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HVAC and Plumbing Distributor
Steven Simmons

Steven Simmons

Technical Services Manager
Southwest District

Common Explained

When you’re still green with experience or if you just entered an HVAC apprenticeship, there will be terms or sayings that you will not understand.  One of these things will be the term “common” when discussing wiring. I’m going to cover what common is and give examples.

The word Common in the HVAC field is used everywhere, it can be almost anything. Almost every electrical device has a common. Common for each device is the same BUT its meaning is only local. What I mean by that is: the definition for what “common” is, changes with each item or device.

Confused yet? Let’s break down some items within the system for a better understanding:

Single Phase Motors & Capacitor

schematic diagram
*Wiring diagrams highlighted with colors chosen for high contrast- not for wire color designation*

For the motors, I have highlighted the L1 & L2 power wires through a basic Carrier 25HCB condenser unit as they feed the compressor and condenser fan motors. The fan motor’s terminals are not labeled but are the same as the compressor’s. I circled the “commons” in purple. For the motors, there are two windings within the motors: Run to C and Start to C. The two windings come together at a “common” terminal as this meeting spot is what they both have in common.

For the capacitor in the center—the capacitor has one input and two outputs. One is (F)Fan and the other is (H)Hermetic (compressor). These two outputs have the same terminal for their input- common.

Relay Contacts

Relay contact terminals are labeled (C) Common, (NO) Normally Open, (NC) Normally Closed. When the relay is not energized C will be closed to NC and C open to NO. When the relay is energized, both will be reversed. Now it will be C closed to NO and C open to NC. Each terminal (NO & NC) have a relatable contact: (C) Common.

Catching on? Let’s discuss a more complicated “Common”:

Carrier Residential Packaged Unit Low Voltage

common wiring schematic
*Wiring diagrams highlighted with colors chosen for high contrast- not for wire color designation*

Low voltage starts in the red box at the transformer, 24vac goes to the Indoor Fan Board (IFB), through a fuse and on to the thermostat. Depending on what the thermostat calls for, 24vac will be sent down a different wire(s) to turn on/off the multiple devices in the system. The other low voltage wire (C) has an unbroken path to EVERY component in the unit. I have highlighted the pathway in blue and circled each component in purple. This wire/path is something each device has in “common”—it is the common wire. In layman’s terms, the common wire is the return pathway for voltage to come back to the transformer since it left out on the 24vac side.

The Common wire/terminal is always the one thing that is relatable or in common with the surrounding wires/terminals. Relays, motors, control boards, and even transformers all have some type of common wire/terminal. Common for a relay isn’t the same as common on a motor, but within each given device they carry the same meaning. Now go back and reread the second paragraph above—it will have a deeper meaning now.

Challenge: Check out the high voltage side of the Transformer. It has a COM terminal…. it’s not low voltage common. What is it?

That’s all.

More to explore

Capacitor Testing

Testing Capacitors

First, let us start by saying there are a variety of capacitors in various shapes, sizes and colors. For the purpose of

1 Stage Outdoor Wiring

38AUZ/D with Twin Furnaces

One popular combination of Carrier equipment is twinning 2 residential furnaces together with a 6-10 ton commercial outdoor A/C unit. When this

Alabama Locations

Anniston (256) 241-3026 105 E D St, Anniston, AL 36201
Auburn (334) 734-8011 2295 1st Ave, Opelika, AL 36801
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South Carolina Locations

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Florida Locations

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Mississippi Locations

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Commercial Sales

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Commercial Parts

 CompanyWeb AddressDescription
Airzonewww.airzoneinc.comCustom and Underfloor Air Handlers
Aladdin Metal Productswww.aladdinmetalproducts.comCurb Adapters and Roof Curbs
Aprilairewww.aprilaire.comIndoor Air Quality Products
Autaniwww.autani.comBuilding Automation and Controls
Bardwww.bardhvac.comHeat Pumps,Gas/Electric, and Wall Mounted Air Conditioners
Bryantwww.bryant.comCommercial, Industrial, Residential Heating & A/C Products and Controls
Carrierwww.commercial.carrier.comCommercial and Residential Heating & A/C Products
Data Airewww.dataaire.comPrecision Air Control for Data Processing Centers
 DXairwww.dxair.comPool dehumidification systems
 First Companywww.firstco.comFan Coil Units
Global Plasma Solutionswww.gpshvac.comAir Purification and Bi-Polar Ionization
 Greewww.greeusa.comPTACs
 Hayes Fluid Controlswww.haysfluidcontrols.comHose Kits and Control Valves
 iAirewww.myiaire.comCommercial Air Purification using ionization with energy savings
 IslandAirewww.islandaire.comPTACs, Vertical Units, Water Source Heat Pumps
 i-Vuwww.commercial.carrier.comBuilding Automation and Controls
 Magic Airewww.magicaire.comFan Coil Units
 Micro Metlwww.micrometl.comRoof Curbs, RTU Accessories, & ERVs
 Modinewww.modine.comGas-Fired Unit Heaters, Duct, Furnaces, Hot Water Cabinet Unit Heaters, Makeup Air, and Dedicated Outdoor Air Systems
 Network Thermostatwww.networkthermostat.comInternet Thermostats, Wifi Themostats, Ethernet Thermostats
Paynewww.payne.comResidential Heating & A/C Products
Rawal Deviceswww.rawal.comContinuous Capacity Modulation and Dehumidification for DX Systems
Renew Airewww.renewaire.comFixed-Plate ERVs
Totalinewww.totaline.comCompressors, Parts, and Supplies
United CoolAirwww.unitedcoolair.comModular All-Indoor HVAC Systems
 Warren Technologywww.warrenhvac.comElectric Heaters
 Wilo Pumpswww.wilo-usa.comPump Systems for HVAC and Water Supply and Sewage