Locate suitable power feed for relay switch

[cwilliams255]

I would like to power something but only when engine is running. Using a relay but where can I find something suitable to use for the switch feed? Relay in back, switch in front ideally

Thanks

[Bob]

Geoff's our man, but maybe a piggy back fuse in the box by driver's right knee?

https://www.halfords.com/technology/das ... fJEALw_wcB

You'll find them cheaper on flea bay, but you get the idea.

[g8dhe]

Simplest is as Bob suggests a piggyback fuse holder on the ignition switch "ON" fuses, but first why do you need a relay ? A relay is normally only used if your switching something needing more current then can ne handled by a switch - up to 5 Amps normally and if you choose the switch maybe as much as 10 Amps. https://www.ebay.co.uk/itm/Add-A-Circui ... 3513228993

Second point do you need it on during Starting itself (IG1 circuit 2nd row far left 5 fuses) or only when the engine is running (IG2 circuit 2nd row far right 3 fuses), if any electronics are involved its best NOT to have it powered up during starting due to the high voltages that can appear!

[cwilliams255]

Thanks guys, need a relay because it’s 25A and the instructions say so to support 240 and 12v. Given the draw I’d only like it to be able to be switched on whilst the engine is running. In case anyone is wondering, it’s an electric calorifier. I would have preferred one connected to coolant loop but they were slightly too big so decided to give it a go so that’s we have a small amount of hot water for a quick shower or more pleasant wash. It might not work out but I didn’t loose any internal space as I moved my water tank underneath

[Tessie]

If the fuse that Geoff refers to is only powered when the engine is running as opposed to when the ignition is switched on, engine running or not, then that is the one to use. I am not doubting you Geoff, you are the man that knows but I know that a relay is used on cars for towing caravans so that the fridge can be powered and the caravan's battery can be charged when the engine of the towing car is running and switches the power off to the caravan if the engine is not running but the ignition is still turned on. Presumably the relay is switched on and off by the alternator output level (guessing). Different relays available but as an example:
https://www.rotatingelectrics.co.uk/uni ... le-mp2881b

Hope I am not leading you down a wrong path.

Just seen your last post so if you do need a relay this 30A one might be suitable
https://www.ebay.co.uk/p/26023789976