Mini split wiring

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MattS87

Senior Member
Location
Yakima, WA
So I just completed my 2nd mini split install and have a question about the wiring. The first one was done about 3 1/2 yrs ago and after completing the wiring the HVAC company said we had to re-pull #14 from the outdoor unit to the indoor units ( we had run #12 as it was a 20A circuit). There reason was its for the signal and MUST be #14. Skip forward to yesterday, I walk into work to find an HVAC tech waiting to show me what he needs for a new mini split in the offices. I mention having done this before and the #14 being required by the other company. He said this is what he was told as well and that it has to do with the signal not working right if its anything other than #14 stranded. Anybody ever heard of this?

Keep in mind this is only between the 2 units, feed is #12 from the disco.
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
MattS87 said:
So I just completed my 2nd mini split install and have a question about the wiring. The first one was done about 3 1/2 yrs ago and after completing the wiring the HVAC company said we had to re-pull #14 from the outdoor unit to the indoor units ( we had run #12 as it was a 20A circuit). There reason was its for the signal and MUST be #14. Skip forward to yesterday, I walk into work to find an HVAC tech waiting to show me what he needs for a new mini split in the offices. I mention having done this before and the #14 being required by the other company. He said this is what he was told as well and that it has to do with the signal not working right if its anything other than #14 stranded. Anybody ever heard of this?

Keep in mind this is only between the 2 units, feed is #12 from the disco.
Click to expand...

I know nothing about wiring mini splits, but my BS meter just pegged red.

Certainly some signals require special cables or such, but they is no way these units could tell #12 from #14 or stranded from solid.
 
M

MattS87

Senior Member
Location
Yakima, WA
jumper said:
I know nothing about wiring mini splits, but my BS meter just pegged red.

Certainly some signals require special cables or such, but they is no way these units could tell #12 from #14 or stranded from solid.
Click to expand...

I agree 100% here which is why I had to ask.
 
S

SkyNetInc

Member
Location
FL
Who is the manufacturer? I just wired a mini-split (Mitsubishi) and it required 16/4awg stranded for the LV control (between head - inside & control unit - outside) I used CL3 stranded for wet locations. For power, outdoor unit requires 15a 2-pole breaker. I ran 14awg stranded for that.

Each manufacturer has different wiring schemes and I believe the NEC indicates the manufacturer wiring instructions are to be followed.
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Not that it is likely to make any practical difference, but it is true that for a given insulation type and spacing, the transmission line impedance of (for example) a piece of NM will depend on the gauge of the wire(s).
This would only be relevant if the control signal were RF, and the transmission line was terminated by the characteristic impedance of the line (probably something between one hundred and six hundred ohms.)

Sent from my XT1585 using Tapatalk
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
GoldDigger said:
Not that it is likely to make any practical difference, but it is true that for a given insulation type and spacing, the transmission line impedance of (for example) a piece of NM will depend on the gauge of the wire(s).
This would only be relevant if the control signal were RF, and the transmission line was terminated by the characteristic impedance of the line (probably something between one hundred and six hundred ohms.)

Sent from my XT1585 using Tapatalk
Click to expand...

Goldy, you be taking back me to schooling now. Stop that.:D
 
C

curt swartz

Electrical Contractor - San Jose, CA
Location
San Jose, CA
Occupation
Electrical Contractor
SkyNetInc said:
Who is the manufacturer? I just wired a mini-split (Mitsubishi) and it required 16/4awg stranded for the LV control (between head - inside & control unit - outside) I used CL3 stranded for wet locations. For power, outdoor unit requires 15a 2-pole breaker. I ran 14awg stranded for that.

Each manufacturer has different wiring schemes and I believe the NEC indicates the manufacturer wiring instructions are to be followed.
Click to expand...

The indoor unit is 208/240 volts NOT low voltage. You should be using a Chapter 3 wiring method between the units.

All the spec's I see for Mitsubishi and Fujitsu call for #14 between the units but I don't see why you couldn't use larger wire. Going smaller is definitely not acceptable.
 
L

Leespark57

Senior Member
Location
Boston, MA, USA
The wiring method of choice around here seems to be tray cable Type ER-JP. I believe some manufacturers are/were requiring a stranded conductor for the signal wire. Otherwise, I personally don't see any reason to not use romex/THHN carflex.

I agree, to require a smaller conductor sounds ridiculous.
 
L

lordofthisworld

Senior Member
Location
Massachusetts
curt swartz said:
The indoor unit is 208/240 volts NOT low voltage. You should be using a Chapter 3 wiring method between the units.

All the spec's I see for Mitsubishi and Fujitsu call for #14 between the units but I don't see why you couldn't use larger wire. Going smaller is definitely not acceptable.
Click to expand...

You can use larger wire, I wired plenty of these. Most of them took a 2P/30A disco feed. I used 10/3 for signaling wire and they worked perfect.
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Only problem with larger wire is that it gets a bit gnarly running it into the back of the indoor units then splicing within the small electrical compartment. I suppose the higher BTU units might be better in that respect because they are larger. Still, I would use the smallest conductors allowed.

-Hal
 
A

Andy Delle

Senior Member
Location
Los Angeles CA
MattS87 said:
So I just completed my 2nd mini split install and have a question about the wiring. The first one was done about 3 1/2 yrs ago and after completing the wiring the HVAC company said we had to re-pull #14 from the outdoor unit to the indoor units ( we had run #12 as it was a 20A circuit). There reason was its for the signal and MUST be #14. Skip forward to yesterday, I walk into work to find an HVAC tech waiting to show me what he needs for a new mini split in the offices. I mention having done this before and the #14 being required by the other company. He said this is what he was told as well and that it has to do with the signal not working right if its anything other than #14 stranded. Anybody ever heard of this?

Keep in mind this is only between the 2 units, feed is #12 from the disco.
Click to expand...

I call it hogwash. Speaking for Midea, but I'm sure most other vendors are similar, the control signal is about 1200 baud serial data. That is mostly immune to 60hz electrical fields found at a typical installation.

In any even case, the gauge of the control wire has nothing to do with performance as long as it's at least 24ga.

My Midea manual specifies to use THEIR cable only and not to splice it. But here is my path.

Condenser 14ga thhn stranded to outdoor Jbox approx 3 feet.
Spliced to 14/3 solid romex to attic, 25 ft run.
Another 4in Jbox in attic with the 14/3 spliced again to #14 stranded THHN approx 3 feet. (Box installed in case AHJ required a service switch)
THHN goes through ENT to air handler.

Works just fine as expected.
 
A

Andy Delle

Senior Member
Location
Los Angeles CA
Another Minisplit myth to bust:

Another Minisplit myth to bust:

If you are required to have a disconnect switch at the air handler, a basic 2 pole wall switch is all you need. You do not need a 3 pole switch. The control line is low voltage and may even be optically coupled further reducing any shock hazard.
 
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