Properly connecting primary side of low voltage transformer in roof top hvac units

Sometimes we find the low voltage circuit of an operating roof top hvac unit with lower than normal voltage.

 

Many manufacturers ship new roof top hvac units with a low voltage transformer that can be connected to either 208V or 230V primary voltage, depending on the line voltage at the installation site.

The image above is of a low voltage transformer installed in a roof top unit. The red and blue wires connected to the spade terminals at the top of the transformer in the photo above are for connecting the secondary winding of the low voltage transformer to the secondary circuit. These two wires (with bare metal connectors) connect the circuit board (low voltage circuit) to the low voltage transformer. The larger red and blue wires with the blue plastic connectors that are connected to the spade terminals at the bottom of the transformer connect to the transformer’s primary winding. The red wire is connected to the spade terminal on the left where the sticker indicates this terminal is the “C” terminal. The C terminal is the common terminal. The blue wire is shown connected to the 230V terminal. There is a third terminal between the C terminal and the 230V terminal. This middle terminal, which has nothing connected to it in this photo, is labeled as 208V.

Confirm the actual line voltage at the installation site by measuring voltage using a digital multimeter (DMM).

In the image above for example, the voltage between two of the incoming leads to the main contactor reads 210V. A similar voltage was obtain between L1 and L2, between L2 and L3, and between L3 and L1. This means that the voltage to the unit is considered 208V. Note that voltage will vary slightly at different points along the electrical grid. At this particular commercial location, the actual line voltage reads a little higher voltage than the nominal voltage of 208V and this is not uncommon.

WARNING: These repair activities involve working on high voltage equipment that poses serious electrical shock risk. Follow all safe electrical work practices and ensure that only qualified, trained service technicians perform these repair activities. Hazardous Voltage! Failure to disconnect power before servicing could result in death or serious injury. Disconnect all electric power, including remote disconnects, before servicing. Follow proper lockout/tagout procedures to ensure the power cannot be inadvertently energized.

Attach the primary wires going to the low voltage transformer to the appropriate primary side spade terminals.

  • For example, if actual measured line voltage of the circuit is 210V, then connect one primary side lead going to the low voltage transformer to the 208V spade terminal. There will be no wire connected to the 230V spade terminal.
  • If the actual measured line voltage of the circuit is 233V, then connect one primary side lead going to the low voltage transformer to the 230V spade terminal. There will be no wire connected to the 208V spade terminal.
  • In either case, the common wire going to the low voltage transformer will connect to the C terminal (for common) of the low voltage transformer.

 

If a low voltage circuit in a roof top unit is found with lower than expected voltage, say a reading of 23.6V, then ensure that the correct primary terminal is being used on the low voltage transformer. If the actual primary voltage, for example, is found to be 210V but the primary lead to the low voltage transformer is found to be connected to the 230V primary spade terminal, then the low voltage transformer primary side is not hooked up correctly.

The photo above shows actual voltage on the low voltage circuit to be 23.6V. This voltage measurement is taken across both of the low voltage terminals of the low voltage transformer.

To resolve the issue, shut off power to the unit by turning the disconnect off, and then remove the primary wire from the 230V spade terminal and connect this primary wire to the 208V spade terminal. Then turn the disconnect back on and measure secondary side voltage, expecting to find a higher voltage reading of say 26.2V.

The photo above shows actual voltage on the low voltage circuit to be 26.2V. This voltage measurement is taken across both of the low voltage terminals of the low voltage transformer.

In this case, utilizing the correct primary winding of the low voltage transformer raised the voltage of the low voltage circuit to a level (26.2V) that is more typical.

There are other reasons why the low voltage circuit may read lower voltage than what is typically found. One possible reason for lower than expected low voltage circuit voltage could be having more loads in the low voltage circuit than what the low voltage transformer can power.

Another reason for lower than expected low voltage circuit voltage could be a loose connection where the leads connect to the low voltage transformer or to the circuit board. There could also be corrosion at electrical connection points.

There could also be a short to ground that is bleeding off some voltage from the circuit. The short to ground would have to be very minor as a short normally would blow the fuse in the low voltage circuit.

While it is very rare, it sometimes happens that a low voltage transformer fails. Normally when this happens, it is the failure of some other component in the circuit that causes the low voltage transformer to fail.

For helpful hints when troubleshooting a failed gas valve in either a residential furnace or a commercial building roof top unit, refer to the blog for troubleshooting a failed gas valve.

There is also a blog for helping to decode the complicated meaning behind the Trane roof top unit model number. Similarly, there is also a blog to help troubleshoot the model numbers for a Trane residential furnace.