IJ. said:
I did mine with a fusible link direct from the battery then relays Ign controlled to the loads.
And you said you weren't electrically inclined
jdub, technically speaking you should feed the block using a conductor sized to match the block's rating, not what loads you're going to put on it. You don't wire a 100 amp panel in a house for 30 amps just because you're only going to pull 30 through it. Plus you have 6 fuse positions. Say your loads total 20 amps so you feed the block with 12 or 14 awg. What's to stop you from adding future goodies? Do that and you'll exceed the original ampcity of the feed.
Wire it for the loads you anticipate and add a bit more for future use because as long as there's an emtpy fuse holder you'll likely have a need for it someday. Based on your loads I'd use 10 awg as long as the run isn't too long. That'll give you a fair margin for future use. It's also why I suggested taking the feed further from that battery. Course, if the feed you tap is smaller than 10 awg it's all for naught so it may be better to to go to the battery. Be sure the relay you use can handle at least 40 amps. The typical 40 amp automotive Bosch type would work but a bit bigger is better for the reasons mentioned below.
Iinductive loads (fans, motors, ect) have a high starting current that drops off as the motor builds speed. Tungsten (lighting) loads have a tremendous inrush current because the filament in a lamp has a very low resistance when cold. Ohm a bulb and you'll see how it can appear as a dead short to the supply at the moment of turn on. For both these reasons stuff technically needs to be bigger than the loads suggest, especially if the load is cyclic with a short duty cycle. No big deal in this case but something to remember when adding futue loads.
Use 10 awg and fuse it for 30 or 40 amps. All this assumes the block itself is rated for at least 40 amps. You have to consider everything in the circuit when dealing with total ampacity. Connections, the fuse springs, everything. For example if the internal buss in the block is rated for 20 amps it's not a good idea to pull 40 through it. This is what I meant when talking about the block's rating. It must have a spec and technically you should wire a big enough feed to use it all, but it's your call.
And again, overcurrent protection is selected based on the conductor, not the load. The same goes for the branch circuits coming off the block. Use the right size wire to handle the load and then use right fuse to protect the wire. Otherwise the car becomes a giant toaster on wheels.
I didn't realize you were going to use the wideband for engine control. In that case they're correct in that the ecu ground is a good choice. Keep in mind though the stock O2 sensor's ground runs through the turbo and exhaust manifold gaskets and a mess of other "crappy" connections and still works OK. Btw, that's what the small ground strap on the PAX side of the firewall is for, the one many guys don't replace thinking it's useless. Me, I don't like like the stock "torturous path" and ground the firewall right to one of the O2 sensor mounting bolts.
The PLX doesn't need to get + from the ecu. No need so why do it? Still, do what they say if there's any question. Just be sure it's fused. If you have room on the block you might want to take it off that instead.
