Add-on fuse box - is this correct?

[hobby-man]

Hey all, I'm fairly comfortable working with electronics but not so much at designing my own circuits so I was wondering if anyone has any input on this. Right now I'm running two accessories off of one +12V switched ignition add-on fuse tap sources. There's a possibility of adding more in the future, so I think it would be smart to just put together a +12V switched fuse box, and run any accessories off of that instead. I think the way to do this is with a relay. My basic idea is below.

Use the +12V switched accessory fuse as the trigger for the relay. System is off until ignition is on.
Once armed, power flows from the battery through the relay to the fuse box.
Individual circuits off the fuse box are still switched, so load will not be instantaneous.


Questions:

1. Does this even work/make sense as described?
2. Is a 30A inline fuse required for the relay if all the outputs are also fused?
3. Is a 30A inline fuse enough given the specs of the fuse box? There would never be a situation where all circuits are fully loaded, and certainly not at 30A per circuit. But better to be over than under protected?

Any ideas or suggestions appreciated!

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[calonzo]

• Does this even work/make sense as described? - Yes
• Is a 30A inline fuse required for the relay if all the outputs are also fused? - The fuse is protecting the single wire leading to the relay, the relay and the wire leading to the fuse box. If you are using a wire that can only handle 30A, then you should put a fuse on it just in case you have 2 circuits that combine to be more than 30A.
• Is a 30A inline fuse enough given the specs of the fuse box? There would never be a situation where all circuits are fully loaded, and certainly not at 30A per circuit. But better to be over than under protected? - See previous comment

 

[hobby-man]

• Does this even work/make sense as described? - Yes
• Is a 30A inline fuse required for the relay if all the outputs are also fused? - The fuse is protecting the single wire leading to the relay, the relay and the wire leading to the fuse box. If you are using a wire that can only handle 30A, then you should put a fuse on it just in case you have 2 circuits that combine to be more than 30A.
• Is a 30A inline fuse enough given the specs of the fuse box? There would never be a situation where all circuits are fully loaded, and certainly not at 30A per circuit. But better to be over than under protected? - See previous comment

Info on the inline fuse makes sense, thank you!

 

[vtecr]

What accessories are you using?
If you're gonna have 6 outputs from the new fuse panel then the total combined would need to add up to max 30amps in your example, not 30 amps each. Even if you don't intend to use them like that, things might change in the future or faults might occur that draw more power and cause the 30amp fuse to blow.

You could omit this fuse entirely, but you should do so by using a high quality wire with good shielding and run it in after the cars main battery fuse rather than directly to the battery itself. The wire would need to be rated equal or above the total capacity of the 6 new circuits you're adding in.

Depending on your spec, you might have an available relay socket in the engine compartment and you might also be able to pick up a switched live there to trigger it. Whether or not you'll need as much as 80a is depending on what exactly it is you're powering.

 

[hobby-man]

What accessories are you using?
If you're gonna have 6 outputs from the new fuse panel then the total combined would need to add up to max 30amps in your example, not 30 amps each. Even if you don't intend to use them like that, things might change in the future or faults might occur that draw more power and cause the 30amp fuse to blow.

You could omit this fuse entirely, but you should do so by using a high quality wire with good shielding and run it in after the cars main battery fuse rather than directly to the battery itself. The wire would need to be rated equal or above the total capacity of the 6 new circuits you're adding in.

Depending on your spec, you might have an available relay socket in the engine compartment and you might also be able to pick up a switched live there to trigger it. Whether or not you'll need as much as 80a is depending on what exactly it is you're powering.

They're all small things. Right now I have a dashcam hardwire kit (5A) and the trigger switch for my methanol injection kit (2A). May add something like footwell LEDs in the future or an oil pressure/temperature gauge.

 

[xRiCeBoYx]

What accessories are you using?
If you're gonna have 6 outputs from the new fuse panel then the total combined would need to add up to max 30amps in your example, not 30 amps each. Even if you don't intend to use them like that, things might change in the future or faults might occur that draw more power and cause the 30amp fuse to blow.

I was thinking that as well.

My electrical theory is a bit rusty, so bear with me..

Assuming all 6 of those loads are in parallel,
IT = I1 +I2 + I3 + I4 + I5 + I6
So if you somehow manage to have all 6 circuits simultaneously draw 30A, that'd draw 180A. Highly unlikely scenario, but still a possibility. Theoretically, with that 30A inline fuse, anything more than 6 simultaneous loads pulling 5A would blow that 30A fuse. I'd imagine you'd put a 5-10A fuse per circuit if they're small loads. Maybe 15A if they're larger loads.

Divvy it all up and/or maybe up that 30A inline fuse to something a bit bigger.

edit: your diagram at first glance confused the shit out of me. I'm used to seeing corresponding pins across from each other (85-86, 30-87), and was thoroughly confused by what I initially though was ground -> add on fuse box -> relay pin -> relay pin -> ground, simply bc I didn't look at pin numbers. lol

 

[calonzo]

They're all small things. Right now I have a dashcam hardwire kit (5A) and the trigger switch for my methanol injection kit (2A). May add something like footwell LEDs in the future or an oil pressure/temperature gauge.

You should uses fuses in the fuse panel that match what you are connecting to them, unless the devices have their own fuse. Fuses prevent the wires from melting in case of short. But if you are allowing 30A through a gauge 16 wire meant for 5A, it could start a fire before the fuse blows.

 

[bikejog]

Why not just tap the 12V socket power wire (20A or maybe 15A) and the option connector in the under dash fuse box (30A I think). You don't need to use any extra fuses.

 

[hobby-man]

Why not just tap the 12V socket power wire (20A or maybe 15A) and the option connector in the under dash fuse box (30A I think). You don't need to use any extra fuses.

I've thought about that as well. I'd have to check if Option is even available, I feel like it's used on the sport touring but not 100% sure.

But as far as the 12V socket - isn't the original circuit only really designed to provide the amperage needed (15A or 20A) +/- some safety margin? Fuse tapping an additional 15A or so might overwhelm the factory wiring? In a situation where the 12V socket were fully loaded + the accessories on. That's how I understood it anyway which prompted this idea.

 

[bikejog]

I've thought about that as well. I'd have to check if Option is even available, I feel like it's used on the sport touring but not 100% sure.

But as far as the 12V socket - isn't the original circuit only really designed to provide the amperage needed (15A or 20A) +/- some safety margin? Fuse tapping an additional 15A or so might overwhelm the factory wiring? In a situation where the 12V socket were fully loaded + the accessories on. That's how I understood it anyway which prompted this idea.

Right, didn't realize you have a touring. Below is from the Type-R. Each have 20A. The 12V Socket 20A fuse is all for the socket so if you got nothing connected to it, then you can draw all 20A, well maybe 15A to be safe. I guess your Touring has auto mirror and the wireless charger? Wireless charger probably don't draw more than 20W. Not sure about the auto mirror.

All the factory options have piggy back power connectors so you could add more as long as you don't exceed 20A total.

 

[calonzo]

I've thought about that as well. I'd have to check if Option is even available, I feel like it's used on the sport touring but not 100% sure.

But as far as the 12V socket - isn't the original circuit only really designed to provide the amperage needed (15A or 20A) +/- some safety margin? Fuse tapping an additional 15A or so might overwhelm the factory wiring? In a situation where the 12V socket were fully loaded + the accessories on. That's how I understood it anyway which prompted this idea.

That's right. I guess if it were only a few amps, it would be okay.

I don't know why they don't make the fuse taps so that all of the current for both circuits still goes through the main fuse. Seems like a no-brainer.

 

[bikejog]

That's right. I guess if it were only a few amps, it would be okay.

I don't know why they don't make the fuse taps so that all of the current for both circuits still goes through the main fuse. Seems like a no-brainer.

A few amps? No, no, no... lol... Says so in the owner's manual, 15A minimum. They wouldn't put a 20A fuse in there if the circuit couldn't handle it.

 

[calonzo]

A few amps? No, no, no... lol... Says so in the owner's manual, 15A minimum. They wouldn't put a 20A fuse in there if the circuit couldn't handle it.

I was talking about getting power from fuse taps, not from the accessory power socket.

IMHO fuse taps are not safe since you can overload the circuit. But if you are 100% sure you won't overload the circuit, then you can do it. But that's a big 'if'.