I hope this little insight into transformers helps in further understanding electrical components in HVAC equipment. The fuse size is based on the existing load! Just because you substitute a larger VA transformer, doesn’t mean you are going to put in a bigger fuse even though the transformer can handler the larger load. Remember, the fuse is the safety for the EXISTING circuit. One word of CAUTION, yes, you can use a larger VA rated transformer on a circuit but if you do this, you should NOT put in a larger fuse because you could be compromising the circuit that is already there. Since the fuse should always be smaller than the VA rating, you would probably need a 250 VA transformer to power this circuit. So this circuit would be fused with a 10 amp maximum fuse.Ĭonversely, if you know the actual LOAD on the transformer in amperes or the existing fuse size, the you can calculate the VA rating for the voltage so you can properly replace a transformer. So this circuit would be fused at a 3 amp maximum fuse. To get this divide the VA rating by the voltage. So, using this formula, a 75 VA rated 24 volt transformer has a maximum load of 3.125 amps. If you know the voltage output of the transformer (load side) and the VA rating of the transformer, you can calculate the amperage of the load side of the transformer. Simply put, the VA rating is the math formula used in determining the amperage at a given voltage. This will then determine the maximum load that can be connected to the transformer. The fuse must be smaller than the VA rating of the transformer. The VA rating is used to select the properly sized fuse for that circuit. The secondary side of the transformer is often labeled LOAD. ![]() The secondary side of the transformer is the new power source. The primary side of the transformer is the light bulb. When we talk about transformers, we talk about primary and secondary sides of the transformer. A transformer is the light bulb end of one circuit ( could be either higher voltage reduced to lower voltage or it could be lower voltage increased to higher voltage ) and a power source for another part of the circuit. If you keep in mind that everything in an electrical circuit is either a power source, switch, or light bulb, transformers are actually light bulbs and power sources. Supco SXT79R 75VA HVAC Transformer 120/208/240/480 Volts x 24 Volt Foot Mount. An inline fuse or a transformer with a circuit breaker can be installed to protect the new transformer while the problem is located.Īre you a homeowner or own a commercial property? Check out mybryantdealer.I often get questions on transformers and blown fuses and asked if they can put in a bigger fuse on the control circuit - of course the answer is NO! This should be found and repaired before replacing the transformer, as a transformer rarely fails on its own. If the transformer fails, you most likely have a short or overload somewhere in the control circuit. ![]() Transformers are designed to fail if a short in the secondary or control side occurs. To determine the maximum transformer current, divide the transformer VA by transformer secondary voltage.Īmperes = VA/Secondary Volts or 40VA/28V = 1.43 amperes. To determine the required transformer VA, multiply the secondary voltage by the required amperes:įor example, VA=Volts x Amperes or 28V x 1.2 amperes = 33.6 VA. This should be checked when installing 208/240 volt package equipment. The low secondary voltage will cause contactor, relay chatter or failures. 208V wired to the 240V tap will result in low secondary voltage. If the primary voltage is 240V, care must be taken to wire only to the 240V primary tap. If the primary voltage is 208V, care must be taken to wire only to the 208V primary tap. To properly size a transformer, first the primary voltage must be selected. The VA needs to be large enough to handle the total control power requirements, so a high enough VA should be selected. The transformer power is rated by VA capability. ![]() The actual output voltage will depend on the actual primary voltage. The secondary voltage or the output voltage will normally be between 24 and 30 volts. The primary voltage is the input voltage, normally 120, 208, 240 or 460 volts. This allows us to run low voltage wiring outside of an enclosure without electrical conduit. In the HVAC industry, we commonly use Class 2 control transformers to lower the voltage to 30 volts or less, with a power rating of 100VA or less.
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