Mini Split system & 14-3

[stevenje]

Had a good discussion with the electrical inspector regarding the wiring of the Mini Split heating/cooling systems that seen to be so popular in this area. Stated in the manufactures electrical installation guidelines, this unit requires a 20A 240V branch circuit to the outdoor unit and then states a 14-3 needed to be installed from the outdoor unit to the indoor unit. A weatherproof fused disconnect (20 amp fuses) is installed next to the outdoor unit. One inspector says that a 12-3 needs to be installed between the units, not the 14-3 wiring that the manufacture calls out for. The other inspector says the 14-3 is fine. After looking at the wiring schematic of the units, I see no 15 amp overload protection in the outdoor unit for the 14-3 wiring running to the indoor unit. Any thoughts on this? Thanks in advance.

 

[augie47]

We have seen some units with OCP for the evaporators built-in and some without.
Whether internal or added we require proper OCP.

(from wiring methods to OCP, mini-splits have been one of our biggest every-day headaches)

 

[petersonra]

I think you have several choices.

1. Change the cable to 12/3.

2. Install a 15A CB on the hot lines of the 14/3.

3. Make the manufacturer fight it out with the inspector.

If the instructions actually dictate a 14/3 (and not a minimum of a 14/3), choice 1 would violate the manufacturer's instructions, and thus violate the NEC WRT installing IAW manufacturer's instructions.

If the whole assembly including the cabling between the units is considered as a listed assembly, the inspector is probably off base, and choice 2 might well violate 110.3(B).

The electrical code allows the AHJ to "approve" an installation. My suggestion is get the inspector's boss to sign off on the idea of violating 110.3 (B), IF the instructions say to use 14/3 and not a minimum size of 14/3.

Personally, this seems like something that comes up on an ongoing basis and ought to be handled somehow by the manufacturer.

 

[petersonra]

because I was curious I called up a manual for one of the Mitsubishi units. It shows a fuse on the 220V going outside that is found on the control board.

 

[Dennis Alwon]

I say Inspector 1 is correct based on 240.4 which directs us to 240.4(G).

 

[stevenje]

because I was curious I called up a manual for one of the Mitsubishi units. It shows a fuse on the 220V going outside that is found on the control board.

Earlier today I pulled up a Daikin mini split wiring schematic and it appeared that the 220v branch circuit was feeding through from the outdoor unit to the indoor unit. I did not have the model number of one of the units in question so I am not sure if they vary much from one to another. It did show an individual internal fuse in each unit protecting that unit. I need to get the exact model number and research it further.

 

[edlee]

Had a good discussion with the electrical inspector regarding the wiring of the Mini Split heating/cooling systems that seen to be so popular in this area. Stated in the manufactures electrical installation guidelines, this unit requires a 20A 240V branch circuit to the outdoor unit and then states a 14-3 needed to be installed from the outdoor unit to the indoor unit. A weatherproof fused disconnect (20 amp fuses) is installed next to the outdoor unit. One inspector says that a 12-3 needs to be installed between the units, not the 14-3 wiring that the manufacture calls out for. The other inspector says the 14-3 is fine. After looking at the wiring schematic of the units, I see no 15 amp overload protection in the outdoor unit for the 14-3 wiring running to the indoor unit. Any thoughts on this? Thanks in advance.

Yeah, my thought, as a electrician in the field(I am not an inspector) is, the manufacturer's listing is good enough for me. Maybe the 14/3 circuit is inherently current-limiting somehow, who knows. If an inspector doesn't like it, then I will either call the manufacturer and have them send me a letter explaining why he is wrong, or I will run the 12/3 whenever I am in his jurisdiction but think to myself that they should work it out between themselves so us guys doing the installations don't get stuck paying for the disagreement.

 

[kwired]

Kind of a confusing area here. OP says the unit needs a 20 amp circuit. What was not mentioned was what is actual MCA and MOCP on the nameplate.

Very possible the MCA is 15 or less amps yet MOCP might be 20 to allow for starting. Also remember 14 AWG@75C is still good for 20 amps and could have even higher OCP especially for art 430 and 440 applications.

On top of that it seems pointless to require 12 AWG between the outdoor and indoor unit when 14 AWG is permitted to supply the whole thing but yet it also makes sense to protect those conductors at least at 15 amps....

 

[hurk27]

We have seen some units with OCP for the evaporators built-in and some without.
Whether internal or added we require proper OCP.

(from wiring methods to OCP, mini-splits have been one of our biggest every-day headaches)

The problem with this is many forget that most of the blower motors in these units has overload protection built in the motor and just like outdoor compressors the conductors are protected from overloads by these built in overload protectors, we should all know that the #14 is also well within the protection from line to line faults and line to ground faults by the 20 amp protection feeding the outdoor unit as even a 40 amp fuse or breaker will be well within the instantaneous area of the trip curve with a #14 as long as the run is not over 100', this is no different then an outdoor AC compressor that has built in overloads where we may see #12 or 14 AWG wire from a 30 amp breaker to the compressor, I would also doubt that the run would ever get close to 100' as the restriction on refrigerant lines would keep this at bay.

We protect from two things overload (long term) and short circuits (short term), overload protection can be at the supply end or at the load end where the fixed load is, but short circuit always has to be at the supply end, and overloads at the load end can only be allowed if no other loads are likely to be added, this is why we can't do this when it comes to lighting and receptacle loads.

So don't always look for overload protection at the supply end of a circuit, as it may very well be at the load end and is allowed by the NEC in 430, 440, as well as in other articles of the NEC.

 

[augie47]

I agree with that premises, but, according to Art 430 Part IV, what would your GFSC rating for a fractional HP motor ?

 

[Greg1707]

3 phase DC

3 phase DC

I was told these units operate on three phase DC current. Hence, a thee pole switch is sometimes installed inside next to the unit as a disconnect.

 

[kwired]

I was told these units operate on three phase DC current. Hence, a thee pole switch is sometimes installed inside next to the unit as a disconnect.

You care to attempt to explain what three phase DC is?

If three pole disconnect is needed it is likely because we have the two line conductors and an ungrounded control lead that need to be opened before all voltage is disconnected from the unit.

 

[GoldDigger]

You care to attempt to explain what three phase DC is?

If three pole disconnect is needed it is likely because we have the two line conductors and an ungrounded control lead that need to be opened before all voltage is disconnected from the unit.

Some (the really good ones) mini splits use "inverter drive" for the compressor to allow soft start and low power mode. I suppose you could call that three phase pulsed DC drive, but the fan in the inside unit would just be AC AFAIK.

 

[Dennis Alwon]

When you use a 14/3 the third wire is a control circuit -= I believe and the others I believe would be the motor some of which are DC motors as stated above but I see the a/c guys wired them with T stat wire. They say that is what the manufacturers state. :?

 

[GoldDigger]

When you use a 14/3 the third wire is a control circuit -= I believe and the others I believe would be the motor some of which are DC motors as stated above but I see the a/c guys wired them with T stat wire. They say that is what the manufacturers state. :?

To the extent that they mention it at all, the instructions I have seen tell you to use SOxx flexible cord (14-3), which is hard to justify under NEC if you consider it to be building wiring rather than part of the equipment.

 

[Dennis Alwon]

To the extent that they mention it at all, the instructions I have seen tell you to use SOxx flexible cord (14-3), which is hard to justify under NEC if you consider it to be building wiring rather than part of the equipment.

I agree -- we will not allow the so cords to be run. I had one heating guy who bought tray cable because that was what their HVAC supplier stocked. He installed about a 75 foot run and I told him tray cable is not compliant in a residence without a raceway. He was made and sleeved the entire run in carflex. I would have just gotten 14/3 nm and be done with it.

 

[Buck Parrish]

I just wired 3 spit LG systems. We only wire to the outside unit. The HVAC guys wire the insides ones via there thermostat wires. I had done some in the past, too. Same thing, back then I questioned the guy about circuit ampacity protection. He said that's how they always wire them. He said all the HVAC guys at his supply house wire them via there thermostat wire. Some may use some other type of stranded wire. But it's real small.
These start real slow.
Some of the older cheap models start fast and are loud like a wall air condition. I have wired them with 14 romex with a disconnect, too

 

[hurk27]

I agree with that premises, but, according to Art 430 Part IV, what would your GFSC rating for a fractional HP motor ?

I would think that 430.53(A) would cover that? just like small portable fans are allowed to be plugged into a 20 amp circuit, I can't see a problem with a fractional HP motor in the wall unit, today most all fractional motors are protected by a thermal fuse embedded into the windings, I think this is a UL requirement when these motors are expected to be operated from much larger circuits, so I think UL took care of the protection of the motor from GFSC, the built in overload takes care of the overload of the 14/3 conductors, and the 20 amp BC breaker takes care of the GFSC of the 14/3 conductors? table 430.52 give us up to 800% over FLA for a Instantaneous Trip Circuit Breaker which I believe most all branch circuit breakers installed today are.

Again it all depends upon how UL listed this equipment and how each section of it is protected, but I would believe if it is UL listed then there should be no problems with the 14/3.