Replacing disconnect for new RTU for minisplit HVAC system

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wondervamp

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Shop technician
Hey guys,

I've never wired up one of these minisplit havac systems. I haven't been up to the roof yet to see the old unit and disconnect but I'm sure the disconnect needs replacing.

I traced out the old 240v branch circuit and it has (2) #10's going up to the roof to the disconnect from the panel for the compressor/condensor.

  • When it comes to replacing the disconnect, can I just use a non-fused 60A disconnect up on the roof instead of a fused disconnect? Why you would want to use fused vs. non-fused?
My plan is to utilize the pre-existing (2) #10's to wire up the new disconnect and use a double pole 40A breaker at the panel for the unit. I'm pushing it a bit on the MCA but really don't want to pull #8's if I can avoid it. I'm attaching a link to the specs.

In a nutshell and listed in the link below: MCA = 31.9A / recommended breaker = 40A

Mitsubishi M series minisplit
 
With 31.9 MCA the #10 would work if it's a 75⁰ conductor

Does the manufacturer call for fuses or breaker, or either? (such language as "max fuse" or "max fuse/breaker")
That will tell you if you can use a pull-out disconnect.
 
James L said:
With 31.9 MCA the #10 would work if it's a 75⁰ conductor

Does the manufacturer call for fuses or breaker, or either? (such language as "max fuse" or "max fuse/breaker")
That will tell you if you can use a pull-out disconnect.
Click to expand...
this is from the link that I posted in my thread: "Recommended Fuse/Breaker Size 40A
 
You should be o.k. but you should check the unit and see if there is any reference to terminal temperature.
The #10 will only be allowable IF the unit has 75° rated terminals.
(If they happen to be 60° you can increase the conductor size from the load side of the disconnect)
 
wondervamp said:
Here's a follow-up question. I don't have much experience with 3 phase system's. Would the above appliance work with this panel?

Here's the link again for reference to the single phase appliance: Click me for appliance specs
Click to expand...
High Leg is required to be the "B phase". But, to answer your question, yes, the system is Single Phase and you should be able to get 120V, 208V or 240V from a "high-leg" delta panel. For your situation, best to use any two "phase" (ungrounded) legs and ignore the grounded conductor.

Mark
 
wondervamp said:
I traced out the old 240v branch circuit and it has (2) #10's going up to the roof to the disconnect from the panel for the compressor/condensor.
[...]
My plan is to utilize the pre-existing (2) #10's to wire up the new disconnect and use a double pole 40A breaker at the panel for the unit. I'm pushing it a bit on the MCA but really don't want to pull #8's if I can avoid it. I'm attaching a link to the specs.
Click to expand...

What wiring method was used? (NM cable, MC cable, THHN in conduit?)

-Jon
 
Exception: Equipment within the same single section or multisection switchboard, switchgear, or panelboard as the meter on 3-phase, 4-wire, delta-connected systems shall be permitted to have the same phase conguration as the metering equipment.

Seemingly this would only apply to factory assembled meter-breaker combo units where it likely makes bussing easier.

1688736535696.jpeg
 
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augie47 said:
Exception: Equipment within the same single section or multisection switchboard, switchgear, or panelboard as the meter on 3-phase, 4-wire, delta-connected systems shall be permitted to have the same phase conguration as the metering equipment.

Seemingly this would only apply to factory assembled meter-breaker combo units where it likely makes bussing easier.

View attachment 2566239
Click to expand...
It also mentions panel boards. Do you think this just means combo units?
 
I don't recall ever connecting a 3 phase meter-breaker combo on a 240v high-leg system so I;m unsure of the factory bussing so I don 't know where the meter "C" phase terminates on the breaker.
 
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