Vetteman61

iTrader: (1)

I ordered a bussman fusebox with dual busses and 20 fuses per side. My plan was to have one side as 12 volt constant battery power and the other side be 12 volt ignition/start/run powered. Now that I've got my wiring figured out more I have found I only have 2 wires that will be 12 volt constant battery power (maybe 3). Because I'm individually wiring a lot of my wires instead of combining them, I have almost 20 fuses for 12 volt ignition/run/start.

My question is, since I only have 2 wires that will be on the 12 volt constant buss, should I reconsider this. Should I divide the two buses up by Ignition/run and Ignition/start, that way when I go to crank the engine all the accessories aren't drawing power during the starter engagement?

Thanks,
Brandon

EFIHARNESS

Your way over thinking this. What about relays for fuel pump and fans ? Bring + into one side and ignition load to the other. Steve

Kawabuggy

iTrader: (1)

Brandon, you can also wire all of your relays for electric fuel pump, fans, and other items to the constant 12V+ side as those items will still need a signal from the PCM in order for them to actuate. With the key off, the only item the PCM will actuate might be the cooling fans if you have it programmed to run after key off. Like you, I have a fuse box split in half, between constant 12V+, and ignition switched 12V+. My fuse box only has 12 fuses total, and I'm 6 on one side, 5 on the other.

Vetteman61

iTrader: (1)

Thanks buggy. I hadn't thought of it that way; That makes a lot of sense. I'll stick with the constant/ignition busses then. What size relay are you using to power your ignition switched buss? I was thinking 40 amp....?

I was concerned of having one whole buss of 20 fuses and only 2 of them used as being wasteful because I won't really have anything else that needs constant power.

-TheBandit-

iTrader: (1)

Which Bussman fuse box are you referring to? One that is setup for fuses only or one that is setup to also accommodate relays?

IIRC the Bussman box uses ATM fuses which are pretty small. They are definitely not appropriate for radiator cooling fans - I would use separate Maxi fuse(s) for the fan(s). ATM fuses will eventually blow due to the high inrush currents used by fans. The fuel pump might be okay to run with an ATM fuse and of course you can do as suggested above for any other 12V constant supplies you need.

1964SS

iTrader: (1)

It really depends on how the circuit is setup. If you are running dual fans off one fuse then yes atm will not do because they do not go over 40 amps and most dual fan setups can pull up to 50 amps, but there is nothing wrong with using one 30 amp atm per fan. The physical size has nothing to do with inrush current capabilities. a 30 amp atm will last every bit as long as a 30 amp maxi.

-TheBandit-

iTrader: (1)

This is incorrect. Below are some links to specifications for mini and maxi fuses from Littelfuse. If you look at the table for % of rating vs. opening time, you will notice that the Maxi fuses have significantly longer opening times for a given %rating. For example, at 135% rating, the maxi fuse will open at a minimum of 60s while the mini will open at a minumum of 0.75sec. That means the Maxi fuse could last 80x as long as the mini fuse at 135% of continuous rating. At 200% of rating the difference is 2s vs 0.15sc or 13x as long.

http://m.littelfuse.com/~/media/auto...lade_fuses.pdf
.http://www.littelfuse.com/~/media/au..._datasheet.pdf

That is measuring a simple continuous current application while the momentary inrush current situation puts a different kind of load on the fuse. Every time the fan starts there will be some number of miliseconds where it's drawing a ton of current and each time it eats away at that rating until finally the fuse finally blows. Here is a video demonstrating just how high the inrush current can be for a fan. This is a Taurus fan and you can see it spikes well above 100amp momentarily before settling back down to around 30amp.


If there was no difference between mini and maxi fuses that have the same continuous current rating, there would be no need to have these different form factors. Alas if you look under the hood of a OEM car today you will see difference form factor fuses used for different applications. Case in point the '00-'02 Camaros use maxi fuses for the fans:



A lot of OEMs uses fuseable links instead of fuses for fans to handle the inrush currents.

1964SS

iTrader: (1)

I see what you mean, I read it as one lasting longer then the other in an application which is not the case. Yes the smaller fuse can not handle over current as long as a larger fuse.