HVAC requirements

M

marmathsen

Senior Member
Location
Seattle, Washington ...ish
Occupation
Electrical Contractor
I'm confused. Is this saying I put a 40A breaker upstream at the panel supplying the branch circuit, and 70A fuses in the disconnect?
Or is it saying I can either used a 40A circuit breaker disconnect OR a 70A fused disconnect? <----That's a big difference

The circuit is powering outdoor and indoor units (ODU +IDU).

Rob G
Seattle

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Also strange that the MCA is 36/42 with breaker of 30/40 (and fuse of 64/70). I don't understand this nameplate either.
 
wwhitney said:
That nameplate is certainly confusing, I think they've put some figures from the spec sheet into the wrong blanks.

The spec sheet is here and is much clearer (page 2, Electrical):


Cheers, Wayne
Click to expand...
This seems to match up with the name plate. What is the difference between "Recommended Fuse/Breaker Size" and "MOCP"? Is it simply that I can install up to 70A OCPD but they recommend 40A? Or is it saying I should install a 40A circuit breaker at the panel AND 70A fuses at the disconnect? If so, why?

I don't know if I'm forgetting previous equipment but is it common to state separate values for circuit breaker vs fuse? Doesn't it normally state Max Fuse OR Circuit Breaker (unless only one type is allowed)?
 
wwhitney said:
The spec sheet is here and is much clearer (page 2, Electrical):
Click to expand...
The relevant excerpt is below. Seems clear enough to me, it just got garbled going onto the nameplate.

OCPD may be breaker or fuse, no difference in values. If the unit is powering a branch box, the recommended OCPD is 40; the MCA is 42; and the MOCP is 70. If the unit is not powering a branch box, the recommended OCPD is 30; the MCA is 36; and the MOCP is 64.

Odd that the recommended OCPD sizes are less than MCA.

Cheers, Wayne


Electrical Power RequirementsV AC / V AC, (Phases) ,Hz208 / 230, 1, 60
Guaranteed Voltage RangeV AC198-253
Voltage: Indoor - Outdoor, S1-S2V AC230
Short-circuit Current Rating (SCCR)kA5
Recommended Fuse/Breaker Size if Branch Box Powered by Outdoor UnitA40
Recommended Fuse/Breaker Size without Branch Box or Branch Box Powered SeparateA30
MCA if Branch Box Powered by Outdoor UnitA42
MOCP if Branch Box Powered by Outdoor UnitA70
MCA without Branch Box or Branch Box Powered SeparateA36
MOCP without Branch Box or Branch Box Powered SeparateA64
 
hbiss said:
Maximum breaker 40A. If you use a fuse instead it can be max 70A. Minimum circuit ampacity is 42A which means #10 wire. There is no fusible disconnect.

-Hal
Click to expand...
No, no no no no we went over this before on my issue. If it says minimum circuit capacity I thought we were under the consensus when I brought this up that minimum circuit capacity you have to have a breaker at least higher than that

Or is it just now since the manufacturer put a lower breaker that it’s perfectly fine.
 
hbiss said:
Maximum breaker 40A. If you use a fuse instead it can be max 70A. Minimum circuit ampacity is 42A which means #10 wire. There is no fusible disconnect.

-Hal
Click to expand...
The two items in bold are mutually exclusive as the circuit size is determined by the rating of the OCPD.
 
wwhitney said:
OCPD may be breaker or fuse, no difference in values. If the unit is powering a branch box, the recommended OCPD is 40; the MCA is 42; and the MOCP is 70. If the unit is not powering a branch box, the recommended OCPD is 30; the MCA is 36; and the MOCP is 64.
Click to expand...

Is that common to show a RECOMMENDED OCPD and a MAXIMUM OCPD? I don't remember seeing that before.

hbiss said:
Nobody caught my typo? "Minimum circuit ampacity is 42A which means #10 wire." No, should be #8 wire.
Click to expand...
I'm glad you corrected it, I had a response written out but hadn't had a chance to finish it. I was trying to figure out how the heck you got 10 AWG!

Rob G
Seattle
 
So I ended up calling Mitsubishi and they simply told me that the "recommended" breaker size ("Breaker" on the name plate) is just that, recommended. You are free to install a larger breaker up to the MOPD (Max Fuse). So if the AHJ requires your breaker to meet or exceed the MCA you can.

Today I just came across this thread that is a very similar scenario.

Proper Labeling

I have a split hvac unit that has me and hvac guy questioning label Say max MOP 49.9 amps Min circuit 29 amp But says Breaker 25 Trying to size feeder
forums.mikeholt.com

But more importantly @DBill64 mentions this article below that is a really good explanation on what is happening with the recent weird nameplate ratings and how we should be thinking about them.

www.hvacrschool.com

Why the Breaker Size May Be Lower than the MCA (and Why That's NOT Necessarily a Code Violation)

If you look at the label, you will see that the breaker size is 25 amps, and the minimum circuit ampacity (MCA or wire size) is 29 amps.
www.hvacrschool.com www.hvacrschool.com

The TLDR is that recent changes to UL standards and motor types have put many units into a position where they calculate a much larger MCA than they would ever realistically see. Mitsubishi has decided to provide a more realistic real world recommended breaker size.

I think for myself in the future I'll just size the breaker based on MCA.

Rob G
Seattle
 
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