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!
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. A transformer is the light bulb end of one circuit ( could be either higher voltage reduced to lower voltage [e.g. 110 Volts to 24 Volts] or it could be lower voltage increased to higher voltage [e.g.. 230 volts to 460 volts]) and a power source for another part of the circuit.
When we talk about transformers, we talk about primary and secondary sides of the transformer. The primary side of the transformer is the light bulb. The secondary side of the transformer is the new power source.
The secondary side of the transformer is often labeled LOAD. The load has a VA (volt-ampere) rating. The VA rating is used to select the properly sized fuse for that circuit. The fuse must be smaller than the VA rating of the transformer. This will then determine the maximum load that can be connected to the transformer.
Simply put, the VA rating is the math formula used in determining the amperage at a given voltage. So how does this work?
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 —-
The basic formula is :
VA ÷ VOLTS = Maximum amp load
So, using this formula, a 75 VA rated 24 volt transformer has a maximum load of 3.125 amps. To get this divide the VA rating by the voltage.
75 VA ÷ 24 volts = 3.125 amps
so this circuit would be fused at a 3 amp maximum fuse.
Or take a larger VA transformer:
250 VA ÷ 24 volts = 10.41 amp
so this circuit would be fused with a 10 amp maximum fuse.
Conversely, 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.
10 amp fuse × 24 volts = 240 VA rating
Since the fuse should always be smaller than the VA rating, you would probably need a 250 VA transformer to power this circuit.
One word of CAUTION, yes, you can use a larger VA rated transformer on a circuit [replace a 40 VA with a 75 VA] but if you do this, you should NOT put in a larger fuse because you could be compromising the circuit that is already there. Remember, the fuse is the safety for the EXISTING circuit. 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. The fuse size is based on the existing load!
I hope this little insight into transformers helps in further understanding electrical components in HVAC equipment.