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.
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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.
York Central Tech Talk 
Interesting article, I once had a rooftop ( Carrier 4 ton ) that would run for about 15 minutes and blow the fuse. It confounded me to no end. I never coud solve the problem, it was after hours and tech support was not available. Sadly, I lost a customer and the chance to add to my arsenal of knowledge. I wonder if you could expound on this problem, and break down the steps to solving this problem. Low voltage blowing fuse. Thank You for any help you can offer.
Roger:
As with any electrical circuit, blow fuses are caused either by shorts in the circuit of be a component drawing high amperage above the fuse rating.
In the case of low voltage circuits blowing fuses, if no apparent shrts are found, it is usually something like a contactor coil being bad or the contatactor is “sticking” causing the contactator to draw high amps.
As with any electrical circuit, it is a matter of finding the component that is causing the problem. this can be done by isolating individual parts from the circuit and retesting the circuit. This can be done by “ohming” out individual parts and seeing it the resistance exceeds the rating. Always keep in mind that a transformer is a power source. Once the power leaves it, either a switch or a “light bulb” (any end load), or any wirining between the switch and light bulb, couled be the source of the fuse blowing.
Our building has 25 York HVAC units which are labeled 460V with min voltage 432 and max 504, we have 480v coming into the building and the meters read 484- 490 on average, we keep blowing circuit boards and York tech support is saying that our voltage is too high it needs to be 460V. That doesn’t seem to match what the equipment specifications read, what else could it mean.
the circuit boards are usually all 24 volts. Not knowing any model numbers, I will assume they are of the 24 volt variety. When you say you are “blowing boards”, what is happening? Is it a specific “circuit” all the time? What is the voltage at R & C on the board? Is it the output to the contactors? How old are the units?
If it is the output circuit to the contactors (blower, compressors, fan motors) if the contactors are sticking, they draw high amperage and you will loose the micro-relays for the output on the boards. From my experience, this is the most commom cause if you are losing outputs — the voltage and board is not bad but the “load” is where your problem is