fine tuning motor windings and the run cap have helped. The internal overload is quick to open if compressor does not take off. If you look inside any AC you have fed with #10, after that point you may see 14 going to the compressor. The rules really change after you are inside the machine.

I'm still considering putting a hard start in my unit even though it's not required.

The reason I haven't yet, is I don't entirely understand what it is.

Does it just provide more phase shift during starting? If so, why not just use a bigger cap?

JUGHNE

A 1965 paperback has a delicious nostalgic aroma to it doesn't it.

Today if you look at ampacity tables, #12 gives you a 30 amp rating...#14 has a 25 amp rating....exceptions apply of course. Many factors are then included in stirring the pot. More than one wants to think about unless needed.

However, elsewhere in the NEC it states that for residential use # 14 shall have no larger than a 15amp CB.....#12 no larger than a 20 amp.

One learns the code book for the work at hand and you may use only 25% of it, but you have to weed out the extraneous material.

Mike Holt and others have made an industry out of explaining a book written in English to people who can, for the most part read that language.

Jamie Hall

Basically kicks in a bigger capacitor for starting, to give more starting torque. Shouldn't really need it -- I hope -- on properly designed modern equipment. It supplements, not replaces, the existing starting capacitor, assuming there is one -- but not the running capacitor.

I do like three phase when you can get it, with proper motor starters...

ChrisJ

JUGHNE said:

A 1965 paperback has a delicious nostalgic aroma to it doesn't it.

Today if you look at ampacity tables, #12 gives you a 30 amp rating...#14 has a 25 amp rating....exceptions apply of course. Many factors are then included in stirring the pot. More than one wants to think about unless needed.

However, elsewhere in the NEC it states that for residential use # 14 shall have no larger than a 15amp CB.....#12 no larger than a 20 amp.

One learns the code book for the work at hand and you may use only 25% of it, but you have to weed out the extraneous material.

Mike Holt and others have made an industry out of explaining a book written in English to people who can, for the most part read that language.

I haven't watched any of his other videos, but I did like the one on this subject. Sometimes, things in a book can be hard to understand unless it is explained.

JUGHNE

Your existing cap is for running use only and always in the circuit. The motors will not start with out the proper matching run cap. If the MFD drops on a run cap than the run current will go up, but it will still seem to run OK. A reason to check run caps when servicing an AC. There are some replacement cheapies that might last only 1-2 years. I save some older run & start caps from change outs as some 20 YO will outlast newer. I will use them for temps if I don't have exact match. I am 100 miles from a supplier.

Start caps do what you imply for phase shift on the start winding...IIUC. They need a potential relay to pull them out of the circuit immediately after up to speed. If left in the circuit they can explode. (usually near your head....I know this to be true!)
There are now solid state devices with spade connections for a quick install that replace the start cap & relay.
What really helps a AC is the 5 minute time delay restart feature in most T-stats. This lets pressures equalize and allow an easy start. Sometimes included in the ACU itself. Manf have realized that short cycling really cooks compressors that are still under warranty. (Playing with T-stat or repeated power drops are an obvious issue.)

JUGHNE

The newer 3 phase AC I have seen have no motor starter with overloads. Only a 3 pole contactor. The OL are built into the compressor.
More than one service tech has been surprised to have piped in and evacuated a compressor and notices on the nameplate it says 3 Phase.....he was replacing a 1 Phase

Or one guy got a great deal on some used ACU's and had them on the roof of his store already when I told him he need a 3 phase service. .......really pays to read nameplates.

ChrisJ

JUGHNE said:

Your existing cap is for running use only and always in the circuit. The motors will not start with out the proper matching run cap. If the MFD drops on a run cap than the run current will go up, but it will still seem to run OK. A reason to check run caps when servicing an AC. There are some replacement cheapies that might last only 1-2 years. I save some older run & start caps from change outs as some 20 YO will outlast newer. I will use them for temps if I don't have exact match. I am 100 miles from a supplier.

Start caps do what you imply for phase shift on the start winding...IIUC. They need a potential relay to pull them out of the circuit immediately after up to speed. If left in the circuit they can explode. (usually near your head....I know this to be true!)
There are now solid state devices with spade connections for a quick install that replace the start cap & relay.
What really helps a AC is the 5 minute time delay restart feature in most T-stats. This lets pressures equalize and allow an easy start. Sometimes included in the ACU itself. Manf have realized that short cycling really cooks compressors that are still under warranty. (Playing with T-stat or repeated power drops are an obvious issue.)

Copeland claims their "compliant scroll" compressors do not need a delay, or for pressures to equalize.

Not sure how true it is.

All monitor tops in stock form do not need a delay either thanks to their unloader.

JUGHNE

One low temp refer unit I service has a scroll compressor the cond unit includes a time delay for restart. Apparently this model would do some back spin on shut down and if immediately restarted would run backwards. Won't work and sounds like hell.

For us the best use of the 5 minute delay is that out here in God's country he throws down a lot of lighting, makes us feel small in the whole schemes of things. We will get a lot of wink outs of power on the grid which runs a 100+ miles back and forth on the prairie. Out for less than 1 second then back on for maybe 10 seconds then repeat. 3 close strikes and you are actually out for maybe 5 minutes. All auto reset designed to protect the grid.

The rapid cycling not only pounding on the compressors but also any start components, the simplest being the contactor.
This cycling also puts a strain on the utility as they try to restart every unloaded compressor every time. The situation snowballs into a major load to pick up all at once considering the starting current required.

ChrisJ

JUGHNE said:

One low temp refer unit I service has a scroll compressor the cond unit includes a time delay for restart. Apparently this model would do some back spin on shut down and if immediately restarted would run backwards. Won't work and sounds like hell.

For us the best use of the 5 minute delay is that out here in God's country he throws down a lot of lighting, makes us feel small in the whole schemes of things. We will get a lot of wink outs of power on the grid which runs a 100+ miles back and forth on the prairie. Out for less than 1 second then back on for maybe 10 seconds then repeat. 3 close strikes and you are actually out for maybe 5 minutes. All auto reset designed to protect the grid.

The rapid cycling not only pounding on the compressors but also any start components, the simplest being the contactor.
This cycling also puts a strain on the utility as they try to restart every unloaded compressor every time. The situation snowballs into a major load to pick up all at once considering the starting current required.

So what you're saying is, required or not, the 5 minute delay is nice.

Koan

@Fred what is the nameplate MCA and MCOP for your 5 ton AC condenser? How long is the run form the box to the unit?

Fred

@Koan, It's on a second floor rear balcony and I can't get to the plate that easily but I will try, in the next day or two. It actually is not the original unit that I wired for which was a Trane 5 ton, 26 years ago. The current one is a Carrier replacement, installed two years ago. Only about 30 ft from the breaker box.

EBEBRATT-Ed

You have to trust that the electrician knows what he is doing...if he has a license. Leaving the 60 amp breaker is fine as long as he puts a fused disconnect in the attic to fuse down the wire. Nothing wrong with it, meets code and is the least expensive way out. The wires are too big for the 30 amp breaker. Pig tailing to smaller wires is done but is frowned upon in a panel due to space requirements (I know everyone does it) If the existing feeder is aluminum which it may be pig tailing to copper is a problem. Most breakers will only take wire 1 size larger than "normal" so a 30 amp breaker which would be "normal" with #10 would be able to take #8 with no problem.

Some would not hesitate to cut a few strands off......not me.

As far as breaker size with wire you cannot over fuse wire. #14...15 am breaker, #12 20 am breaker, #10 30 amp etc etc. That is the standard rule.

UNLESS IT"S A MOTOR CIRCUIT!! Motor circuits are completely different.

Follow the name plate on the unit, MCA (minimum circuit amps) is your wire size, MOCP (maximum overcurrent protection is your fuse or breaker)

What everyone forgets is that the motor, compressor whatever it is has thermal overload protection in it. This protects the circuit from serious overload.

The breaker or fuse is for short circuit protection.

Think of this.

If you size an equipment ground conductor (green or bare) you will find that a #10 equipment ground is legal with up to a 60 amp breaker.

WHY???

because they know that a #10 is plenty large enough to trip a 60 amp breaker

EBEBRATT-Ed

Why are we all banging on an old post

ChrisJ

question. #6 Nm b is good up to 55a. Can a 60a breaker be used on a sub panel fed by it?

hvacfreak2

Just to clarify , the purpose of the breaker in the panel is to protect the wire ( feeder ) connected to it and not the device served ( that may have been stated ) . Thermal protection in motors is to protect the device ( windings ). Fuses and thermal motor starters offer the best protection for motors ( and the overall motor circuits ) and are usually required for most 3 phase circuits. I am a fan of the ICM phase monitor products , they even have single phase monitors that seem worth the expense.

EBEBRATT-Ed

@ChrisJ 55 amps is the 60 deg C rating of the wire. I don't have my book handy but I think NMB is rated for a higher temperature probably 75 deg C. That being said to use the 75 deg rating all the wiring and terminations in that panel would have to be rated at 75 deg.

So after all that (sigh) probably not, at least not legally. Can you put a 50 amp breaker on it????

ChrisJ

> @EBEBRATT-Ed said:
> @ChrisJ 55 amps is the 60 deg C rating of the wire. I don't have my book handy but I think NMB is rated for a higher temperature probably 75 deg C. That being said to use the 75 deg rating all the wiring and terminations in that panel would have to be rated at 75 deg.
>
> So after all that (sigh) probably not, at least not legally. Can you put a 50 amp breaker on it????

I can and did. Was just curious.

hvacfreak2

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