Sizing relays

Good day all,

I wanted to know how you all go about sizing relays. What size relay should be used as the main relay. What should be used as the enabled relay etc. Is their a formula or a "rule of thumb" when selecting the correct amperage relay?

I know for fuses we is 150% of the peak current draw. Was wondering if its the same for relays

A relay has a rated current which is the maximum current it is capable of continuously handling - In the case of the common Bosch relays the rating is 30 amps. Check the current draw on the circuits the relay will be controlling and make sure that you're below the maximum, otherwise you may need to incorporate more relays or split the circuit load differently. With the exception of the likes of fuel pumps and radiator fans there shouldn't be much in the way of high current draw from the engine.

thanks Andre, it's much clearer now

Hi Andre. I am in the process of wiring my vehicle, and wondering the best way to configure the following items

EFI fuel pump

Electric Vacuum pump ( brakes)

Electric water pump/fan

Motec M48 EFI

Headlights

How many can be grouped together on one switch and or relay or should they all be separate ?

most of those are power hungry devices. look at the specs of your fan. the fans i have in my car (spal) can draw upwards of 30A @ 12V mind you i am 16V system so slightly different for me. fuel pump can draw upwards of 25A/pump water pump i would estimate 15 amp (my meizer advertises 15A) although i have to get setup to collect data for actual current draw you motec M48 depending how loaded it is can draw like 5A headlights 55W roughly 5 amp/per. if it were me each one of those would have its own relayand fuse. and use the outputs on the ecu to trigger fuel pump water pump and fan. headlights if you have lots of outputs can be ecu triggered. have you taken the club level coarse? worth its weight in gold. actually all the courses i have taken have something to offer. your motec should be powered by a relay triggered by acc relay