In my hivoy, the prior owner put a metal butt connector in a switched spot that controls my heater blower, cab lights, signal lights and fuel pump. I'd like to correct this and put a fuse in it. Any thoughts on what size fuse I should use?

 

You would have to add together the Amp draw on all those items and get a fuse with just a tad higher rating. Off the top of my head I'd guess a 35 amp.

 

The heater itself needs 10 amps.
The cab lights and signal lights need very little, a few amps.
You should run a separate wire from the switched panel to the fuel pump and fuse it per manufacturer's recommendations with an inline fuse.
Fuses protect wires, not stuff on wires. Do not put a 35 amp fuse on a 10 amp circuit, especially to a fuel pump.

 

Since you need those circuits at different stages of operation, they shouldn't be all mixed together.

It sounds like the PO was trying to find a valid power source for the electric fuel pump (and missed) during a fuel pump upgrade.

 

Hey Thipdar. I was thinking just the oposite. Please help me understand. I thought having things used at different stages would actually be better. That way the chance of all parts in the curcuit on at the same time would be low. But with a fuse strong enough to protect the curcuit just in case they are. Which is why I suggested measuring the draw on all the items in the curcuit. And Cougarjohn warned of putting too high a fuse in the curcuit.
But I suppose all this is Moot Point. I think the fuel pump should have it's own curcuit,relay, and it's own fuse.

 

While it might be o.k. to mix some of these four circuits together, it doesn't make sense to have (for instance) the Dome light and the Fuel Pump on the same circuit. You may want to use the Dome Light before and/or after the engine is running. Leaving power available to the Dome Light all the time is o.k. because it is only used intermittently (not counting when someone switches it on via the Light Switch and then forgets to switch it off). Leaving power to the Fuel Pump all the time would not be o.k., because the Fuel Pump would then drain the battery. These two circuits need to operate independently from each other.

If you only needed a fuse for the circuit with the highest draw, there would only be one fuse in the Fuse Block.

You don't want Starter Motor current running through your radio, you'd be listening to "The Sounds of Silence" in fairly short order.

Different fuses protect different parts of the system. Given a mix of Fuel Pump, Dome Light, Heater Motor and turn signal circuits, a short in the turn signals could cause fuel starvation and kill the engine. Setting up multiple fuses prevents a failure in one subsystem from causing a failure in a different subsystem.