aphelps Loc: Central Ohio

Never replace a smaller fuse with a larger one. You could have a wiring fire if the downstream wires (designed for 15 A max) are subjected to currents of 20 to 30 A.

agillot

It all depend of the size / diameter of the pre fuse wires . It may be safer to go to the battery , if you are going to connect a device that use a certain amount of power .Basically , dont melt the car fuse box .

Longshadow Loc: Audubon, PA, United States

?? The 20 amps, for example, will be split between the two fuses in parallel, ≈10amps each. Either 15 amp fuse will not blow until the total draw is over 30 amps, being over 15 for each leg, then the second will blow because it cannot handle the load alone.

While SO true about never replacing a smaller fuse with a larger one, the question asked was for a NEW circuit.

Sirsnapalot Loc: Hammond, Louisiana

Longshadow Loc: Audubon, PA, United States

Last I knew fuse ratings were selected for the load.
But hey, I'm just an electronics engineer.

n4jee Loc: New Bern, NC

If you are installing something that could draw 30 amps run minimum of 10 gauge wire (8 would be better) directly from the battery. Put a fuse in both the positive and negative leads. This is how I wired my 100 watt ham radio transceiver in my truck.

TriX Loc: Raleigh, NC

I think the some of the disagreement here is based on understanding of the proposed change and some on the background of the person answering. An electronic engineer or an electrical engineer working in electronics thanks of fuses as protecting the load device, and of course, he’s correct, while an electrician is thinking of the National electrical Code and house wiring where the breaker or fuse is designed primarily to protect the wire feeding the load from overheating from too heavy a load or a direct short. It may also protect the load from over current, but that’s not the primary reason for it’s existence. The load in a home may or may not be separately fused to protect the device.

As far is paralleling the fuses or changing the size, my assumption was that a new wire would be run from the fuse block to the load, so no worries there. and yes, you could parallel two 15 amp fuses although I’ve never done it. The fuse is just a piece of metal (with a specific resistance) that heats and finally melts somewhere close to its rating. Think of it as paralleling resistors. Both see the same voltage, and since they are both the same, or nearly the same resistance, both see the same current and the same power dissipated. And since each fuse sees 1/2 the current and dissipates 1/2 half the power, each shares half of the 30A load.

And while changing to a larger fuse in most cases is verboten - the wire in a house would no longer be properly protected and if it were a fused device, the device could draw too much current and perhaps be damaged before the fuse blows. But here, assuming new wire to the load, neither of those things are true, and the fuse block in a modern car is supplied with enough amperage for it not to be an issue. But, fuse blocks may be protected with a fusible link upstream, so you might exceed that capacity, and of course if the alternator output is marginal, tha5 could be an issue, but in general, in this case no worries.

Net-net Longshadow is correct assuming a new wire (or wires) is run to the load.

jerryc41 Loc: Catskill Mts of NY

Just to add a comment. I added two fuse panels to my car. One runs directly off the battery, and the other is connected to the car's fuse panel. I have switches to turn the always-on circuits on and off. They power two or three 12v outlets. The on-with-car-power panel powers the dash cams, GPS, and a string of LED lights. No accessories are tapped into the car's wiring.

Longshadow Loc: Audubon, PA, United States

I believe his question did say theoretically and reference unused fuse locations, implying no existing wire(s).
And yes, one would have to use wire of a sufficient gauge to accommodate the load.

fetzler Loc: North West PA

You like playing with fire. Remember the wires themselves are rated up to a certain current. Drawing too much current though a too small wire can cause a fire. If you have an unused fuse space then you can use the slot up to its rated current. If there are two things that WILL NOT AND CANNOT be used at the same time you can double up. Many car accessories are very low current. Lighter plugs are usually good to 10 amps.

TriX Loc: Raleigh, NC

It’s exactly how my VHF transceiver in my car is wired as well - good practice.

73
DE K4CKB

JBRIII

Agree, I actually did this to solve USB power issues with a telescope. This is also how dummy batteries running on USB power work, at least for Canon cameras, they come with a Y cable, two inputs to battery. Of course final combined output line needs to be able to handle the amperage.

Toby

Exactly right. Look at your house. The fuse size is determined by the wire gauge in the wall. The fuse is to shut off the power if the load is so great that it could burn the house wires.

rbourque2 Loc: Portland, Maine

I totally agree with Najataagihe as a fuse is in line to protect the wire and not the equipment tied on to the wire. To use a 30 amp fuse you would have to upsize the wire to the proper wire gauge.

Longshadow Loc: Audubon, PA, United States

In a house because they don't know what will be plugged into it, so they set a general limit to 15 amps for general receptacles, usually 20 amps for kitchen. Normal runs for 15 amp breakers is usually 14 gauge wire in a house. if they need to run it a long distance they will use 12 gauge (to reduce the IR drop for heavy loads over the distance), but still a 15 amp breaker. Wire is selected to provide the required power with a zero or negligible IR drop for the projected devices.

Car accessory fuses are similar, but the total of the items (usually more than one) that are connected to the line. Fused (and wired for the summation of all device requirements. The A/C fuse for example is rated for the A/C system load. The A/C is usually the only thing on that fuse. There is a method to their madness, not a general "covers all" method.
It depends on what/how many items are being fed and their load ratings as to what wire and fuses are utilized.

Beware armchair electrical engineers. Yes, they do have opinions.