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Mini split wiring check-bad/unclear instructions

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PCustoms

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Quoting some of my posts from over in the HVAC sub forum as it's purely electrical:

PCustoms said:
Check my plan for wiring a Mr. Cool 18k unit, as I think I'm wrong/their chart *****:

12-2 NM-B
25A breaker
Non-fused disconnect

Screenshot_20240525-174318.png
Click to expand...

PCustoms said:
Well physical unit arrived and, surprise, is marked differently then the manual online.

IMG_20240531_072803820.jpg
  • new manual calls for 15.5A MCA, MOP 25A
    • My plan is still 12/2 on a 25A breaker
  • unit is marked Max Fuse 25A
Question 1: other then code, what is the purpose of a fuse instead of breaker? Seems to be a lot of thought online this is an unintentional discrepancy with the manual

Question 2: is there a minimum height for the exterior disconnect? I'd like to exit the basement in a joist bay, which leaves the disconnect lower then I'd normally mount one. Other option (I see) is a interior junction box, thwn and an LB/conduit up the exterior, which is less then desirable.
Click to expand...
 
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dscheidt

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PCustoms said:
I know.

Code says if spec says "MOP" then fuse or breaker is OK, but if spec says "Max Fuse" then a fuse is required.

Spec says both....
Click to expand...

nameplate on equipment is controlling. Specs change faster than the documents fairly often, or components are changed without updating the user manual.
 
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mm08822

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It looks like the online manual is referring to 60C rated conductor ratings. BTW, their 36KBTU sizing is wrong.

The manual that came with the unit appears to be following 90C conductor ratings.

MCA = 125% of largest load + 100% of the sum of any other load.

36K Example:
Assume condenser = 20A, fan = 2A
MCA = 1.25*20A + 2A = 25A = 2A = 27A >> #10 @ 60C
MOP = 2.25 * 20 + 2 = 45 + 2 = 47A, so MOP derates to 45A = std cb/fuse size.)

For OP's situation:
MCA = 15.5A (assume 10.8 condenser + 2A fan = 1.25* 10.8 + 2 = 13.5 + 2 = 15.5A > #12 @ 60C, #12 @ 90C)
MOP = 25A (2.25 * 10.8 + 2 = 23.8 + 2 = 25.8A, so MOP derates to 25A = std cb/fuse size.)
 
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PCustoms

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wyliesdiesels said:
Screenshot the nameplate and post it here
Click to expand...
I stupidly didn't snap a pic when I had it out of the box.

I clearly says "max fuse"....

I'll get a pic later today or tomorrow.

While I know it's not "correct", with the BS between their labels and the manual, what's the potential risk of using just a breaker?

Also, how low can a disconnect be mounted?
 
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Norcal

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If the listing calls for fuses then fuses are required, as to the disco height, use common sense, somebody servicing the unit/disco is not going want to roll around in the dirt.
 
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mm08822

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Read my reply in the same thread posted in Heating/Cooling forum.......come up into the stud bay and then out wall.
 
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PCustoms

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mm08822 said:
Read my reply in the same thread posted in Heating/Cooling forum.......come up into the stud bay and then out wall.
Click to expand...
Apologies on the 2nd thread. The other one died off.

I've had limited success coming up the stud bay, based on past attempts not sure I want to try that again.

As for future service, that would be me. It won't exactly be rolling around on the ground, but I'd much prefer setting it at 3-4' from ground level.
 
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PCustoms

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PCustoms said:
I stupidly didn't snap a pic when I had it out of the box.

I clearly says "max fuse"....

I'll get a pic later today or tomorrow.

While I know it's not "correct", with the BS between their labels and the manual, what's the potential risk of using just a breaker?

Also, how low can a disconnect be mounted?
Click to expand...

IMG_20240531_123128021.jpg
 
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dscheidt

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PCustoms said:
Also, how low can a disconnect be mounted?
Click to expand...

in general, the NEC doesn't specify a minimum mounting height. The center of the handle needs to be at or below 2m, in whatever the highest position is. They usually get mounted at roughly the same height as the compressor. In VT, I think you'd worry about being high enough to not get it covered in snow.
 
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PCustoms

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Tundra1 said:
Keep in mind you can get a fused disconnect for about the same price as nonfused. So other than buying fuses it isn't much difference.
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I already bought the non-fused based off the instruction manual.

Now I need to return it and buy a fused. And fuses.
 
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wyliesdiesels

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klassenl said:
Where I come from a breaker would be sufficient. In this setting "max fuse" would be interpreted as meaning maximum setting of the over current protection (be it a fuse or breaker).
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Wrong

A breaker and a fuse have different trip/blow characteristics. They are not the same. Theres a reason the manufacturer says to use fuses and code requires you to follow manufacturer instructions. So what youre doing is a code violation
 
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PCustoms

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wyliesdiesels said:
A breaker and a fuse have different trip/blow characteristics. They are not the same. Theres a reason the manufacturer says to use fuses and code requires you to follow manufacturer instructions. So what youre doing is a code violation
Click to expand...

But what if the mfg says to use a breaker on one spot, and further in they say use fuses?

There's a bunch of other discrepancies in the manual too.
 
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klassenl

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wyliesdiesels said:
Wrong

A breaker and a fuse have different trip/blow characteristics. They are not the same. Theres a reason the manufacturer says to use fuses and code requires you to follow manufacturer instructions. So what youre doing is a code violation
Click to expand...

I don't think we have established in this instance that the manual calls for fuses. It just says maximum fuse.

But if we're going to say that the manufacturer requires fuses, what kind of fuse should we use. Different kinds of fuses have different characteristics as well. Surely the manual will enlighten us.
 
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wyliesdiesels

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klassenl said:
I don't think we have established in this instance that the manual calls for fuses. It just says maximum fuse.

But if we're going to say that the manufacturer requires fuses, what kind of fuse should we use. Different kinds of fuses have different characteristics as well. Surely the manual will enlighten us.
Click to expand...
did you miss the nameplate pic in #12
 
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PCustoms

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wyliesdiesels said:
you go by the nameplate not the ****** manual which may have been translated a few times
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I bought a fused disconnect today. Just annoying....

So with a MCA of 15.5, would you breaker this at 20 or 25A?

If 20, try a 20A fuse?

I've got s bunch of 20A breakers unused. The 25A cost me almost $20...
 
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wyliesdiesels

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PCustoms said:
I bought a fused disconnect today. Just annoying....

So with a MCA of 15.5, would you breaker this at 20 or 25A?

If 20, try a 20A fuse?

I've got s bunch of 20A breakers unused. The 25A cost me almost $20...
Click to expand...
i would just use what the nameplate says- 25a fuse. for the breaker in your panel you could try a 20a
 
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PCustoms

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Closing this out.

I ended up going off a sub panel in my basement to shorten the run, so used the 25a breaker (the 20's are unused in my main panel).

After a second measurement I drilled through the top of my band joist and came out high enough to mount the disconnect. With the amount of spray foam on my sills and only having 2x4 wall there was no way I was drilling blind from below and feeding wire up through the wall without a struggle.higher might have been nicer, bu you can pull the fuse standing up so I'm fine with it.
 
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