Replacing disconnect for new RTU for minisplit HVAC system

[wondervamp]

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

 

[James L]

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.

 

[wondervamp]

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.

this is from the link that I posted in my thread: "Recommended Fuse/Breaker Size 40A

 

[James L]

this is from the link that I posted in my thread: "Recommended Fuse/Breaker Size 40A

So you're covered with a breaker in the panel, and you can use a non-fused pull-out disconnect (even though hvac tech will curse you)

 

[augie47]

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]

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

 

[busman]

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

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

 

[James L]

High Leg is required to be the "B phase".

I believe that changed in 2008.

 

[Little Bill]

I believe that changed in 2008.

It says "B" is to be the high leg in 2005 up to the 2017. That's all the books I have. So I don't think that has changed.

 

[winnie]

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.

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

-Jon

 

[James L]

It says "B" is to be the high leg in 2005 up to the 2017. That's all the books I have. So I don't think that has changed.

This 2011 article says since 1975 it has to terminate on "B"

StackPath

However, since 2002 (not 2008) there's an exception in 408.3(e) which says it can be terminated on the same leg as the utility. That's usually "C"

 

[Little Bill]

Never even seen the exception, nor ever heard anyone mention it. Just "C" on the meter and "B" in the panel.
Glad someone wised up to allow the same in both meter and panel.

 

[LarryFine]

Glad someone wised up to allow the same in both meter and panel.

Yes, but read the entire exception.

 

[Little Bill]

Yes, but read the entire exception.

I did, what is your point? (not being rude or smart, just wondering what you were alluding to)

 

[augie47]

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.

 

[Little Bill]

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

It also mentions panel boards. Do you think this just means combo units?

 

[augie47]

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.