Pre Wire for 36K Mr Cool

[EP24]

I have just installed and wired a new Mr Cool Mini Spit - DIY 3 ton 36K BTU (4 zone). Only 1 zone currently installed.
Unit turns on and blows cold for 30+ minutes, then the fuse in the electrical panel flips and turns everything off.

These are the specifications for the outdoor condenser (attached): Minimum Circuit Amp 30 Amp, Max Fuse 45 Amp.

30 amp double pole breaker in the electrical box.
30 amp AC shutoff box
10/2 wire

What can I do to fix this situation?

 

[American Locomotive]

You cannot upgrade to a bigger breaker on 10/2 wire.

I would do two things first:
- Check for voltage drop at the unit.
- Measure the current after 10-15 minutes when the unit is at full speed. See if its actually drawing 30+ amps, or if your breaker is tripping early.

Edit: You can put a larger breaker because there are exceptions for air conditioner. But a 30A breaker is well over the unit's 20A running current, so something isn't right.

 

[mike93lx]

You cannot upgrade to a bigger breaker on 10/2 wire.

I don't think this is true. My ac's are breakered at 25 (edit, correcting as I said 35 originally) with 12/2 and it's compliant. I'll have to dig up the code reference

Edit: here is the discussion from my inquiry

25A breaker with 12/2 NM-B

I think I know the answer already, but asking anyway. House was built in 2003, a/c systems are all original. I had previously noticed that my 1st and 2nd floor a/c systems (identical systems) used 10/2 nm-b and 25 a breakers. It caught my eye as it wasn't a 30a breaker but smaller is ok In...

www.garagejournal.com

 

[kbeefy]

I was under the impression a 36k would run off a 10/2, but reading the install guide it calls for 6-8 guage.
I haven't plugged mine in yet, but hopefully by mid week.

 

[American Locomotive]

I don't think this is true. My ac's are breakered at 35 with 12/2 and it's compliant. I'll have to dig up the code reference

Edit: here is the discussion from my inquiry

25A breaker with 12/2 NM-B

I think I know the answer already, but asking anyway. House was built in 2003, a/c systems are all original. I had previously noticed that my 1st and 2nd floor a/c systems (identical systems) used 10/2 nm-b and 25 a breakers. It caught my eye as it wasn't a 30a breaker but smaller is ok In...

www.garagejournal.com

Actually, you are correct. Wire size is based minimum circuit current (30A). In this case the largest breaker he could run would be a 45A.

I would still check for current draw and voltage drop, because a 20A unit should not be tripping a 30A breaker.

 

[mike93lx]

Actually, you are correct. Wire size is based minimum circuit current (30A). In this case the largest breaker he could run would be a 45A.

I would still check for current draw and voltage drop.

I'd check the same, and barring anything way off, I'd be throwing in a 40a breaker.

@EP24 is this a brand new install or was it working fine for a while? Is the coil dirty?

 

[EP24]

I'd check the same, and barring anything way off, I'd be throwing in a 40a breaker.

@EP24 is this a brand new install or was it working fine for a while? Is the coil dirty?

Brand new install.

 

[EP24]

FYI....This is a 4 zone unit. Only one zone currently installed.

 

[sparky 1971]

The minimum circuit ampacity is 30, you have that covered with the 10/2 wire. Put in a 40 or 45 amp breaker. The 40 will be much easier to get. This goes against what everyone thinks based on 310.16, but motors and AC units have a different set of rules.

The breaker is oversized for the start up current of the condensing unit.

 

[EP24]

The minimum circuit ampacity is 30, you have that covered with the 10/2 wire. Put in a 40 or 45 amp breaker. The 40 will be much easier to get. This goes against what everyone thinks based on 310.16, but motors and AC units have a different set of rules.

The breaker is oversized for the start up current of the condensing unit.

Can I have a 40 in the breaker and only 30 on the AC cutoff?

 

[mike93lx]

Can I have a 40 in the breaker and only 30 on the AC cutoff?

Sure. Might mean the fuse pops next, but that can be dealt with separately. I don't have fuses in my disconnects, but it's an older install so maybe that requirement has changed

 

[American Locomotive]

The minimum circuit ampacity is 30, you have that covered with the 10/2 wire. Put in a 40 or 45 amp breaker. The 40 will be much easier to get. This goes against what everyone thinks based on 310.16, but motors and AC units have a different set of rules.

The breaker is oversized for the start up current of the condensing unit.

This is an inverter unit, there really isn't a "start up inrush current" and the fact that the unit trips after 30 minutes of continuous operation shows that start up isn't the issue.

A really don't think a unit rated for 20 amps nominal should be tripping a 30a breaker after 30 minutes. Plus, I don't think Mr. Cool makes a 3-ton indoor head, so the unit shouldn't be getting anywhere near maximum capacity.

OP needs to put a clamp meter to measure current and check voltage drop before slapping a bigger breaker in.

I did flip through the manual for the unit, and for some reason the manual says 6 gauge wire minimum. So there is a discrepancy there,

 

[sparky 1971]

Can I have a 40 in the breaker and only 30 on the AC cutoff?

Yes, but if you have a 30 amp fused disconnect, get rid of it and install a 60 amp non fused. They are only about $15. Kind of hard to find at times though.

 

[wyliesdiesels]

Theres something everyone missed here

The unit calls for fuses ONLY.

He should have max 45a fuses in the disconnect.

 

[bluedog225]

I would have thought that a multi head unit would be precharged with enough refrigerant for all lines. With only one installed, the reservoir may be overflowing and putting liquid refrigerant into the compressor. Bad or very bad.

I’m just an onlooker in the mini-split world. May be worth some thought. And maybe stop running it until you pin this down.

Good luck.

 

[sparky 1971]

Theres something everyone missed here

The unit calls for fuses ONLY.

He should have max 45a fuses in the disconnect.

I always hate to assume, but I'm willing to bet the name plate says max fuse or breaker size. I looked up the install manual for a 36k unit and it says MOP of 40 and MCA of 25. It also says 12 or 10 AWG wire.

EDIT. I looked up the single zone, not the four zone. The four zone says MOP of 45, so a breaker is fine.

 

[wyliesdiesels]

I always hate to assume, but I'm willing to bet the name plate says max fuse or breaker size. I looked up the install manual for a 36k unit and it says MOP of 40 and MCA of 25. It also says 12 or 10 AWG wire.

EDIT. I looked up the single zone, not the four zone. The four zone says MOP of 45, so a breaker is fine.

He posted the nameplate in the OP. It says nothing about a breaker, only a fuse. Literally says “max fuse 45a”

 

[sparky 1971]

He posted the nameplate in the OP. It says nothing about a breaker, only a fuse. Literally says “max fuse 45a”

I believe you. I can't zoom in with my tablet for some reason so I can't read it. I went to the install manual. Weird that they are calling it DIY but specified fuses.

 

[mike93lx]

He posted the nameplate in the OP. It says nothing about a breaker, only a fuse. Literally says “max fuse 45a”

Does that mean "fuse required"? I would take it more as "the max fuse allowable is 45a", which wouldn't preclude a breaker. If a fuse was required, I'd expect that to be stated.

But I am not an electrician

 

[grounded-b]

Does that mean "fuse required"? I would take it more as "the max fuse allowable is 45a", which wouldn't preclude a breaker. If a fuse was required, I'd expect that to be stated.

But I am not an electrician

If it says Max FUSE size, then you MUST use fuses.

Fuses have a different "trip curve" than circuit breakers.

 

[sparky 1971]

Does that mean "fuse required"? I would take it more as "the max fuse allowable is 45a", which wouldn't preclude a breaker. If a fuse was required, I'd expect that to be stated.

But I am not an electrician

If it says fuse, that's what it means. Usually it's "maximum overcurrent protection" or "max fuse or breaker size . I can't say that I've ever seen max fuse size only, and since I can't zoom in, still haven't.

 

[mike93lx]

Thanks guys

 

[vrinner]

I would have thought that a multi head unit would be precharged with enough refrigerant for all lines. With only one installed, the reservoir may be overflowing and putting liquid refrigerant into the compressor. Bad or very bad.

I’m just an onlooker in the mini-split world. May be worth some thought. And maybe stop running it until you pin this down.

Good luck.

I'm the same as @bluedog225 and just an onlooker in the mini-split world but I do remember reading that with a 4zone unit you have to hook up at least 2 of them.

This is from Mr. Cool website.

• Q. On the Multi-Zone do you have to connect all of your air handlers to your condenser?
  A.Some bundles have fewer air handlers than available ports on the condenser, so it is based on the capacity of the air handler.
  
  You only need to have a minimum of ⅔ load – i.e. if it’s a 3 zone and you buy 3 9K air handlers for it – you can connect it with just 2 to start

Not sure this has anything to do with the OP's question/issue but might have something do do with it.

 

[bluedog225]

Well. If the compressor is under load trying to pump fluid, that may create additional draw and blow the fuse (or trip the breaker). OP said it happens after 30 min.

I suspect this is covered in the manual.

My understanding is also that such a condition would destroy (or damage) the compressor. But maybe (hopefully) that happens over time.

I’ve got a little contact anxiety for the OP just participating in this thread. Hope it turns out ok.

 

[wyliesdiesels]

Does that mean "fuse required"? I would take it more as "the max fuse allowable is 45a", which wouldn't preclude a breaker. If a fuse was required, I'd expect that to be stated.

But I am not an electrician

if a breaker was allowed the nameplate would say max fuse or breaker.

in this case the nameplate only lists fuses so fuses are required.

 

[EP24]

Quick update.... threw a 40 amp in the breaker and system seems to be running without a problem.
Ill be completing the install of the second indoor unit tomorrow, lets see if its still working well.

Thanks all for your tips and recommendations.

 

[American Locomotive]

You really should measure the current being used.

 

[PWilks]

Well. If the compressor is under load trying to pump fluid, that may create additional draw and blow the fuse (or trip the breaker). OP said it happens after 30 min.

I suspect this is covered in the manual.

My understanding is also that such a condition would destroy (or damage) the compressor. But maybe (hopefully) that happens over time.

I’ve got a little contact anxiety for the OP just participating in this thread. Hope it turns out ok.

This…

 

[Viper98912]

I'm also in the camp that something isn't working right if you're theoretically running a low load and your current draw is exceedingly high

 

[paranoid56]

i would call mrcool also, when i talked about them about this exact thing, i was told i couldnt run a single unit on their 4 zone setup, i had to have 2 zones hooked up.