Hi,

I have a question that I'm struggling with and was hoping for some help.

I'm trying to select the correct circuit protection for a 240/24VAC Transformer and field cabling.

The transformer I use has specifications for which breakers to use. If I use a 250VA transformer the manufacturer specifies a 2A 230VAC CB but I know the transformer can produce 10A at 24VAC.

If the lower voltage rating of the CB changes the circuit protection required to protect the TX due to the power to heat and overload the circuit. will this have the same effect on the field cable and allow me to run more current on a smaller cable?

 

Unsure of what you are asking about. Yes, you must to fuse the primary input (240VAC) circuit to protect the utility wiring to the transformer and maybe size the fuse to limit the power drawn from the secondary 24VAC. The 24VAC secondary lines can also be fused to protect wiring and maybe components on that low voltage side also.

 

Calculate the prospective fault current: the current that will flow if there is a short circuit at the end of the cable connected to the secondary. This requires you to know the output impedance of the transformer,
Whatever protective device you choose must be able to trip at the prospective fault current in less time than it takes the transformer to overheat or the wires to catch fire.
If necessary, you may have to increase the size of the cables on the secondary.

 

If you are fusing the 240 volt input of a transformer, the fuse size is determined by the input CURRENT. That is related to the output POWER, not the output CURRENT rating. A 250 VA transformer at 240 volts in will draw just a bit over 1 amp, so a two amp fuse is a good choice. 250 VA out of a 24 volt secondary will be just a bit more than TEN amps, so a 2 amp fuse there will be a problem.

 

read electrical code that applies to country where this is to be used. it tells you exactly what to do and how. while there is a lot of similarities, national standards are not harmonized and universal.

 

Electrical codes are concerned with avoiding fires and shock hazards. Advice on fusing is all towards those goals.

Understand that with transformers, it is the input POWER that mostly appears at the output. Watts in = watts out, and WATTS= voltage times amps. (assuming 100% efficiency)