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12v source from battery - Wire gauge?

 
Old 05-02-2019, 07:32 PM
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Default 12v source from battery - Wire gauge?

In the interest of doing things "the right way", I'm wanting to run a dedicated wire directly from the battery, to the cabin (probably somewhere under the dash) to a fused relay, then to a power distribution block. That way, I won't be tapping off other sources to power add-on sensors, gauges, inverters, etc.

The relays I'm using support up to 30 amps so that's my max power target draw - accounting for 15-20 feet to neatly run a wire from the trunk to the dash, and provide 20-30 amps, online calculators are telling me I'll need somewhere in the range of "impossible" to 6AWG wire for this.

Anyone have anecdotal evidence of what's required/what you've done for this?
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Old 05-02-2019, 07:40 PM
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Two questions:

1: What loads are we talking about? "Sensors / gauges" and "inverters" are at totally opposite ends of the spectrum in terms of current draw, sensitivity to voltage fluctuation, and contribution to noise in the power supply.

2: Do you intend for these to be used primarily when the engine is off? If not, you should consider that the alternator, not the battery, is the source of current when the engine is running.
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Old 05-02-2019, 07:40 PM
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How many relays and what are they powering. What is the amp draw for each.
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Old 05-02-2019, 07:46 PM
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Wire gauge depends on length - and continuous draw of max amps produces heat, which eventually burns insulation. But 30amps sounds like you're planning on fans. Fans spike on turning on (inrush) and then level off.

Easiest place to tap - underhood fuse block - runs from the starter (4awg(?) from the battery to 8awg to the fuseblock, 80a(?) jpac fuse to the alternator.

Wire gauge though.. for the most part - run what fits and won't burn.
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Old 05-02-2019, 08:02 PM
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It's really intended to just be a general power source for anything I install - currently that includes a power source for wideband, digital voltmeter, and a 12v to 5v step down inverter that powers a USB hub w/ raspberry Pi. The inverter is quite beefy (capable of 150 watt output). I'm also probably calling this the wrong thing, I believe "buck converter" is more appropriate terminology.

I may add more digital gauges in the future, power additional things over USB, etc so I wanted margin. I'm using
these relays these relays
.

I figured running from trunk to cabin would be an "easier" wire run since there's access holes already there.

I think I wouldn't want to run from the under hood fuse block - as my intention is to not add additional load to existing wires/fuses - correct me if I'm wrong here.
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Old 05-02-2019, 08:03 PM
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6AWG isn't too far off base for a 15ft run with a 30A current requirement.

Being more realistic about the actual max current draw from that fuseblock will allow you to be more realistic with the wire gauge too.

Realistically, unless you are adding an auxiliary heater under the dash, 30A is probably an excessive amount of current to need.
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Old 05-02-2019, 08:09 PM
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You're way overengineering it. Rpi is 600mA, wideband is gonna be 5a with heater on. Buck converter is DC to DC step down (as opposed to boost, which is DC to DC step up). Watt is volt * amp - 150watt at 12v is going to be 12.5amp - or if it's 150watt at 120v - it's gonna be like.. 2amp.

Run a 14ga tap-a-fuse and be done with it. Your application description isn't making the alternator even breath hard.
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Old 05-02-2019, 08:26 PM
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Originally Posted by Savington View Post

Being more realistic about the actual max current draw from that fuseblock will allow you to be more realistic with the wire gauge too.
Originally Posted by gooflophaze View Post
You're way overengineering it.
Yeah, I think this is what I need to hear. I'm budgeting like 2 amps for Pi at full load + LCD, and I guess ~5 amps for the gauges, and maybe another 5 amps for anything else I plug into the hub. So maybe like 15 amps worst scenario.

I keep reading posts from people on here saying not to overload factory wiring, fuses etc. I'm currently powering these things with an add a fuse tap indeed at 14awg - just thought there was probably a better way to do it.
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Old 05-02-2019, 08:38 PM
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Oh - those warnings are there for a reason. One of the more subtle mods I've done over the years has been to run speaker wire from my stereo head unit to the speakers in the doors. Now - I know the miata will never be a bangin stereo stage - but running some 16awg stereo wire has always seemed to make the speakers "better" to my ear - and it's almost essentially a "free" mod. Well, I just so happen to have my dash wiring out right now and I'm making the mod more permanent - and what should my eyes behold, but the stock power wiring on the Accessory fuse is.. 20awg. And it splits off 4x times in the harness.

So yeah - don't overload stock wiring. It's possible - but it's generally ******** who are adding everything to that piddly 20awg accessory wire. And you really need to learn the difference between your IG1 and IG2 fuses.
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Old 05-02-2019, 09:07 PM
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Re-stating: the alternator is the principal source of current when the engine is running, not the battery.
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Old 05-02-2019, 09:37 PM
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Originally Posted by Joe Perez View Post
Re-stating: the alternator is the principal source of current when the engine is running, not the battery.
Thanks Joe - I think I pulled up a post from you saying this from like 10 years ago in my searches too.

If I still decide to run the dedicated wire I think I'll do that - should be a shorter run anyways.
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Old 05-03-2019, 08:18 AM
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go to the nearest lowes. grab some 12 AWG wire. 15 feet should be about 15$.
done.
doesnt sound like you will ever really be drawing more than 10 amp from the misc stuff you want to use.
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