# Split Mitsubishi AC units



## Salvatoreg02 (Feb 26, 2011)

Last week I had the pleasure of installing one of these units for a data center.. As per specs and being advised by HVAC company I was told to run 10/3 to condenser and then, 14/3 condenser to fan unit. 

Now, I've done over a dozen of these in the past and have always questioned the manufactures specs for installing a 14/3 from a unit that was being fed from a 30 amp circuit. I also snooped around the inside of the condenser unit to check if the 14/3 was protected by fuses and it wasn't. 

Has anyone installed these units in this manner???

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## administr8tor (Mar 6, 2010)

Yes, perfectly normal, condenser has transformer in it, low voltage goes over your 14/3:thumbsup:

Great a/c's:thumbup::thumbup::thumbup:


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## Salvatoreg02 (Feb 26, 2011)

administr8tor said:


> Yes, perfectly normal, condenser has transformer in it, low voltage goes over your 14/3:thumbsup:
> 
> Great a/c's:thumbup::thumbup::thumbup:


Transformer? Im getting 120volt reading at the fan unit!! The outdoor unit is 208-240volt.

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## Dennis Alwon (May 9, 2009)

I believe the 14/3 is used for communication and low voltage motor. The motor on these are often a DC motor that I have seen run on T-stat wire.

I guess I didn't read you were getting 120v at the fan-- not sure about that


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## Salvatoreg02 (Feb 26, 2011)

All of the fan units I installed either required a separate power source, with a control wire from condenser. Or a 14/3 from condenser. Is this code complaint?

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## Dennis Alwon (May 9, 2009)

Salvatoreg02 said:


> All of the fan units I installed either required a separate power source, with a control wire from condenser. Or a 14/3 from condenser. Is this code complaint?
> 
> Sent from my iPhone using ET Forum


I have seen them that way but I always thought the units were low voltage motors. Call the manufacturer. Without seeing the info I don't know but I bet it is compliant. Whats the model.


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## Salvatoreg02 (Feb 26, 2011)

Let's simplify this I'm taking a 30 amp circuit 10ga wire and tapping it with 14 ga wire. Legal or not??

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## Dennis Alwon (May 9, 2009)

Salvatoreg02 said:


> Let's simplify this I'm taking a 30 amp circuit 10ga wire and tapping it with 14 ga wire. Legal or not??
> 
> Sent from my iPhone using ET Forum


I know what you are saying and if that is a factor requirement then it is probably compliant. The 14 gauge wire is probably protected with overload protection in the unit.


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## Salvatoreg02 (Feb 26, 2011)

Dennis Alwon said:


> I know what you are saying and if that is a factor requirement then it is probably compliant. The 14 gauge wire is probably protected with overload protection in the unit.


That's the thing, I snooped around that entire condenser unit and there were no fuses anywhere. Anyway !!!

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## Dennis Alwon (May 9, 2009)

Salvatoreg02 said:


> That's the thing, I snooped around that entire condenser unit and there were no fuses anywhere. Anyway !!!
> 
> Sent from my iPhone using ET Forum


They could be thermal overload protectors not fuses.


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## macmikeman (Jan 23, 2007)

I've studied Mitsubishi split ac schematics at great length in the past, based on exactly the situation the OP is talking about. There is no secondary overcurrent protective devices present for the control. Mitsubishi does not encourage the practice of using an undersized conductor for the field installed control wiring, and usually calls for separate overcurrent protection if required by "local codes" in the documentation. I do think that they might be aware of the common practices that prevail and do not concern themselves about it much.


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## guest (Feb 21, 2009)

I see this situation as no different than the rules that allow a #12 to feed an outside condensing unit with a 30 or 40 amp breaker in the panel. 

_*Overload*_ protection is provided by the condensing unit's thermal protection. 

The breaker provides _*short circuit*_ protection for the wire. 

As for the split units, some may have a transformer built in to feed the fan coil unit....but I still see no problem with the 14/3 as it is very unlikely to see an *overload *high enough to present a hazard. (Unless some fool taps some receptacles off of it.) And if there ever is a short in that 14/3, it is very likely that even a 30 or 40 amp breaker would trip fast enough to prevent damage to the wire.


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## mgraw (Jan 14, 2011)

I looked at a couple of wiring diagrams for Mitsubishi splits. They had fuses on the circuit board before the S1 & S2 terminals.


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## Salvatoreg02 (Feb 26, 2011)

It just seems as though it's not the proper wiring method. I don't think any of the installs I did in the past the inspector ever pick up on the 14/3 feed/control wiring was noticed for the fan unit..

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## mgraw (Jan 14, 2011)

I don't see a problem with 14/3. The circuit board fuses for S1 & S2 are 6.3 amps, at least for the two I looked at. I don't know what model you have.


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## Dennis Alwon (May 9, 2009)

Salvatoreg02 said:


> It just seems as though it's not the proper wiring method. I don't think any of the installs I did in the past the inspector ever pick up on the 14/3 feed/control wiring was noticed for the fan unit..


So the inspector didn't pick up on it either because it is legal or he doesn't care. I am quite certain it is compliant.

What I don't like is some of these units come with a rubber cord used to install between the outside unit and the inside unit- usually 14/3 cord. The size is not the issue but the rubber cord passing thru the wall is. I called them on it and they said some areas will allow it.  Why would UL or whomever allow this to be part of their listing?


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## macmikeman (Jan 23, 2007)

mgraw said:


> I don't see a problem with 14/3. The circuit board fuses for S1 & S2 are 6.3 amps, at least for the two I looked at. I don't know what model you have.


I think they must sell different units to different parts of the country. Here, we never see lv control like some here talk about, the control is always three wire 208-230 volt. I have seven split Musubi's set up at my house, and none of them have a protective fuse at S1 and S2. It is straight off the contactor out to the fan coil units. If this makes anybody feel happy, I'm a hack so I never let it slow me down much, I use 14-3 if the cable is run thru indoor dry locations. The fan coils actually draw about 2 amps......


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## Dennis Alwon (May 9, 2009)

macmikeman said:


> I'm a hack .


 You are a big bonehead too. :laughing:


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## macmikeman (Jan 23, 2007)

Dennis Alwon said:


> You are a big bonehead too. :laughing:


I see I made you feel happy


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## mgraw (Jan 14, 2011)

Here is a partial wiring diagram from a Mitsubishi split. Notice F65 and F66 between the S terminals and the line terminals. These are fuses soldered to the circuit board.


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