amps at 240 amps at 120

Hi guys just a quick question
If a set of contacts are rated for 3 amp at 240 volts how do you figure out what it would be for 120 volts
Thanks
Rick

 

I'm not sure I understand your question. But.... Watts = volts x amps. So 240 volts x 3 amps = 720 watts. To carry the same wattage at 120 volts, you would do 720 watts / 120 volts = 6 amps.

If you keep the wattage the same, a wire can be thinner with higher volts. And vice versa. That is why electric transmission lines run at such high voltage...it allows them to use thinner wire.

 

Are you sure it is 3 amps @ 240 volts, and not 3 amp and 240 volts?

Most contact ratings I have seen are rated for X voltage and X Current. In example of your contacts, they would be rated 3 amps max current and 240 max voltage.

 

Hey thanks guys
Maybe I asked the wrong question
Im looking at this on ebay
1pcs ZG3M 308B Solid State Relay 8A Output 90 480VAC | eBay
This is the discription
1PCS ZG3M-308B Solid State Relay 8A Output 90-480VAC
So Im really after how many watts or amps I can draw at 120 volts
Is 8 the max at any voltage
Thanks

 

8A is the maximum no matter the voltage.

 

Electrical components including switches and contacts have both a maximum amperage rating and a maximum voltage rating.

But the maximum voltage and maximum amperage as stated in the specifications or instructions should be such that you can use both of the maximums safely

Meanwhile if you do not use the maximum voltage stated, you may be able to use more than the maximum amperage stated, or vice versa. Because of the complexity of describing this relationship i.e. answering the OP's initial question, "safe" maximum voltage and amperage are given instead of "absolute" maximum voltage and amperage.

Exceeding the absolute maximum voltage leads to flashover, or current jumping the switch or relay contacts in the fully open or off position (or between other subcomponents) with this arc most likely to commence and persist as the switch is flipped off. Exceeding the absolute maximum amperage leads to overheating where the most wear and tear happens at the contacts during the arc that always occurs as the switch is flipped off.