OBA to fuse box or directly to Aux battery

[goldeneagle1976]

I have a Viair Heavy Duty OBA compressor. Trying to decide if I can hook the power to the positive main (not one of the 30a fused outputs) on my Blue Sea fuse panel. Or if I need to connect directly to my Aux battery? Does it matter either way? The compressor has an in-line fuse in its wiring harness.

 

[Herbie]

You need *a* fuse, but there are a couple of factors to determine if it's better to go through the panel or direct:

* Is the in-line fuse closer to the compressor or to the "battery" end of the wiring - it's useless if it's too close to the compressor to prevent smoking a wire if the short happens "upstream" closer to the battery, fuses need to be as close to current source as possible
* Does the feed from battery to panel support the total current you need? (i.e. if you wired the blue sea panel w/ a 30A cable but plan to pull 30A for the compressor, plus 5A for the fridge, plus 10A for the water pump, plus...)
* Do you have spare terminals on the fuse panel for now and the future?
* Would it be cleaner (and thus safer, avoiding wiring issues like chafing or shorting) to wire through the panel?
* Does your battery have a terminal setup where you can safely wire directly to it without using flaky "add-on" connectors or splices?
* Does the inline fuse use the same fuse type as elsewhere in your vehicle? (Or will you be carrying unique spare fuses just for compressor?) - a minor point but the kind of thing that trips my OCD

The name of the game is "AVOID ELECTRICAL FIRE". All other considerations are really secondary. So if one method or the other ensures you can better make appropriate and safe connections, then that's the method I would choose.

I have everything wired to my fuse panel, because the fuse panel is mere inches away from the battery - meaning that every run of cable is fused in a way to protect the wire from a short. If there were some weird situation involving running a ton of extra wire across multiple positions, then I'd consider connecting to a battery terminal, but you still need to sweat all the details to make sure it's safe.

 

[DaveInDenver]

@Herbie covers the main points about the inline fuse only being there to protect the compressor, not the wiring. I'll make one point more, though.

If your circuit goes battery->large fuse->heavy cable->fuse panel->smaller fuse->smaller cable->inline fuse->compressor the question of safety is covered but you have a fair number of connections and wire size changes that might result in significant voltage drop.

FWIW, I have a 6-position fuse block fed with a 8AWG/60A that has a misc 12AWG/25A tap. This is technically sufficient to run a portable compressor.




But I ended up hanging a 8AWG/40A fuse (Maxi-Fuse) straight from the starting battery that I actually use for my compressor. It's a little overkill (there's no real noticeable improvement in air up time although the compressor sounds better, I should borrow an IR temp gauge and see if it runs cooler). But it's turned out to be handy to have a bigger tap.

I can run a big charger on the batteries, for example. I also carry a 10AWG/25 foot SB50 extension cord that works out well if I have to use the compressor next to someone else's truck.

 

[goldeneagle1976]

You need *a* fuse, but there are a couple of factors to determine if it's better to go through the panel or direct:

* Is the in-line fuse closer to the compressor or to the "battery" end of the wiring - it's useless if it's too close to the compressor to prevent smoking a wire if the short happens "upstream" closer to the battery, fuses need to be as close to current source as possible
* Does the feed from battery to panel support the total current you need? (i.e. if you wired the blue sea panel w/ a 30A cable but plan to pull 30A for the compressor, plus 5A for the fridge, plus 10A for the water pump, plus...)
* Do you have spare terminals on the fuse panel for now and the future?
* Would it be cleaner (and thus safer, avoiding wiring issues like chafing or shorting) to wire through the panel?
* Does your battery have a terminal setup where you can safely wire directly to it without using flaky "add-on" connectors or splices?
* Does the inline fuse use the same fuse type as elsewhere in your vehicle? (Or will you be carrying unique spare fuses just for compressor?) - a minor point but the kind of thing that trips my OCD

The name of the game is "AVOID ELECTRICAL FIRE". All other considerations are really secondary. So if one method or the other ensures you can better make appropriate and safe connections, then that's the method I would choose.

I have everything wired to my fuse panel, because the fuse panel is mere inches away from the battery - meaning that every run of cable is fused in a way to protect the wire from a short. If there were some weird situation involving running a ton of extra wire across multiple positions, then I'd consider connecting to a battery terminal, but you still need to sweat all the details to make sure it's safe.

I will have my starter battery to Redarc FK40 fuse then 6AWG cable to Redarc BCDC1225D Charger to FK40 fuse to Auxiliary battery. The Redarc BCDC1225D and Aux battery will be in the bed of my truck in a battery box. Then the plan is Aux battery to 50A Circuit breaker (to be able to shut it all down if needed) then to Blue Sea fuse panel. The Compressor will be within 5 feet of the Auxiliary battery.
I didn't know if I could run off of the main power-in terminal of the fuse panel or if I needed to go directly to the Aux battery.
To answer your questions above:
The in-line compressor fuse will be closer to the Aux battery.
The cable from my Aux battery to panel with be at least 8 AWG but most likely 6 AWG
Yes, the panel will have 12 terminals, plenty of room
The battery I will be using is a BassPro Pro Series Dual Purpose cranking and deep cycle battery 110aH, 190 min reserve and 773CCA and it has terminals to be able to hook up directly.
The compressor in-line fuse is different (blade type) but I could be switched up if I wanted.

 

[DaveInDenver]

I didn't know if I could run off of the main power-in terminal of the fuse panel or if I needed to go directly to the Aux battery.

Do you mean using the terminal itself as just a common point? If the battery-side is fused and you use the same size wire between the fuse, fuse block terminal and the compressor it could be safe. But like Herbie says, you have to add up all the loads to make sure that works.

If you're supplying 60A to the fuseblock and it will use most of it then there's nothing left for the compressor. But daisy chaining a 6AWG with a 60A fuse is probably safe and I can appreciate not wanting to parallel two runs.

If you don't want to use 6AWG to the compressor you could in theory hang a smaller fuse from the terminal and use 10AWG (or whatever). But at that point you might just as well wire up the compressor as a regular load from the fuse block.

If you wanted to reduce the run size from the fuse block to the compressor you'd have to size the front end fuse for the smallest wire in the circuit. IOW 6AWG to 10AWG would require using a 30A or so. Which makes the 6AWG redundant. There's one benefit in reducing voltage drop but for such a short run that's not a major issue.

 

[goldeneagle1976]

Thank you for the replies, it's appreciated. I'm slowly grasping the electrical side of this operation. Learning a lot before putting it into action.

 

[OllieChristopher]

If you get in the mindset to fuse only for the wire then it makes installs much easier. An example is my 4 AWG wire. It is rated for 150 amps at 50 feet. My fuse panel is rated at 100 amps. I have my 4 AWG wire going to fuse panel on a 120 amp breaker before the panel. This makes servicing the panel and adding/removing circuits safe by shutting off breaker.

The 4 AWG wire going from battery to back of my truck is fused 150 amps at battery. All my 1/0 cables are fused 300 amps at battery.

Another little tip is to run every single device ground directly to battery. I use a buss bar for all my grounds. Everything that is powered has a ground that goes directly to negative battery terminal. This way you will never have a grounding problem that you have to chase down.

Too many times I have seen where installers will shortcut by using chassis or engine block for a ground just to save on cable. Very poor practice.

The only time I fuse for other than the wire are power outlets and switches which almost always have a lower amp rating than the wire.

 

[DaveInDenver]

Too many times I have seen where installers will shortcut by using chassis or engine block for a ground just to save on cable. Very poor practice.

Every OEM would disagree on using the chassis for ground. Poor workmanship is poor workmanship. It's possible to have issues even running a ground all the way back to the battery so don't get lazy about your crimps, protection and weatherproofing either way. One issue with doing full runs is you can create a sneak path ground if the engine block cable breaks and your appliance case isn't insulated and mounted on the chassis. This is why you see fuses on both the positive and negative terminal in some harnesses with both wires in them, like with ham radios. They'll have 12AWG wires that might seem heavy for a 15A radio but it isn't nearly heavy enough to carry the starter current if the heavy cable corrodes or breaks from the block.

 

[OllieChristopher]

Every OEM would disagree on using the chassis for ground.

Unfortunately OEM has a bunch of stuff grounded to the chassis. My answer to that was adding two additional 1/0 grounds from battery to chassis as well as leaving the OEM strap from block to chassis on. This is a total of three grounds to the chassis from battery.

One issue with doing full runs is you can create a sneak path ground if the engine block cable breaks and your appliance case isn't insulated and mounted on the chassis. This is why you see fuses on both the positive and negative terminal in some harnesses with both wires in them, like with ham radios.

I have 3 full on 1/0 grounds to engine block from buss bar. My system is redundant and precludes any "sneak path" grounding issues. I can never see an instance where fusing a ground wire is of any benefit with a properly built system. Sneak ground circuits are nothing more than losing a ground or loose ground with nothing to back it up.

The best way to test it is to disconnect ground from battery and measure resistance to chassis. The resistance should be the same. This way if a cable breaks you have the redundancy of your chassis as a secondary grounding source.

 

[DaveInDenver]

Unfortunately OEM has a bunch of stuff grounded to the chassis. My answer to that was adding two additional 1/0 grounds from battery to chassis as well as leaving the OEM strap from block to chassis on. This is a total of three grounds to the chassis from battery.

I have 3 full on 1/0 grounds to engine block from buss bar. My system is redundant and precludes any "sneak path" grounding issues. I can never see an instance where fusing a ground wire is of any benefit with a properly built system. Sneak ground circuits are nothing more than losing a ground or loose ground with nothing to back it up.

The best way to test it is to disconnect ground from battery and measure resistance to chassis. The resistance should be the same. This way if a cable breaks you have the redundancy of your chassis as a secondary grounding source.

I tend to agree with you on all your principle statements. I'm of the opinion that it's generally better to design a proper chassis ground and run appliance negatives to your chassis nearby, e.g. like the OEM does it (despite the question of "proper" notwithstanding). I don't have 1/0AWG cabling, instead I use 2AWG in my truck. I just don't have the need for anything that heavy. I have the common buss point on my left frame horn rather than the block, though.

With ham radios two fuses and a full run of both power and return is kind of a unique situation due to DC and RF path concerns. Doing it that way reduces chance of common mode noise from the power side with an unknown supply situation. IOW, getting interference or alternator whine is slightly less likely using the included harness rather than relying on the end user doing things right or possibly assuming the OEM built in sufficient interior body ground for sometimes adding as much as 25 amps for a 100 watt HF radio. So it's easier for the radio manufacturer to give you a double wire harness than to tell you to upgrade your body ground and bond your body panels with braid.

It also may be a carryover from *way* back when some vehicles had positive grounds and the harness had to accommodate either one. Putting radios in vehicles isn't a new phenomenon and hams tend to do stuff a certain way because it's the way Hiram Percy did it in his Model A.

 

[OllieChristopher]

That's cool Dave, There is more than one way to skin the cat for sure. I did the 1/0 because that's what Mechman recommended. I will tell you it sure was a pain to get it to fit though!! All that big wire takes up a lot of room!!

 

[DaveInDenver]

That's cool Dave, There is more than one way to skin the cat for sure. I did the 1/0 because that's what Mechman recommended. I will tell you it sure was a pain to get it to fit though!! All that big wire takes up a lot of room!!

There's nothing at all wrong with oversizing. Even if you never need the ampacity it'll have next to zero voltage drop. Plus if you ever do need the capacity you've done it right, done it once. Ultimately everything is compromise of weight, budget and space, though. So people also shouldn't feel they have to do 1/0AWG when 4 or 2 or something else is sufficient. I recommend people approach antennas with the same practicality. Of course the middle of your roof is ideal but so what if you're annoyed every day pulling in and out of your garage? A bumper or fender might give up some performance but if it keeps you sane that's the right way to go.

 

[Ngneer]

Thank you for the replies, it's appreciated. I'm slowly grasping the electrical side of this operation. Learning a lot before putting it into action.

ANd in turn I am learning as well, Thanks to all who have responded to this thread.Please continue with updates I have a fridge to get to workin.