Wiring for 36K MrCool DIY Mini-Split

[Slowbra]

Question for the electrical aspects of the DIY kits.

Does the 36K units require an 8 AWG wire to be ran or a 10 AWG? From what I’ve read previously everyone said 8, however, my electrician says otherwise. Read on..

I had an electrician come out today to provide an estimate for wiring. He looked at my 36K MrCool DIY unit and said that in North Carolina they do not base the wiring off of the maximum amps. They base the wiring off of the minimum circuit capacity.

The sticker on my 36K unit has 25A minimum capacity and they want to run a 10 AWG wire.

There are two versions of the manual. I’ve attached the screenshots of both. One is from online and the other is came with the unit.

So, do I need to run 10 AWG or 8 for this unit? If 8, why would a trusted electrician say otherwise? I was told I would need to have/show the 8 AWG requirement in writing to justify why it needs to be run (I can use the online version of the manual to prove that if I need to, but it would be more expensive especially because I already have an unused 10 AWG wire in that area they can use).

 

[dcg9381]

I believe your electrician is right - at least in terms of doing it the "least" expensive way and having it be compliant. You've got a good question though. It's definitely confusing.

For A/C units:
The breaker can be sized according to the "Maximum overcurrent device".
The conductors can be sized according to the "Minimum circuit ampacity".


I've got Daikin 24k units on a 15A breaker (manual says max breaker 20A) - so I know they are pulling under 12A constantly. I'm kinda surprised by the "big breaker" that this thing requires.

Note, your electrician is supported by the sticker on the A/C unit - "Minimum Circuit Ampacity 25A". As 10GA supports 30A, you're all good.

NEC has "special rules" - I believe - for motors and A/C that allow what I consider to be "odd combinations" of circuit breaker any wire. Odd combination meaning 200%+ more amps on the circuit breaker than what the wire will support sustained. Special cases for starting in-rush current would be my guess.

 

[Slowbra]

I believe your electrician is right - at least in terms of doing it the "least" expensive way and having it be compliant. You've got a good question though. It's definitely confusing.

For A/C units:
The breaker can be sized according to the "Maximum overcurrent device".
The conductors can be sized according to the "Minimum circuit ampacity".


I've got Daikin 24k units on a 15A breaker (manual says max breaker 20A) - so I know they are pulling under 12A constantly. I'm kinda surprised by the "big breaker" that this thing requires.

Note, your electrician is supported by the sticker on the A/C unit - "Minimum Circuit Ampacity 25A". As 10GA supports 30A, you're all good.

NEC has "special rules" - I believe - for motors and A/C that allow what I consider to be "odd combinations" of circuit breaker any wire. Odd combination meaning 200%+ more amps on the circuit breaker than what the wire will support sustained. Special cases for starting in-rush current would be my guess.

So what do you think the risk may be, if any, on using 10 versus 8? I’ll have a fuse right outside of the unit.

 

[LS6 Tommy]

Go with the what is on the printed documents that came with it and the unit tag- 25A minimum circuit capacity & 10Ga.

Tommy

 

[Slowbra]

Go with the what is on the printed documents that came with it and the unit tag- 25A minimum circuit capacity & 10Ga.

Tommy

Tommy,

It’s confusing. Printed documents are different between the online version and the version I received. Online specifically says the 36K takes 8 AWG. The manual that I have just talks about amps and size - suggests 25A takes 12 AWG. Both of these are in the original post above.

I just need to tell the electrician what the hell I want. And if it’s more than what he thinks I have to validate why. Last thing I want to do is run something that’s not gonna be ideal because of some confusion in the paperwork.

I am leaning toward just going with what the sticker on the unit says and the electricians professional experience - similar to what you are saying.

 

[LS6 Tommy]

I'd go with the material you received and the data plate on your unit- 25A minimum ampacity and 10Ga wire, 35A protection.

Tommy

 

[Bert_]

Nothing confusing here. The unit clearly says your circuit needs an ampacity of 25A. You don't do any extra math or anything you just look at the number. #10 Nm is good for 30A. 30 is greater than 25...

Now you size the overcurrent protection. Technically the unit calls for a fuse so the correct thing to use is a 35A fuse, not a breaker.

Most people would be fine with using a breaker. 35A is not a standard size for breakers so you could try a 30A or if it nusince trips switch it to a 40.

Those breaker/fuse sizes are all ok on #10 wire because this is a motor circuit.

The manual is clearly written by someone who doesn't understand sizing motor circuits. The unit really only pulls about 17.5A (fan + compressor)

 

[Slowbra]

Nothing confusing here. The unit clearly says your circuit needs an ampacity of 25A. You don't do any extra math or anything you just look at the number. #10 Nm is good for 30A. 30 is greater than 25...

Now you size the overcurrent protection. Technically the unit calls for a fuse so the correct thing to use is a 35A fuse, not a breaker.

Most people would be fine with using a breaker. 35A is not a standard size for breakers so you could try a 30A or if it nusince trips switch it to a 40.

Those breaker/fuse sizes are all ok on #10 wire because this is a motor circuit.

The manual is clearly written by someone who doesn't understand sizing motor circuits. The unit really only pulls about 17.5A (fan + compressor)

Thank you for the great explanation.

 

[mm08822]

I always use the nameplate values for sizing. Manuals can be slow to get updated.

If the electrician runs #10 NM-B and puts 35 amp fuse in, you are meeting the requirements of the unit.

Depending upon your panel mfr, you may be able to locate a 2 pole 35 Amp cb.

You also need a gfci receptacle with 25' of the unit. Maybe an existing recept covers this requirement. If not, have sparky add one.

 

[LS6 Tommy]

Now you size the overcurrent protection. Technically the unit calls for a fuse so the correct thing to use is a 35A fuse, not a breaker.

Thank you for bringing that up. I missed that when I posted my reply. If the manufacturer states "maximum fuse", or "maximum breaker", that's what needs to be used. If they just specify "maximum overcurrent protection", you can use either. What I would do in your case is to use a 35A HACR breaker in the panel, 10AWG NM-B cable to a 35A fused disconnect, then 10 AWG THHN in liquid tight flex to the unit.

Tommy

 

[dcg9381]

So what do you think the risk may be, if any, on using 10 versus 8? I’ll have a fuse right outside of the unit.

I do not think there is any risk. See above.

It just "seems odd" because we associated 35A with 8 gauge wire. As mentioned, you're fused and this unit pulls 25A (probably less) and your 10GA circuit is rated for 30A. The fuse is within correct specs for what's on this circuit.

 

[mm08822]

I believe your electrician is right - at least in terms of doing it the "least" expensive way and having it be compliant. You've got a good question though. It's definitely confusing.

For A/C units:
The breaker can be sized according to the "Maximum overcurrent device".
The conductors can be sized according to the "Minimum circuit ampacity".


I've got Daikin 24k units on a 15A breaker (manual says max breaker 20A) - so I know they are pulling under 12A constantly. I'm kinda surprised by the "big breaker" that this thing requires.

Note, your electrician is supported by the sticker on the A/C unit - "Minimum Circuit Ampacity 25A". As 10GA supports 30A, you're all good.

NEC has "special rules" - I believe - for motors and A/C that allow what I consider to be "odd combinations" of circuit breaker any wire. Odd combination meaning 200%+ more amps on the circuit breaker than what the wire will support sustained. Special cases for starting in-rush current would be my guess.

Yes, dedicated motor ckts have this degree of freedom to prevent nuisance trips upon start-up. Motor thermals protect the motors themselves.

The minimum circuit ampacity is 125% of the compressor motor rla + the blower fan amps.

The maximum opc value is based upon 175% of compressor motor rla's. Can even go to ~200-225% or so, IIRC.

 

[Slowbra]

Got it all wired up today - works great using the plan discussed in this thread.