breakers too large for appliance

[ericm]

I had an HVAC company install 24k Mitsubishi mini splits in my new shop. The day after they called and told me it went fine but that I need to replace the breakers with 20a ones. The breakers the electrician installed for those circuits was 25a or 30a, I can't remember which. Wiring was supposed to be suited to match and I assume it is. The spec I gave him was for Mr Cool 24k mini splits.

Of course I will replace the breakers with 20a ones. But I was wondering as a theoretical question, what would be the risk of using the larger breakers, or advantage of using the 20a ones? Do they trip sooner on a fault?

 

[mm08822]

Take a pic of the unit nameplate. There are 2 values of concern: the MCA and MOP...minimum circuit ampacity and maximum overcurrent protection. With that info, the correct changes can be determined.

 

[larry_g]

Maybe give us the model number of the unit? If it has more than one way to configure then give those details also.

lg

 

[mikedodge]

Check the manual or info sticker on it, one should say what the max breaker size is.

 

[kbuhagiar]

What is the size/gauge of the wire feeding the units? Shouldn't that determine the breaker size?

 

[mm08822]

What is the size/gauge of the wire feeding the units? Shouldn't that determine the breaker size?

No, it's a motor circuit basically. Has different requirements for short circuit and overload protection than a general purpose circuit.

 

[mike93lx]

What is the size/gauge of the wire feeding the units? Shouldn't that determine the breaker size?

Wire gauge is used to determine the max breaker size but the circuit doesn't require that size be used

 

[mm08822]

Wire gauge is used to determine the max breaker size but the circuit doesn't require that size be used

No........not in a motor circuit. AC unit is just a special case of a motor circuit. This random ac unit label is for 2 motors in the unit on the same circuit and calculated accordingly.

As a random example:

 

[PCustoms]

I bet $1 the OP unit has a nameplate that calls for a fuse and this can be fixed (if it's even wrong) at the disconnect...

 

[mike93lx]

No........not in a motor circuit. AC unit is just a special case of a motor circuit. This random ac unit label is for 2 motors in the unit on the same circuit and calculated accordingly.

As a random example:

I know that. Wire gauge is part of determining that limit

 

[manwithtools]

I bet $1 the OP unit has a nameplate that calls for a fuse and this can be fixed (if it's even wrong) at the disconnect...

Not likely with a modern Mitsubishi unit. They will typically allow a fuse or HACR breaker. Photo of the nameplate on my Mitsubishi 15k unit, note: "APPROVED FOR HACR BREAKERS OR TIME DELAY FUSES"

 

[PCustoms]

Not likely with a modern Mitsubishi unit.

I had Mr. Cool in my mind from reading it last night. Now that I read OP again I see it's a Mitsubishi installed...

I remember my Mr. Cool being a PITA if conflicting information until I had the actual unit/label in front of me

 

[mm08822]

I know that. Wire gauge is part of determining that limit

Not really.
MCA determines min conductor size from 125% of comp FLA plus 100% of fan fla.
MOC is determined from 175- 225% of comp FLA plus 100% of fan fla depending upon type of circuit protection permitted.

 

[mike93lx]

Not really.
MCA determines min conductor size from 125% of comp FLA plus 100% of fan fla.
MOC is determined from 175- 225% of comp FLA plus 100% of fan fla depending upon type of circuit protection permitted.

So I can use whatever size wire I want as long as I breaker to what the compressor mfr states?

That's my point. It's part of the equation

 

[mm08822]

So I can use whatever size wire I want as long as I breaker to what the compressor mfr states?

That's my point. It's part of the equation

You can use whatever size wire equal to or greater than required by mca.

Moc has nothing to do with wire size.

FLA's are the input into each different equation.
310.16B gets you to wire size once you have mca.

Moc could care less about 310.16B.

 

[justsam]

Interesting that the label states "minimum circuit capacity" is 23Amps. What would you get if you asked an electrician to install a 23 Amp minimum circuit? Seems like to be installed compliment with manufacture spec. That is what you would need to do and arguing over inductive or resistive, 80% rule etc. would have no bearing. Would it force a 10ga , 30 Amp circuit?

 

[mm08822]

Interesting that the label states "minimum circuit capacity" is 23Amps. What would you get if you asked an electrician to install a 23 Amp minimum circuit? Seems like to be installed compliment with manufacture spec. That is what you would need to do and arguing over inductive or resistive, 80% rule etc. would have no bearing. Would it force a 10ga , 30 Amp circuit?

Conductor ampacity limits are based on insulation temp rating and termination temp ratings.
Other derating factors could be present also.
For the simplest case, Sparky will run 10-2 nmb.

Mca covers the continuous load/125% branch circuit upgrade that a random motor circuit would otherwise need.

MCA and MOC are just dumbbed-down NEC code requirements for the HVAC crowd.

 

[wyliesdiesels]

post a pic of the nameplate...

 

[ericm]

Maybe I used too many words in the original post. The manual says 20a. What would be the advantage of a 20a vs 25a breaker?

 

[PCustoms]

Maybe I used too many words in the original post. The manual says 20a. What would be the advantage of a 20a vs 25a breaker?

Manual says 20a what?

Post the nameplate...

 

[mm08822]

Nameplate needed. The rest is just wasting time.

 

[sparky 1971]

It also wouldn't be the first time the HVAC contractor didn't have a clue. Twice I've installed a circuit for an AC when I had the spec's before it was installed. Once the spec was #14 wire and a 25 amp breaker, (I used #12 because that's what I had), the other was #12 and a 30. Both times the AC contractor told the customer I effed up because #12 is limited to a 20 amp breaker and no amount of explaining could convince anyone that I was right so I gave up and put 20's in. AFAIK, the smaller unit was fine, but the larger unit would occasionally trip the breaker on start up; of course, that was my fault. After about two weeks of back and forth I finally called an inspector friend of mine from another jurisdiction and had him tell them that I was right and the reason a 30 amp breaker was suggested by the manufacturer was to allow the compressor to start. I could have easily installed #10 just to shut everyone up, but out of principal I wasn't gonna do it, that would have been no different than admitting I was wrong in the first place.

 

[mm08822]

It also wouldn't be the first time the HVAC contractor didn't have a clue. Twice I've installed a circuit for an AC when I had the spec's before it was installed. Once the spec was #14 wire and a 25 amp breaker, (I used #12 because that's what I had), the other was #12 and a 30. Both times the AC contractor told the customer I effed up because #12 is limited to a 20 amp breaker and no amount of explaining could convince anyone that I was right so I gave up and put 20's in. AFAIK, the smaller unit was fine, but the larger unit would occasionally trip the breaker on start up; of course, that was my fault. After about two weeks of back and forth I finally called an inspector friend of mine from another jurisdiction and had him tell them that I was right and the reason a 30 amp breaker was suggested by the manufacturer was to allow the compressor to start. I could have easily installed #10 just to shut everyone up, but out of principal I wasn't gonna do it, that would have been no different than admitting I was wrong in the first place.

I would not have changed anything.

I would have shoved the mca/moc concept and the nameplate into their face and told them to go learn their trade.

 

[sparky 1971]

I would not have changed anything.

I would have shoved the mca/moc concept and the nameplate into their face and told them to go learn their trade.

The AC guys were long gone both times and I was dealing with a pissed of HO that had been convinced that I was a hack (which is true, but not in this case). I did call the HVAC company and when whoever I talked to pointed me to 310.16 I just gave up.

 

[theoldwizard1]

Maybe I used too many words in the original post. The manual says 20a. What would be the advantage of a 20a vs 25a breaker?

Breakers/fuses are sized to protect the wire to the load. If the wire is sized for 25A, no benefit for going to a 20A breaker.

The load "should" protect itself.

 

[mm08822]

The AC guys were long gone both times and I was dealing with a pissed of HO that had been convinced that I was a hack (which is true, but not in this case). I did call the HVAC company and when whoever I talked to pointed me to 310.16 I just gave up.

It should have been inspected mechanical and electrical.

An HVAC company that knows nothing about NEC 440 should shut down.

 

[Norcal]

Interesting that the label states "minimum circuit capacity" is 23Amps. What would you get if you asked an electrician to install a 23 Amp minimum circuit? Seems like to be installed compliment with manufacture spec. That is what you would need to do and arguing over inductive or resistive, 80% rule etc. would have no bearing. Would it force a 10ga , 30 Amp circuit?

If THHN/THWN conductors were used 12 AWG would be fine.

 

[sparky 1971]

It should have been inspected mechanical and electrical.

An inspection would require a permit... and that's something I don't do very often unless the POCO is involved.

An HVAC company that knows nothing about NEC 440 should shut down.

You wouldn't believe the number of "lectricians I've had that argument with. I almost made one guy cry when he saw I used #8 on an 80 amp breaker for a 7-1/2 HP single phase air compressor...

 

[mm08822]

An inspection would require a permit... and that's something I don't do very often unless the POCO is involved.

Wouldn't the AC replacement have triggered a mechanical permit required from HVAC contractor and then rolled into needing an electrical permit?

 

[sparky 1971]

Wouldn't the AC replacement have triggered a mechanical permit required from HVAC contractor and then rolled into needing an electrical permit?

Yes but this was a new install on an old house with no central AC. It should have been permitted, but nobody does it; get in, get done, get out. The only time I've ever pulled a permit for an AC install was a rental house and only because the rental inspectors check for things like that.

 

[micromind]

An inspection would require a permit... and that's something I don't do very often unless the POCO is involved.

You wouldn't believe the number of "lectricians I've had that argument with. I almost made one guy cry when he saw I used #8 on an 80 amp breaker for a 7-1/2 HP single phase air compressor...

That's the wire and breaker I would have chosen too.

Perfectly code compliant.

P.S. I don't get permits unless I absolutely have to.

 

[mm08822]

You wouldn't believe the number of "lectricians I've had that argument with. I almost made one guy cry when he saw I used #8 on an 80 amp breaker for a 7-1/2 HP single phase air compressor...

Maybe he was related to the lectrician who was the buddy of the GC doing a kitchen model for a previous customer. He told the customer my earlier 48a evse charger wiring was wrong. I got a call from the customer. He told me wires were undersized!
What??? I ran 2 #6 and 1 #10 grn. (Didn't I?, oh ****....)
I show up and find I did run what I mentioned. I calm down the HO and tell him all is ok. You want it inspected, no problem. Let's do it.
I find Sparky on the job and ask him WTF are you talking about? That's to code. Why are you slamming me with my customer.

He said we always run the same size grd as hot wire. We dont look at any table in 250.
DUMB MF'r. Ever looked at a piece of 6-2 nmb?? Huh??? Deer in headlights look.

 

[wyliesdiesels]

Maybe I used too many words in the original post. The manual says 20a. What would be the advantage of a 20a vs 25a breaker?

another request for a pic of the nameplate

 

[wyliesdiesels]

Breakers/fuses are sized to protect the wire to the load. If the wire is sized for 25A, no benefit for going to a 20A breaker.

The load "should" protect itself.

for AC motor circuits the breaker only provides protection for ground faults and short circuits

 

[wyliesdiesels]

He said we always run the same size grd as hot wire. We dont look at any table in 250.

wow what an idiot

 

[sparky 1971]

Maybe he was related to the lectrician who was the buddy of the GC doing a kitchen model for a previous customer. He told the customer my earlier 48a evse charger wiring was wrong. I got a call from the customer. He told me wires were undersized!
What??? I ran 2 #6 and 1 #10 grn. (Didn't I?, oh ****....)
I show up and find I did run what I mentioned. I calm down the HO and tell him all is ok. You want it inspected, no problem. Let's do it.
I find Sparky on the job and ask him WTF are you talking about? That's to code. Why are you slamming me with my customer.

He said we always run the same size grd as hot wire. We dont look at any table in 250.
DUMB MF'r. Ever looked at a piece of 6-2 nmb?? Huh??? Deer in headlights look.

He would have really beat me up when he saw I didn't pull a ground at all if EMT was used.

 

[Bert_]

An inspection would require a permit... and that's something I don't do very often unless the POCO is involved.

Glad I'm not the only one.

I like doing farm wiring, a permit and inspection is usually not required. Sometimes the service needs it. Once an inspector told me what came down the chain, "don't get in the way, unless it's a serious issue". They usually look at grounding and bonding then it's see you later!

I'm the next couple weeks I'll start installing a 600A 480V service. No inspection required.

 

[mm08822]

Glad I'm not the only one.

I like doing farm wiring, a permit and inspection is usually not required. Sometimes the service needs it. Once an inspector told me what came down the chain, "don't get in the way, unless it's a serious issue". They usually look at grounding and bonding then it's see you later!

I'm the next couple weeks I'll start installing a 600A 480V service. No inspection required.

Lucky you! I'm being harassed over a permit/inspection for changing a friggin meter pan!! POCO threatens no re-connect w/o cut-in card.

How hard can 6 wires be to connect and confirm correct? The lineman are laughing their tales off.

 

[theoldwizard1]

for AC motor circuits the breaker only provides protection for ground faults and short circuits

I have trip a few AC motor breaker from overloads !

 

[ericm]

another request for a pic of the nameplate

I'm not on site.

I wasn't asking for you guys to figure out what breakers to use. The manual says 20a and the HVAC company says 20a, so I will replace the 25 or 30a ones in there with 20s. I'm pretty sure they're 25s. What I was asking was what would the theoretical benefit be to using 20s on a circuit made for 25s. Is it just following the manual? Do the 20s trip easier? What sort of situation would happen where the 25s would not trip and the 20s would, and would it matter?

With automotive fuses, the smaller fuse burns out quicker, more likely saving the wiring in the case of a short. Is it the same for breakers? Or does a short trip the 20 and 25a breakers equally fast?