Automotive Electrics Question: This is strictly theoretical, having popped into my mind a couple of days ago. If you want to add an electrical accessory to your car, you can connect it...

jerryc41 Loc: Catskill Mts of NY

This is strictly theoretical, having popped into my mind a couple of days ago.

If you want to add an electrical accessory to your car, you can connect it into an empty space in the fuse box. If there are no empty spaces, you can use one of those adapter wires, although that has two things running off the same fuse. Here's the question. Let's say I have two empty spaces in my car's fuse box. I connect the device to one empty 15 amp space. Then I run a wire from another unused 15 amp space and tie that wire into the power for the device. Does that mean the device has 30 amps of protection? That would seem to be the case, but I suspect I'm missing something.

Soul Dr. Loc: Beautiful Shenandoah Valley

No, you still only have 15 amps of protection.
When the 1st one trips the other one will not get any power.

Will

Longshadow Loc: Audubon, PA, United States

Soul Dr. wrote:

No, you still only have 15 amps of protection.
When the 1st one trips the other one will not get any power.

Will

This will happen if the fuses are in series.

If the fuses are in parallel, theoretically you can have 30 amps, 15 through each fuse.
The current splits between each fuse in parallel. But they probably won't be precisely equal.
(Provided each fuse can actually handle 15 amps.)

From your description, I'm inferring that you mean in parallel.

alawry Loc: Timaru New Zealand

Hi Jerry longshadow is correct. As you say it's a theoretical question so that's fine. In practice you would pull the 15a fuse and inserta 30a fuse and there you go. Cheers.

Longshadow Loc: Audubon, PA, United States

alawry wrote:

Hi Jerry longshadow is correct. As you say it's a theoretical question so that's fine. In practice you would pull the 15a fuse and inserta 30a fuse and there you go. Cheers.

Hmmmm. I wonder what size fuse protects (feeds) the fuse block.

TriX Loc: Raleigh, NC

alawry wrote:

Hi Jerry longshadow is correct. As you say it's a theoretical question so that's fine. In practice you would pull the 15a fuse and inserta 30a fuse and there you go. Cheers.

Right answer.

BebuLamar

Longshadow wrote:

This will happen if the fuses are in series.

If the fuses are in parallel, theoretically you can have 30 amps, 15 through each fuse.
The current splits between each fuse in parallel. But they probably won't be precisely equal.
(Provided each fuse can actually handle 15 amps.)

From your description, I'm inferring that you mean in parallel.

Yes it's true but the current may not split evenly.

tradio Loc: Oxford, Ohio

Longshadow wrote:

This will happen if the fuses are in series.

If the fuses are in parallel, theoretically you can have 30 amps, 15 through each fuse.
The current splits between each fuse in parallel. But they probably won't be precisely equal.
(Provided each fuse can actually handle 15 amps.)

From your description, I'm inferring that you mean in parallel.

Correct.

Longshadow Loc: Audubon, PA, United States

BebuLamar wrote:

Yes it's true but the current may not split evenly.

You are absolutely correct! It depends on the effective resistance of each fuse wire.
Which, I personally, would consider to be negligible.
(What's a few micro joules between friends.)

And I did state that above, as in not being equal. (paragraph 2 line 2)

Najataagihe

Soul Dr. wrote:

No, you still only have 15 amps of protection.
When the 1st one trips the other one will not get any power.

Will

Actually, the other fuse will blow, too, because the load will be more than 15 amps and will try to draw all of it from that second circuit.

Also, don’t EVER replace a 15 amp fuse with a 30 amp fuse!

Those fuses are rated for the WIRE, not the LOAD.

TriX Loc: Raleigh, NC

Najataagihe wrote:

Actually, the other fuse will blow, too, because the load will be more than 15 amps and will try to draw all of it from that second circuit.

Also, don’t EVER replace a 15 amp fuse with a 30 amp fuse!

Those fuses are rated for the WIRE, not the LOAD.

A good general rule, but not applicable in this case. Actually, the fuse block will typically have 1-2 dozen fuses typically rated from maybe 10 amps to 30 amps for multiple circuits/loads, and the block will be fed with a wire likely rated at least 50 amps (because the block will have to supply current to multiple circuits) and protected by a fusible link. The individual fuses protect the wire and load attached to each one, and if the wire to the load (remember this is a user installed accessory, so the new wire can be rated appropriately) is rated for say, 35-40 amps, there is no issue.

BTW, although the NEC specifies that the conductor must be fused/breakered and loaded to no more than 80% of the ampacity of the wire, the NEC doesn’t apply to automobile wiring, but good practice none the less. This is no different from changing the breaker size in a panel and running the new appropriate size branch circuit wire from the new breaker,

Manglesphoto Loc: 70 miles south of St.Louis

jerryc41 wrote:

This is strictly theoretical, having popped into m... (show quote)

Jerry: My old 2006 and my latest 2018 Grand Caravans have a place at the main fuse box were you add an accessory with an external fuse (under the hood) and the answer to your last question is NO , it's a toss up of which fuse would blow first. It is never a good idea to daisy chain fuses!!!

Manglesphoto Loc: 70 miles south of St.Louis

Longshadow wrote:

Hmmmm. I wonder what size fuse protects (feeds) the fuse block.

None!!! on most cars, years ago they tried fusable link , but they were just a PITA.

Bunko.T Loc: Western Australia.

jerryc41 wrote:

This is strictly theoretical, having popped into m... (show quote)

The load you put on it is the issue.

tcthome Loc: NJ

Soul Dr. wrote:

No, you still only have 15 amps of protection.
When the 1st one trips the other one will not get any power.

Will