Name: Resideo AT20A1123/U Foot Mounted 120 V AC Transformer with 9 in Leadwires
Brand: Resideo
SKU: HONE-AT20A1123/U
Price: 49.43 USD
Availability: InStock

Foot mounted 120 V AC Transformer with 9 in leadwires

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Frequency	50 Hz; 60 Hz
Electrical Ratings	Primary voltage--120 Vac; Secondary voltage--26.5 V.O.C.
Mounting	Channel Frame Mounting allows for Direct, Horizontal, or Vertical Foot mounting.
Dimensions	2 in. high x 2 1/4 in. wide x 1 7/8 in. deep
UL	UL Component Recognized.
CSA	Certified
Temperature Range (F)	-20 F to +105 F
NEMA Rating	NEMA type B
Electrical Connections (Primary)(in.)	9 in. color coded primary and secondary leadwires
Electrical Connections (Secondary)(in.)	9 in. color coded primary and secondary leadwires
Electrical Ratings, Output	24 Vac at 19 VA

Product Data for AT20, AT40, AT72, AT87, AT88 Transformers (English) - Legacy Product Docu PDF ↗
Installation Instructions for AT Transformers (English, French) PDF ↗

The AT20A1123 is a control transformer from Resideo (Honeywell Home), 20 VA, 120 V primary, foot mount.

Almost every heating and cooling control runs on 24 volts, and the transformer is what makes it. It takes line voltage in and delivers the low-voltage supply that the thermostat, the zone valves, the relays and the gas valve all share.

Size it by VA, not by voltage. VA is the load the transformer can carry, and every device on the control circuit adds to it — a zone valve pulls current while it drives, a relay coil holds current the whole time it is energized, and a Wi-Fi thermostat draws continuously. A transformer sized for the original equipment will not carry the same system after two zones and a smart stat have been added, and the symptom is intermittent: things work alone and fail together.

Mounting matters more than it looks. The transformer has to land where the enclosure has room and where the leads reach, and a foot-mount body will not sit in a knockout that was made for a plate mount.

Leadwires are 9 inches.

Brand: Resideo (Honeywell Home)
Manufacturer Part #: AT20A1123/U
Type: Control transformer
Rating: 20 VA
Primary: 120 V
Secondary: 24 Vac
Mounting: Foot
Leadwires: 9 in
UPC: 085267035613
Category: Transformers

Full Specifications

NEMA Rating	Yes
Primary Connection Size	9 in, 229 mm
Primary Connection Type	Leadwires: Black and White
Primary Input Voltage	120 V ac
Secondary Connection Size	9 in, 229 mm
Secondary Connection Type	Color coded primary and secondary Leadwires
Secondary Open Circuit Voltage	26.5 V.O.C.
VA Rating	19 VA
Frequency	50 Hz; 60 Hz
Approvals and Standards	UL Component Recognized, CSA Certified, NEC Class 2
Ambient Operating Temperature (Max)	105 °F, 41 °C
Ambient Operating Temperature (Min)	-20 °F, -29 °C
Product Height	2 in, 50.5 mm
Product Width	2.25 in, 57 mm