We are installing 6 Mitsubishi split a/c units. 6 outdoor units, with 6 corresponding indoor units to cool electric rooms and i.t rooms. The outdoor units are 208 volt single phase fed from 20 or 30 amp breakers, depending on size of unit, as per manufacturer. The branch circuits land on L1 and L2 in the outdoor unit. According to the installation manual and the HVAC tech onsite, the outdoor unit wiring going to the indoor unit needs to have 3 conductors 16 AWG. These terminals are labeled S1, S2, S3 at both indoor and outdoor units. The "Field electrical wiring" chart in manual shows 208V between S1-S2, and 24VDC between S2-S3. No idea how this is accomplished or what S1-S3 might read. The problem is, there is no protection in the outdoor unit for the 16 AWG conductors to go to the indoor unit. They are protected only by our breakers ahead of the outdoor unit. The HVAC tech insists they have to be wired this way for the 24VDC communication to work, and they've had problems with the indoor units not running correctly when wired with 12 or 10 AWG wiring. I say, I can't run 16 AWG protected by a 30 amp breaker. The wiring has been done already, to code (12 & 10 AWG). Now the HVAC tech says he can't do start up on the units because we haven't wired them per manufacturers instructions (16 AWG to indoor units). We're at a bit of a stalemate now. Has anybody run into this, or dealt with these units much? We've installed them previously, and never had a problem that we know of.
Mitsubishi split a/c unit
- Thread starter bob75
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mcclary's electrical
Senior Member
- Location
- VA
I've done a few of the exact same Mitsubishi package units and we did not run 16, we ran 12 to the inside unit.
dwellselectric
Inactive, Email Never Verified
- Location
- Cape Cod, Massachusetts
I've done many of them and usually run a 12/3 SO or SJ cord between the two units. I haven't had any issues with the ones that i've done.
bphgravity
Senior Member
- Location
- Florida
Upsizing conductor sizes is usually never a problem, otherwise following the installation instructions should'nt be a problem either...
These type systems have been discussed here previously and it seems questions remain as far as disconnecting means (for the inside unit) and wire sizes. It seems unlikely that a larger wire size would prevent the unit from operating properly, and from an inspectors viewpoint, I would need some verification of proper overcurrent before I would accept a smaller gage wire.
side-note to DSilansks: We view SO cord between the units as a violation of 400.8
side-note to DSilansks: We view SO cord between the units as a violation of 400.8
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
There is an assumption here that the electrician (assisted by code and experience) knows better than the manufacturer.
Are you absolutely certain that there is direct connection with nothing non-solid-metallic intervening between the supply to the outdoor unit and where the power turns up in the indoor unit? For example, some inverter splits have the inverter in the outdoor unit, and it manages the feed to the indoor unit fans. I suspect that if the indoor unit shorts the inverter will act as a fuse, based on the construction of the last (made in RoC) unit I saw...
Having said that, upsizing conductors shouldn't cause functional problems, providing, of course, the terminals are designed and listed for #12 or #10; I think the A/C guy is full of it.
Are you absolutely certain that there is direct connection with nothing non-solid-metallic intervening between the supply to the outdoor unit and where the power turns up in the indoor unit? For example, some inverter splits have the inverter in the outdoor unit, and it manages the feed to the indoor unit fans. I suspect that if the indoor unit shorts the inverter will act as a fuse, based on the construction of the last (made in RoC) unit I saw...
Having said that, upsizing conductors shouldn't cause functional problems, providing, of course, the terminals are designed and listed for #12 or #10; I think the A/C guy is full of it.
c2500
Senior Member
- Location
- South Carolina
I wired one of these (residential) units back in December. There is a requirement in the instructions to provide a means of disconnect for the interior head unit. I used a double pole switch, and did not cut the wire used for controling the system.
c2500
c2500
gotmud
Senior Member
- Location
- some place cold
when we wire these we use a cord that you can get that is an option per manufacture specs in the spec book you get with it.. If I remember right it's 14 or 12 gage....We buy this cord per foot and have had no problems with the inspectors and if we did I would show them the spec book...I think it's a 4 wire cord though...It's not as flexable as SO cord but pretty damn close...
mcclary's electrical
Senior Member
- Location
- VA
Jim W in Tampa
Senior Member
- Location
- Tampa Florida
Having both electronic and electrical background find it hard to believe that using #12 or #10 wire to replace the factory wires would cause a problem. Now that is assuming the #16 cable is indeed just normal wire and has no other factors involved. As to protection, if the inside unit has internal protected motor then it could be fused higher in the same way we do normal AC units. As an electrician or inspector i would want to see this in the specs. Also what is the temp rating and type of insulation on the factory #16 ?
problemsolver600
Member
- Location
- South Chicago suburbs
AC or DC
AC or DC
I have worked with european based split units which operated both the inside and outside units on DC.
If the power is modulated, it might also be modified (lower voltage).
I suggest checking the schematics.
Gary
AC or DC
I have worked with european based split units which operated both the inside and outside units on DC.
If the power is modulated, it might also be modified (lower voltage).
I suggest checking the schematics.
Gary
Ditto here. We use #14 instead of #16.
hvac units
hvac units
The ones I have installed the outside unit at s1 s2 s3 are tied directly to the s1 s2 s3 on the inside units. The s1 is actually the (communication) leg and power is on the s2 and s3.
If you do not size your wire as you did you will be hit by the inspector. I have done about fourty of these at 240 volt and they do work this way. If the unit is not heating or cooling have them check and make sure they pulled the rubber plug in the line before hooking up. You can test yourself by feeling the line at the unit and if it is warm but at the fan coil it is not then they have a blockage.
Terille
hvac units
The ones I have installed the outside unit at s1 s2 s3 are tied directly to the s1 s2 s3 on the inside units. The s1 is actually the (communication) leg and power is on the s2 and s3.
If you do not size your wire as you did you will be hit by the inspector. I have done about fourty of these at 240 volt and they do work this way. If the unit is not heating or cooling have them check and make sure they pulled the rubber plug in the line before hooking up. You can test yourself by feeling the line at the unit and if it is warm but at the fan coil it is not then they have a blockage.
Terille
hvac
hvac
The latest decision from the inspectors is if it is residencial you need a disconnect, due to it not being written in the code book under the residencial side. if commercially the you can use a lockout as long as the circuit and associated unit is on a plate on the unit so who ever is working on it knows where to turn power off at.
hvac
The latest decision from the inspectors is if it is residencial you need a disconnect, due to it not being written in the code book under the residencial side. if commercially the you can use a lockout as long as the circuit and associated unit is on a plate on the unit so who ever is working on it knows where to turn power off at.
Split system
Split system
Under the provisions of 440.8 these units can be treated as a single machine as per 430.112 Exception. The problem we have found with them is that the indoor unit sometimes calls for a lower overcurrent protection than the outdoor unit. Rubber cord is not allowed as per 400.8. Mixing class 2 and light and power in one cable assembly is also a violation
Split system
Under the provisions of 440.8 these units can be treated as a single machine as per 430.112 Exception. The problem we have found with them is that the indoor unit sometimes calls for a lower overcurrent protection than the outdoor unit. Rubber cord is not allowed as per 400.8. Mixing class 2 and light and power in one cable assembly is also a violation
- Location
- Massachusetts
It is not 'class 2' and it is no different then using 14/3 between home smoke alarms. The white and black carry 120 AC and the red is 9 VDC.
charlietuna
Senior Member
They usually come as a package unit and the cable length matches the pre-charged refrigerent lines. They have a few options on distance. We always use the factory cable, but installed in an approved raceway and terminated as the manufacturer directs. Never had a problem with them. They are perfect for IT rooms.
Update to original post
Update to original post
Back on the job with more info in hand. As it turns out, there is fusing for S1, S2, S3. The fuses look like the kind you would use for your car stereo, though they're 250v rated. The fuse holders are soldered to a printed circuit board and have a translucent cover over them. We had looked right over them figuring they were for the circuit board and wouldn't be for field wiring. The one line diagram provided to us shows the fuses. We can't trace out the wiring to and from the fuses due to the circuit board, so we're taking it on faith. We repulled #16's to the units, protected by the 6.3A fuses, at the HVAC tech's insistance. Still not sure why the wire size matters. Everybody I've talked to has used #12's.
Update to original post
Back on the job with more info in hand. As it turns out, there is fusing for S1, S2, S3. The fuses look like the kind you would use for your car stereo, though they're 250v rated. The fuse holders are soldered to a printed circuit board and have a translucent cover over them. We had looked right over them figuring they were for the circuit board and wouldn't be for field wiring. The one line diagram provided to us shows the fuses. We can't trace out the wiring to and from the fuses due to the circuit board, so we're taking it on faith. We repulled #16's to the units, protected by the 6.3A fuses, at the HVAC tech's insistance. Still not sure why the wire size matters. Everybody I've talked to has used #12's.
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