Transformer Sizing

Sizing the transformer for a zoning system is often overlooked because of the trivial nature of the role it plays in the zoning application.  It is, however, critical to provide a properly sized transformer on any Zoning system.  Zoning panels can misbehave and even cause the equipment to do some erratic things if the transformer has been undersized.  Over sizing is not typically an issue, so this should be re-named MINIMUM Transformer Sizing.

NOTE: Included Transformer - Don’t be deceived by a zoning system that has an included transformer, this doesn’t mean you do not have to calculate the necessary transformer size.  The included transformer may not be enough to handle all of the dampers or the type of thermostats that the specific application demands.

Calculation

[Zoning Panel VA]+[All Thermostats VA]+ [All Dampers VA]=Total VA<Transformer Size

The Transformer Size must be GREATER than the Total VA. Simple, right? Yes, it is simple, but crucial to calculate accurately. So, take into consideration ALL of the pieces being attached to the zoning panel. Refer to the manufacturer for exact VA ratings on each of the pieces.

• Dampers         -  2 to 12 VA (each Damper)
• Thermostats    -   1 to 3 VA (each Thermostat)
• Zone Panel      -   10 to 20 VA

See SmartZone Guide for VA ratings

Fuse Size

In all zoning applications the transformer secondary (24VAC) side should be fused with a standard in-line fuse.  This is very important with zoning because the zoning panels can be sophisticated electronics that need protection from the vulnerabilities in the field.

Some transformers have breakers on board but it is still recommended to provide a field installed in-line fuse to protect the zone panel.  The resettable breaker built on some transformers is sized to only trip if the voltage exceeds the maximum of the transformer.   The inline fuse, on the other hand, can be sized according to the specific installation.

The inline fuse size is calculated by using the TOTAL VA requirement for the system (in the transformer sizing calculation section).  This number is divided by 20 to get the FUSE SIZE in amps.  The 20 comes from 24VAC X power factor (assuming a power factor of 0.83).

FUSE SIZE (AMPS) = (Total VA)/(24V*0.8333) = (Total VA)/20

This means a 40 VA transformer should never have more than a 2 AMP fuse on the secondary side.

NOTE: Wall plug transfomers typically have a built in fuse that is sufficient but it wouldn't hurt to put an inline fuse even with this style of transformer.