When a transformer that powers the control circuit of a chiller or rooftop unit (RTU) fails, and the original part is no longer available, an obsolete HVAC transformer replacement can be selected from the original’s nameplate ratings. Six ratings are especially important: secondary voltage, primary voltage and tap, volt-ampere (VA) rating, Class 2 status, overcurrent protection, and mounting. A replacement that meets each of these can generally be installed in place of the original, regardless of the manufacturer.
Functional Devices, Inc. manufactures RIB® (Relay-In-a-Box®) transformers from 20 VA to 375 VA, with primary voltages of 120, 208, 240, 277, and 480 Vac and 24 Vac or 120 Vac secondaries. This guide covers how each of the six ratings is matched, where a replacement can differ from the original, and where it cannot.
Reading the Nameplate on the Original Transformer
The original transformer’s nameplate or label usually lists its primary voltage or voltages, secondary voltage, VA rating, and frequency. Class 2 transformers are typically marked as Class 2. Overcurrent protection is visible on the unit itself as a circuit breaker or fuse holder, or is noted as inherent limiting on the label. Mounting style and dimensions are measured from the unit and the panel it sits in.
When the nameplate is unreadable, the same ratings can usually be recovered from the equipment’s wiring diagram, which shows the transformer’s primary connection, secondary voltage, and often its VA. The equipment manufacturer’s part number identifies the original part, but it does not by itself give the ratings a replacement has to meet.
Matching Secondary Voltage
The secondary voltage should be matched exactly. Many RTU and chiller control circuits that power thermostats, controllers, and relays run at 24 Vac; some equipment uses 120 Vac for contactor coils and control relays. A 24 Vac transformer cannot replace a 120 Vac one or the reverse, regardless of VA.
Functional Devices’ TR series covers both. Most models have a 24 Vac secondary. The 120 Vac secondary models, such as the TR100VA008 (100 VA, 480/277/240/208 Vac primary, circuit breaker, foot and dual threaded hub mount), are general-purpose transformers rather than Class 2.
Matching Primary Voltage and Selecting the Tap
The replacement’s primary must include the voltage the equipment supplies to the transformer. Multi-tap primaries cover several supply voltages with one part: each tap is a separate color-coded lead, and the lead matching the supply is connected along with the common. On the TR100VA008, for example, the primary leads are gray for 480 Vac, brown for 277 Vac, orange for 240 Vac, and red for 208 Vac, with black as the common.
The tap is selected from the measured supply voltage, not the equipment’s nameplate range. A unit rated 208/230 V may be running on either, and the secondary voltage scales with the ratio between the supply and the tap. A 24 Vac transformer connected on its 240 Vac tap to a 208 Vac supply delivers about 20.8 Vac (24 × 208 ÷ 240), which can leave relays and contactors below their pull-in voltage.
Sizing the VA Rating
The replacement’s VA rating is equal to or greater than the original’s. A larger VA rating does not force more current into the load, since the load draws only what it uses; it raises the load the transformer can carry before its voltage sags or its protection trips.
When the original’s VA rating is unknown, the load is added up from the devices on the secondary: controllers, relay coils, contactor coils, and any valves or dampers. Contactor coils draw several times their sealed current at the moment they pull in, so a transformer sized only to sealed current can sag when several coils energize together.
Class 2 vs. General-Purpose Transformers
A Class 2 transformer is power-limited: at 24 Vac, a Class 2 power source is limited to a 100 VA nameplate rating under NEC Chapter 9, Table 11(A). The low-voltage wiring on a Class 2 secondary is installed under Class 2 wiring methods, which are permitted because the source is power-limited, so a Class 2 original is replaced with a Class 2 transformer. Replacement parts should be confirmed against the NEC edition in force and any additional requirements of the Authority Having Jurisdiction.
Functional Devices’ Class 2 models with a 24 Vac secondary run up to 96 VA. The TR100VA015, for example, is a 96 VA Class 2 transformer with a 480/277/240/208/120 Vac multi-tap primary, a circuit breaker, and foot and single threaded hub mounting. When the original transformer is rated above 100 VA, it was not Class 2, and the replacement is a general-purpose model of at least the same VA.
Overcurrent Protection
Transformers are protected in one of three ways: inherently limited, fused, or by a circuit breaker. Small Class 2 transformers such as the TR20VA002 (20 VA, 208 to 24 Vac) are inherently limited, meaning the transformer’s own construction limits its output under a fault. Larger models use a fuse or a circuit breaker.
The replacement provides at least the protection the original had. If the original had a circuit breaker and the equipment’s wiring relies on it, the replacement either includes a breaker or is installed with equivalent protection on the secondary.
Mounting and Physical Fit
Functional Devices’ TR series transformers mount by a foot, by one or two 1/2” NPT threaded hubs, or both, and some models have side or bottom openings instead of hubs. Foot mounting fastens the transformer to a panel; hub mounting attaches it to a junction box through a knockout. The replacement is checked against the space available in the equipment’s control panel and the original’s mounting holes or knockout, using the dimensions on the datasheet. A different footprint may need new mounting holes; a replacement that does not fit the enclosure cannot be used.
Frequently Asked Questions About Obsolete HVAC Transformer Replacement
Can a multi-tap transformer replace a single-voltage transformer?
Yes, when its secondary voltage, VA rating, Class 2 status, and protection match or exceed the original’s. Only the tap matching the supply voltage is connected; the multi-tap primary lets one stocked part replace transformers from equipment on different supply voltages.
Can a larger VA transformer replace a smaller one?
Yes. A 75 VA Class 2 transformer can be replaced with a 96 VA Class 2 transformer, for example. The higher rating raises the load the transformer can carry; the devices on the secondary draw the same current as before. A Class 2 original is still replaced with a Class 2 transformer, which limits the replacement to 100 VA.
What happens if the wrong primary tap is used?
The secondary voltage changes in proportion. A 240 Vac tap on a 208 Vac supply delivers about 20.8 Vac from a 24 Vac transformer, which can prevent contactors and relays from pulling in. A 208 Vac tap on a 240 Vac supply delivers about 27.7 Vac, which runs coils and controls above their rated voltage.
Why match the nameplate instead of the original part number?
An equipment manufacturer’s part number identifies the original part, not its ratings. The nameplate gives the six ratings a replacement has to match, and any transformer that meets them can be installed regardless of who made the original.
Sourcing a Replacement Transformer
To source a replacement transformer for a chiller or RTU, find an authorized Functional Devices distributor near you or browse the RIB® transformer lineup.