American Auto Wire
Username Post: Using Ign Switch to power a fuse block        (Topic#341297)
LGK 
Contributor
Posts: 189
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
12-17-16 07:08 AM - Post#2666455    

I posted this in the Electrical section but putting it here as well:

Guys- As I redo the wiring in my 54 pickup, I have made up a panel assy that among other things includes a switched and unswitched fuse block to cover all situations. I was going to install a relay to power the switched block which has about 55 amps of stuff in it. An experienced electrician (I know his work in both cars and commercial building electrical), claimed that you can simply use the ignition switch to power the fuse block on and off. No need for a relay. If that is true then I would assume you would need to connect like a 4 gauge cable from the ign switch to the "+" of the fuse block. That wire length is only going to be about 2 feet max. Certainly don't see that size cable going to a ign switch right? Maybe I am wrong and a 10 or 12 gauge would work. Yes I have looked up the chart on what gauges can carry in terms of amperage. How have you done it? Thanks.



 
aawtech 
American Autowire "Site Sponsor"
Posts: 4960

Loc: Bellmawr, NJ
Reg: 09-11-06
12-18-16 08:48 AM - Post#2666600    
    In response to LGK

  • LGK Said:
I posted this in the Electrical section but putting it here as well:

Guys- As I redo the wiring in my 54 pickup, I have made up a panel assy that among other things includes a switched and unswitched fuse block to cover all situations. I was going to install a relay to power the switched block which has about 55 amps of stuff in it. An experienced electrician (I know his work in both cars and commercial building electrical), claimed that you can simply use the ignition switch to power the fuse block on and off. No need for a relay. If that is true then I would assume you would need to connect like a 4 gauge cable from the ign switch to the "+" of the fuse block. That wire length is only going to be about 2 feet max. Certainly don't see that size cable going to a ign switch right? Maybe I am wrong and a 10 or 12 gauge would work. Yes I have looked up the chart on what gauges can carry in terms of amperage. How have you done it? Thanks.



No good. You are now using that ignition switch to carry the load of all those additional items. Unless they are really low draw items, that switch will get overloaded in a hurry, and will eventually fail.

Donny




 
Bel Air kiwi 
"2nd Year" Silver Supporting Member
Posts: 2292
Bel Air kiwi
Loc: New Zealand
Reg: 04-24-14
01-03-17 02:31 AM - Post#2669105    
    In response to aawtech

Hi LGK, Donny is 100% on this one. You want as little load as possible going through the ignition switch itself.
Otherwise it will heat up and last no time at all.
Just out of curiosity what circuits do you want to run un-fused?

Normally the starter is not, but that is already a high current relay (Solenoid) controlled circuit.
The ignition primary is not normally fused but that is only 3 Amps max with points and about 7 Amps with street Electronic ignition.
Sometimes the stoplights are not fused, or the fuel gauge.

Generally everything else is fused.

But what I think you should be really thinking is at what times various elements are power feed relative to key position. That is, what runs when on accessories, what is live in key on, and what is live while cranking.
Having made the first decision, then you also need to determine what should be run off relays and what will switch this. Typically anything over 10 Amps continuous.

So on a modern style system there is a relay for everything like head lights, electric fans, fuel pumps, wipers, horn, etc.
Indicators, park lights, hazards, courtesy and dash lights don't need a relay.

The safe rule of thumb is the switched circuit of the relay should be twice the nominal current rating of the circuit. So 15 Amp horns have 30 Amp relays. 30 amp electric fans run a 60 amp relay. This is to avoid relay damage with startup spikes and run them well below design capacity for a good long service life.

One of the smart things to do is get rid of the ammeter as they are a great place to start a dashboard fire and bring unnecessarily large current circuitry into the cabin. Go alternator and if you must monitor it, use a voltmeter gauge.

With all due respect to your Buddy, automotive DC systems are nothing like domestic or commercial AC systems. You need someone with a whole lot of Automotive Background to design a wiring loom for a modern vehicle, or an older vehicle with modern accessories. There are more differences than similarities.

I put up a little relay circuit I use in one of my vehicles to show another poster, (actually I think he wanted to know if I was full of BS or not). If you can't tell me how it works or why it's wired this way you probably need a pro to design your new loom.

Cheers Kiwi






Attachment: Electric_fuel_pump_with_Prime_and_cut_out_jpeg.jpg (37.82 KB) 7 View(s)




48 3100 RHD, 51 Deluxe 4DR RHD, 51 Bel Air parts car, 52 Bel Air P-Glide LHD. Others 23T, 32 Tudor, 58 Edsel pacer 4DR HDT, 79 F250 351C RHD. 69,70,82 Capri. No mobile, no TV, and no Jap cars


Edited by Bel Air kiwi on 01-03-17 02:40 AM. Reason for edit: No reason given.

 
LGK 
Contributor
Posts: 189
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
01-03-17 06:48 AM - Post#2669123    
    In response to Bel Air kiwi

  • Bel Air kiwi Said:
Hi LGK, Donny is 100% on this one. You want as little load as possible going through the ignition switch itself.
Otherwise it will heat up and last no time at all.
Just out of curiosity what circuits do you want to run un-fused?

Normally the starter is not, but that is already a high current relay (Solenoid) controlled circuit.
The ignition primary is not normally fused but that is only 3 Amps max with points and about 7 Amps with street Electronic ignition.
Sometimes the stoplights are not fused, or the fuel gauge.

Generally everything else is fused.

But what I think you should be really thinking is at what times various elements are power feed relative to key position. That is, what runs when on accessories, what is live in key on, and what is live while cranking.
Having made the first decision, then you also need to determine what should be run off relays and what will switch this. Typically anything over 10 Amps continuous.

So on a modern style system there is a relay for everything like head lights, electric fans, fuel pumps, wipers, horn, etc.
Indicators, park lights, hazards, courtesy and dash lights don't need a relay.

The safe rule of thumb is the switched circuit of the relay should be twice the nominal current rating of the circuit. So 15 Amp horns have 30 Amp relays. 30 amp electric fans run a 60 amp relay. This is to avoid relay damage with startup spikes and run them well below design capacity for a good long service life.

One of the smart things to do is get rid of the ammeter as they are a great place to start a dashboard fire and bring unnecessarily large current circuitry into the cabin. Go alternator and if you must monitor it, use a voltmeter gauge.

With all due respect to your Buddy, automotive DC systems are nothing like domestic or commercial AC systems. You need someone with a whole lot of Automotive Background to design a wiring loom for a modern vehicle, or an older vehicle with modern accessories. There are more differences than similarities.

I put up a little relay circuit I use in one of my vehicles to show another poster, (actually I think he wanted to know if I was full of BS or not). If you can't tell me how it works or why it's wired this way you probably need a pro to design your new loom.

Cheers Kiwi








As always thanks for the useful, and when you are talking electrical (just like fuel), critical information.

So I will respond to your questions and advice (all good). If I have not said it already, the truck is wired and items work. My plan is to make sure that it is upgraded without having to necessarily do a Painless (brand) rewire so to speak. From what I saw in the 2 weeks I had it before it went to the shop for mechanicals (power strg, pwr brakes, new auto shifter, etc) things worked and "looked" ok, but still had that stock fusebox on the firewall, and some other things I did not like.

I built a panel with 2 fuse blocks (both marine type). That panel will be located somewhere under the dash. Both have ground busses on them. One constant hot, and one switched. I also built a small assembly that at this point houses the 30 amp horn relay, and the flasher (EPL 27 for LED signal/running lights). I wnat as little on the firewall as possible. That assembly has room for other relays. Headlight relays are resident in the new harness (from Painless) I have for the H4 headlights I am upgrading to.

From the battery, for power (and ground), I am running #4 cables to a pos junction . From there, for the switched fuse box, it goes to a protector fuse (at this point that fuse is 60 amps) and from the protector fuse it goes to a Tyco 75amp heavy duty relay with screw on terminals. On that switched fuse box to answer your question (at this point) resides the fuses for headlights, heater, gauges, and coil...although I am not sure the coil needs to be fused. But anyway that is all that is not the switched fuse box to make sure there is not too much power in one fuse box.
The other constant hot fuse box of course gets power directly from the junction post I mentioned above. Just to be organized, to that box I have everything else...tail, sig, reverse lights; radio, cig lighter (for accessories not cigs!), dome light, etc. I figured as long as I am going thru this fuse what I can.
I AGREE....ammeters are an accident waiting to happen.

So as far as the wire that comes from the ignition switch, that will be a nominal gauge wire that goes to the 75 amp tyco relay mentioned above.

Also parking, running, backup and turn signal lights will all be low power LED's. The only higher power lights are the H4's mentioned above.
You mentioned alternator. If I haven't mentioned it, this truck is non stock as far as the power train. 350 sbc with TH350 auto trans, dual exhausts, all in good order. So alternator..yes. Electronic ignition...yes.

As far as relays, to review, at this point they are: the master one that switches on the fuse box, headlights, horn, the flasher for turn signals. I do not know the condition of the stock heater although the previous owner said it worked. I will have to hunt down where that relay is. Same goes for the stock wipers and what the fuel pump set up is at this point.
To your point about the relay being 2x the capacity of the switched circuit, then the FUSE for a circuit, to protect the relay, needs to be 2x the capacity of the circuit as well. E.G...horn at 15 amps has a 30 amp relay and 30 amp fuse.

the other thing that there is much talk about is the "final inch" of connection, i.e. the spade or crimp on connector that connects to a fuse location. I am using Packard 56's (I think they are called) for the push on ones. And a quality connector for the screw down ones.

Am I missing anything?



 
Bel Air kiwi 
"2nd Year" Silver Supporting Member
Posts: 2292
Bel Air kiwi
Loc: New Zealand
Reg: 04-24-14
01-03-17 10:55 PM - Post#2669250    
    In response to LGK

Hi LGK. To answer your questions and issues I will explain it from the circuits down from the biggest power to the smaller ones.
But first I must specify that a fuse is pure and simply a safety device, however the compromise is they can fail in service without a fault through corrosion and service life. So vital circuits like starting and ignition are not fused.
A relay is a electrical device to turn on or off a much higher current circuit using a smaller switching circuit. It is not a direct safety device.

The starter has a relay called the solenoid which switches the much higher amperage starting circuit. This is the king daddy of demand in a vehicle. So it must have the big fat cables to work. This means the battery power post on the solenoid is already a convenience power post, and this is why the alternator normally delivers its out put to there to save on a duplicate run of wire to the battery.
This is where you would pick up power and run off to everything else in the car/truck normally as nothing else normally draws more than 50 amps.
This is a permanently live point but only becomes active when the starter is engaged or the alternator is charging.

IMHO everything else should be both switched and fused with the exception of a light power feed to the key and back to the ignition which is not normally fused.

Once again you need to plan what is powered up with each key position. The only exception is you can have courtesy lights and stop lights un-switched.
If you have lots of un-switched circuits you will be constantly leaving things on and flattening your battery.
So inside the truck you can run a fuse panel and a relay panel both powered off the starter feed.
Normally you run a simple separate circuit to power the key for the ignition and the the stoplights and courtesy.

So if you get your list of what you want switched and when then you can design a dash loom to connect all the switch gear to the relays and fuse them out from there.

Personally I would fuse using the continuous rating of the fuse at the same as the appliance.

Cheers Kiwi

48 3100 RHD, 51 Deluxe 4DR RHD, 51 Bel Air parts car, 52 Bel Air P-Glide LHD. Others 23T, 32 Tudor, 58 Edsel pacer 4DR HDT, 79 F250 351C RHD. 69,70,82 Capri. No mobile, no TV, and no Jap cars


 
LGK 
Contributor
Posts: 189
LGK
Loc: Cincinnati, OH
Reg: 04-26-14
01-04-17 06:55 AM - Post#2669273    
    In response to Bel Air kiwi

  • Bel Air kiwi Said:
Hi LGK. To answer your questions and issues I will explain it from the circuits down from the biggest power to the smaller ones.
But first I must specify that a fuse is pure and simply a safety device, however the compromise is they can fail in service without a fault through corrosion and service life. So vital circuits like starting and ignition are not fused.
A relay is a electrical device to turn on or off a much higher current circuit using a smaller switching circuit. It is not a direct safety device.

The starter has a relay called the solenoid which switches the much higher amperage starting circuit. This is the king daddy of demand in a vehicle. So it must have the big fat cables to work. This means the battery power post on the solenoid is already a convenience power post, and this is why the alternator normally delivers its out put to there to save on a duplicate run of wire to the battery.
This is where you would pick up power and run off to everything else in the car/truck normally as nothing else normally draws more than 50 amps.
This is a permanently live point but only becomes active when the starter is engaged or the alternator is charging.

IMHO everything else should be both switched and fused with the exception of a light power feed to the key and back to the ignition which is not normally fused.

Once again you need to plan what is powered up with each key position. The only exception is you can have courtesy lights and stop lights un-switched.
If you have lots of un-switched circuits you will be constantly leaving things on and flattening your battery.
So inside the truck you can run a fuse panel and a relay panel both powered off the starter feed.
Normally you run a simple separate circuit to power the key for the ignition and the the stoplights and courtesy.

So if you get your list of what you want switched and when then you can design a dash loom to connect all the switch gear to the relays and fuse them out from there.

Personally I would fuse using the continuous rating of the fuse at the same as the appliance.

Cheers Kiwi



Thanks Kiwi-
Yes, I am aware of the starter solenoid (and the general function of a relay and a fuse).
The truck is set up like that currently (power to the fusebox and other locations from that solenoid positive post). There is NO additional cable running from the "+" battery post itself. But good suggestion. No need to run an additional battery post to fuse panel cable.

As far as the (ign) key positions and the circuits I want running, in thinking about it, there is really no pressing need to have anything "constant hot", that is, unswitched.
So having said that, should I simply make both fuse boxes on the panel I am building switched? To do that I add another (75 amp) relay to the fuse box that is unswitched currently.







 
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