Transformer Sizing for Unit Controllers

Issue:

Transformer Sizing for Unit Controllers

Resolution:

Transformer Sizing for Unit Controllers

If you are providing a new transformer for power, use a UL listed Class 2 power transformer. The transformer must be sized to provide adequate power to the controller and output devices, including relays and valve actuators.

- Know your systems total power draw in volt-amps (VA)

- Convert all your devices ratings to this standard (VA) and add them up to get your total power requirement.

- Consider - inrush current (maximum, instantaneous input current drawn by an electrical device when first turned on)

- Is a good practice to calculate 30% to 50% more than the total VA for future expansion.

 

Note: Loading a Transformer fully, even though it is supposed to be designed for it, will shorten its useable life due to excessive heat buildup inside the unit.

 

Notes:

What is a Class II Transformer?

Transformer used to supply Class II circuits. Rating of less than 100 VA and a maximum secondary output of 30 VAC. The maximum VA generally offered is 75 and the most common secondary voltage is 24 VAC. Class II Transformers are either inherently or non-inherently limited.

Note: Class 2 circuits are limited to voltage and current values that will not usually present a shock or fire hazard.

 

Inherently Energy Limited Class II Transformers - Class II transformers up to 50 VA are "Inherently Limited" which means that the transformer, if overloaded, will short itself out and fail safely, not requiring a fuse.

 

Non-Inherently Energy Limited Class II Transformers - 60 thru 100 VA Class II transformers are generally protected by a resettable circuit breaker or a fuse within the transformer secondary. Without overload protection, the transformer would not satisfy the safety requirements for a Class II circuit.

 

What is VA rating?

A volt-amp (VA) is basically just another unit for measuring power. Describes a quantity of electrical power used, supplied, or otherwise handled by an electrical or electronic device VA is the unit of choice for controls manufacturers.

VA vs Watts

On a DC circuit, 1 VA is the equivalent of one watt (1 W). On an AC circuit, to convert from W to VA, multiply by 1.4, or to go the other way, multiply by 0.714. Note: Because most AC circuits contain reactance, the VA figure is greater than the actual dissipated or delivered power in watts.

 

Other Question/Problem/Cause Statements:

• Transformer Sizing for Unit Controllers.

Facts:

• Sensors and Accessories
• Terminal Unit Controller (TUC)
• Thermostat Control Module (TCM)
• Tracer MP501
• Tracer MP503
• Tracer MP580/581
• Tracer UC400
• Tracer VV550/551
• Tracer ZN510/511
• Tracer ZN517
• Tracer ZN520/521
• Tracer ZN523
• Tracer ZN524
• Universal Programmable Control Module (UPCM)
• VariTrac Comfort Manager
• VariTrac II Central Control Panel (CCP)
• VariTrac III Central Control Panel (CCP)

Original wave: wave48451