Wiring a mini split Heat/ A/C unit

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howardrichman

Senior Member
This unit is rated 208/240V Max HACR breaker 20 -- 17.2 min. CKT ampacity. do I figure a #12 copp. is rated @16 amps @80%, and must run #10 to compensate? Also, a 14-4 tray cable, or a 14-3 romex is required to the internal fan. The installation recommends a disconnect on that line to the fan. Is a disconnect required either @ the condenser or cut in @ the internal fan for these conductors? it would have to be a double pole, cause 1 conductor is a communicator, and can probably be spliced in box.

Please reply;
Thanks; HR...
 

iwire

Moderator
Staff member
Location
Massachusetts
This unit is rated 208/240V Max HACR breaker 20 -- 17.2 min. CKT ampacity.

MCA (Minimum Circuit Ampacity) is compleate, you don't add anything to it at all.

You need a conductor with 17.2 amps of capacity, it could be done with 14 AWG and a 20 amp breaker.

Also, a 14-4 tray cable, or a 14-3 romex is required to the internal fan. The installation recommends a disconnect on that line to the fan. Is a disconnect required either @ the condenser or cut in @ the internal fan for these conductors? it would have to be a double pole, cause 1 conductor is a communicator, and can probably be spliced in box.

We put a disconnect at the outdoor unit for it and another disconnect at the indoor fan unit as a disconnect for that.

Often we use a three pole motor switch but I agree that you could use a 2 pol switch for the power conductors and just splice the communication wire straight through.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The wiring method between the unit and the fan has been an interesting point of contention here.
If Romex is used, a transition to a "wet location" wiring method is needed once you go outside and the installers advise some manufacturers require "no splices". TC cable is not allowed in a residential setting unless its in a raceway. Cord is prohibited.
Lately most of our mini-split installers have been using a jacketed MC designed for mini-split installs
http://www.southwire.com/products/ez-in-mini-split-cable-600-volt.htm
 

howardrichman

Senior Member
MCA (Minimum Circuit Ampacity) is compleate, you don't add anything to it at all.

You need a conductor with 17.2 amps of capacity, it could be done with 14 AWG and a 20 amp breaker.



We put a disconnect at the outdoor unit for it and another disconnect at the indoor fan unit as a disconnect for that.

Often we use a three pole motor switch but I agree that you could use a 2 pol switch for the power conductors and just splice the communication wire straight through.

Please give reference to the use of 14 GU for 17.2 amps.
Thanks for replies; HR...
 

ActionDave

Chief Moderator
Staff member
Location
Durango, CO, 10 h 20 min from the winged horses.
Occupation
Licensed Electrician
Please give reference to the use of 14 GU for 17.2 amps.
Thanks for replies; HR...

TABLE 310. 15(B)(16) tells you that 15AWG THHN is good for 20A, but there is an asterisk that tells you to look at 240.4(D).

240.4(D) tells you to look at 240.4(G).

240.4(G) tells you to look at Table 240.4(G) Specific Conductor Applications. The first thing listed on that table is Air-conditioning and refrigeration equipment circuit conductors. Then you go to Art. 440.

In Art. 440 there are steps to follow to calculate the wire size you need and the size of the overcurrent/breaker to use, or you can do the easy thing and look at the name plate on the A/C equipment.

It's a lot of hoops to jump through but when you boil it all down the code says, "Wiring rules for A/C equipment are different". The unit is has its own thermal protection so the circuit conductors can never be overloaded, the breaker is only providing short circuit and ground fault protection.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
TABLE 310. 15(B)(16) tells you that 15AWG THHN is good for 20A, but there is an asterisk that tells you to look at 240.4(D).

240.4(D) tells you to look at 240.4(G).

240.4(G) tells you to look at Table 240.4(G) Specific Conductor Applications. The first thing listed on that table is Air-conditioning and refrigeration equipment circuit conductors. Then you go to Art. 440.

In Art. 440 there are steps to follow to calculate the wire size you need and the size of the overcurrent/breaker to use, or you can do the easy thing and look at the name plate on the A/C equipment.

It's a lot of hoops to jump through but when you boil it all down the code says, "Wiring rules for A/C equipment are different". The unit is has its own thermal protection so the circuit conductors can never be overloaded, the breaker is only providing short circuit and ground fault protection.
One fly in the ointment. Unless there is a misprint in my 2011 & 2014 Code, 310.15(B)(16) shows the ampacity for #14 at 60? to be 15 amps. So NM, with the 60? limit, would not allow 17.2 on a #14.
That's why I mentioned the wiring method is my post. NM, no, THHN, yes. IMO
 

Joe Villani

Senior Member
The wiring method between the unit and the fan has been an interesting point of contention here.
If Romex is used, a transition to a "wet location" wiring method is needed once you go outside and the installers advise some manufacturers require "no splices". TC cable is not allowed in a residential setting unless its in a raceway. Cord is prohibited.
Lately most of our mini-split installers have been using a jacketed MC designed for mini-split installs
http://www.southwire.com/products/ez-in-mini-split-cable-600-volt.htm

Augie,

Have the installers been using connectors designed for use with jacketed mc cable? They're a bit pricey.

Thanks,

Joe Villani
 

kwired

Electron manager
Location
NE Nebraska
This unit is rated 208/240V Max HACR breaker 20 -- 17.2 min. CKT ampacity. do I figure a #12 copp. is rated @16 amps @80%, and must run #10 to compensate? Also, a 14-4 tray cable, or a 14-3 romex is required to the internal fan. The installation recommends a disconnect on that line to the fan. Is a disconnect required either @ the condenser or cut in @ the internal fan for these conductors? it would have to be a double pole, cause 1 conductor is a communicator, and can probably be spliced in box.

Please reply;
Thanks; HR...
MCA already has 125% of the compressor factored into it. You do not need to adjust that number any, so a 17.2 amp minimum copper conductor needs to be 12 AWG if it is a 60C conductor, or can be a 14 AWG if a 75C conductor. 240.4(D) doesn't apply to art 430 or 440 applications, so that breaker could be even higher setting and still on a 14 AWG and be code compliant. Of course name plate max of 20 is the limit for this particular model.
 

WirenutGP

Member
The Mitsubishi split systems recommend a 3 pole disconnect at the fan so you also break the communicator conductor.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Augie,

Have the installers been using connectors designed for use with jacketed mc cable? They're a bit pricey.

Thanks,

Joe Villani

They have. Some on the initial install, some after the rejection note :)
 

rt66electric

Senior Member
Location
Oklahoma
As a service guy who comes along after the install.... USE THE LARGER WIRE SIZE!!!!!.... The smaller wire size generates more HEAT. The large amp draw causes the small wire to act like a heater element, and, overheating problems occur at the termination points.
Copper is an excellent conductor of heat , oversize wire sizes will draw heat away from the termination points.

The smaller wire has a higher resistance and causes the compressor to work harder. Think of running two light bulbs in series, a element will burn out quickly because of the additional amp draw of the other bulb.

The old industrial rule of thumb is run the wire at 125% and fuse at 80%. The fuse should blow before the equipment tears itself up.

If a problem occurs such as low freon or control failure , then the fuse should blow before the compressor overheats and shorts out.

A recent troubleshooting call proved to me that 14ga wire can only pull aprrox 28 amps, it was a dead short, "Down the line" and the amp gauge would hold at 28-30 amps,then the breaker would trip in approx 2 mins.

Is customer is paying you to "do a good job that will last a long time" and provide enough room to upsize if necessary.

---OR------

Is the customer paying you to " do the minimum amount and let the next guy worry about it" ??
 

kwired

Electron manager
Location
NE Nebraska
As a service guy who comes along after the install.... USE THE LARGER WIRE SIZE!!!!!.... The smaller wire size generates more HEAT. The large amp draw causes the small wire to act like a heater element, and, overheating problems occur at the termination points.
Copper is an excellent conductor of heat , oversize wire sizes will draw heat away from the termination points.

The smaller wire has a higher resistance and causes the compressor to work harder. Think of running two light bulbs in series, a element will burn out quickly because of the additional amp draw of the other bulb.

The old industrial rule of thumb is run the wire at 125% and fuse at 80%. The fuse should blow before the equipment tears itself up.

If a problem occurs such as low freon or control failure , then the fuse should blow before the compressor overheats and shorts out.

A recent troubleshooting call proved to me that 14ga wire can only pull aprrox 28 amps, it was a dead short, "Down the line" and the amp gauge would hold at 28-30 amps,then the breaker would trip in approx 2 mins.

Is customer is paying you to "do a good job that will last a long time" and provide enough room to upsize if necessary.

---OR------

Is the customer paying you to " do the minimum amount and let the next guy worry about it" ??
I agreed with what you started saying then you mentioned the light bulbs in series - they should last much longer in series - but not get nearly as bright either - assuming applied voltage remains same.

then you mentioned fuse at 80% - with motors and hermetic compressors - that doesn't even allow them to start in many instances - besides we are supposed to have motor overload protection to protect the motor

I'd also expect 14 AWG to be able to carry even more then 30 amps without damaging insulation - exactly how much more IDK, but one needs to factor time in there as well. 50 amps for 5 minutes may be pushing it to near the limit, but this is for a one time event, if you do that kind of thing often damage does accumulate.
 

kwired

Electron manager
Location
NE Nebraska
Fuse or HACR breaker
HR...

If a breaker is used it will need to be a HACR rated breaker - not too difficult to comply with that with just about all common branch circuit breakers made the past 25 years or so. What was mentioned however is that if the equipment nameplate only mentions fuses - then it must be protected by fuses. I have not seen a new unit in years that didn't allow either fuses or HACR breakers though, so unless you have an old unit - likley not an issue.
 
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