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Electrical Wire Gauge discrepancies - Help!! - for dual ductless mini-split

Electrical Wire Gauge discrepancies - Help!! - for dual ductless mini-split

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  #1  
Old 05-14-13, 07:30 PM
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Electrical Wire Gauge discrepancies - Help!! - for dual ductless mini-split

I am having a 2-zone ductless mini-split system installed. In order to save on cost, I am doing as much work as I can before HVAC guy comes over.

Here is where I encounter an electrical wire gauge discrepancy.
Person A is telling me the following:
From House Main Breaker get a 2-pole 20 AMP breaker...
from breaker to outside disconnect box, use 14/2 wire...
(disconnect box is a pull out -- 30 AMP)
from disconnect to outdoor condenser will be a whip of 14/2 wire...
from condenser to each of the two indoor units will be a 14/3 wire.

Person B is telling me the following:
Replace ALL 14/2 with 12/2 and ALL 14/3 with 12/3.

Here is some of the specifics of the unit being installed:
indoor 1: Mitsubishi MSZ-FE12NA-8 (12,000 btu)
indoor 2: Mitsubishi MSZ-FE09NA-8 (9,000 btu)
outdoor unit: Mitsubishi MXZ-2B20NA-1

outdoor unit Electrical Data:
Voltage (Volts) 208-230
Phase 1
Frequency (Hertz) 60
Recommended Breaker Size (A) 20

indoor unit Electrical Data:
Voltage (Volts) 208-230
Phase 1
Frequency (Hertz) 60
Recommended Breaker Size (A) 15


Who is correct? Person A or Person B?
I thought, because breaker is 20 AMP, that wire should be a 12 gauge... thus, making person B correct.
However, unit sold by ecomfort dot com has as an accessory needed to install, a 14 gauge wire.

I'm a bit confused and any help and advise would be awesome!!
 
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  #2  
Old 05-14-13, 07:37 PM
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You should be able to find out the minimum circuit ampacity and the maximum overcurrent protection size from the nameplate.

A/C wiring falls under a different set of sizing rules than general purpose circuits.
 
  #3  
Old 05-14-13, 07:48 PM
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According to the installation instructions.....it's a 20A breaker with all #14 wiring.
If you had a long run from the AC to the outside unit.....you could use #12 wiring. Stay with #14 wiring between the inside and outside unit.


Check out page 5 of the installation instructions.

Mitsubishi install pdf
 
  #4  
Old 05-14-13, 08:17 PM
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but isn't a #12 required with a 20 amp breaker?

I've been told the wires will burn before the breaker trips.
 
  #5  
Old 05-14-13, 08:32 PM
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With air conditioner service the code is a little different. They go by minimum circuit ampacity and overcurrent protection ampacity. You can use #12 from the panel to the compressor.
 
  #6  
Old 05-14-13, 09:23 PM
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Another way to look at this is the running current of the compressor is less than 15 amps so #14 is adequate. The 20 amp breaker is permitted so the initial high starting current which lasts only a few seconds won't trip the breaker.
 
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Old 05-15-13, 07:16 PM
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Person B is telling me the following:
Replace ALL 14/2 with 12/2 and ALL 14/3 with 12/3.
Using 12-2, 14-3 or 12-3 implys that you should use NM B cable (aka Romex). You can use NM B cable between your main panel and the disconnect, but any further, to the condensing unit or inside unit, you cannot use NM B cable. NM B cable cannot be used outside even in conduit. Here you would use THHN/THWN conductors in conduit. The electrical specs look to me like you need two circuits, one to the condensing unit and a second circuit to the inside unit.
 
  #8  
Old 05-15-13, 07:21 PM
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Here is some of the specifics of the unit being installed:
indoor 1: Mitsubishi MSZ-FE12NA-8 (12,000 btu)
indoor 2: Mitsubishi MSZ-FE09NA-8 (9,000 btu)
outdoor unit: Mitsubishi MXZ-2B20NA-1

outdoor...

indoor unit Electrical Data:
Voltage (Volts) 208-230
Phase 1
Frequency (Hertz) 60
Recommended Breaker Size (A) 15
Which indoor unit is that for?
 
  #9  
Old 05-15-13, 09:59 PM
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they are for both...
same numbers for the 9K and 12K
 
  #10  
Old 05-15-13, 10:08 PM
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In post #3 I left a link for the installation of this system. This system works as most splits do...... power is sent to the outside unit and it in turn feeds the inside units. Therefore only one circuit is required.

I run a 14-3 w/ground NM cable from the inside unit..... down thru the wall.... and thru the outside wall into a weatherproof j'box. From there to the compressor in seal tite (weatherproof flex tubing).

With a dual system you bring both wires from inside into a w.p. j'box and a single piece of seal tite.
 
  #11  
Old 05-15-13, 10:12 PM
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forgive my ignorance... is the THHN/THWN similar to SOOW?
 
  #12  
Old 05-15-13, 10:25 PM
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forgive my ignorance... is the THHN/THWN similar to SOOW?
Not even in the same family. THWN is single conductor wire used in conduit. SOW is a type of flexible cable used as cords on appliances and in extension cords. It would not be code compliant for this.

In this case from the service disconnect to the AC you would use a "whip" which is a piece of waterproof flexible conduit such as Sealtite that contains two THWN conductors and a THWN ground.

[ATTACH=CONFIG]12694[/ATTACH]
Image source: Fox Electric Supply
 
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  #13  
Old 05-15-13, 10:28 PM
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No......not similar

THHN is thermoplastic/higher heat wire.
THWN is thermoplastic/water resistant.

SOOW is a multi-conductor rubber jacketed cable like what an extension cord would be made of.
THHN/THWN are single conductor wires and used inside of an electrical conduit system like pipe or flex conduit.(seal-tite)
 
  #14  
Old 05-17-13, 06:34 PM
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This system works as most splits do...... power is sent to the outside unit and it in turn feeds the inside units. Therefore only one circuit is required.
That's what I thought too although I have never seen a split like this. What puzzled me was when the OP posted specs that called for separate circuits for the inside and outside units.
 
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