SnowHater

Any of us that have add "things" to our trucks that require power may have used the fuse adapter shown below. I showed one to a friend of mine and he came up with an interesting thought. Lets say you pulled out a 25 amp fuse and insert it into the adapter. The other slot, you insert a 20 amp fuse. For this example the original fused device pulls 20 amps. The new added device pulls 15 amps. Now you have a 35 amp draw on the wires that powered the fuse box. If that fuse box had been wired to handle 30 amps you would have a draw that exceeds the safe wire size. Wire gets hot and starts a fire. With this scenario, what are your thoughts of this happening?
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dont slow down

Very plausible. If you have something that pulls 15 continuous amps it would be better to wire it straight to the battery on its on circuit. Or most F150 fuse boxes have plenty of Spare circuits already wired, I would use one of those first before adding on to another circuit. Any time you exceed the original fuse limit you increase the risk of over loading the circuit.

Also take a multimeter and check your add a fuse. On the ones I have the bottom slot wasn't even connected to the output wire.

SnowHater

For sure with that much of a draw.
Looking at the two pins, the one on the left side are connected, but not on the right. Would not work if it was.

moparado

Rule number one with electrical stuff, don't overload the wiring or any connectors with excessive amperage draw.

As mentioned, if the added accessory draw is excessive use an unused fuse tap in the fuse block.
If way over excessive like for installing auxiliary fog or running lights use the fuse tap to energize a relay coil where the relay contacts will supply the heavier amperage directly from the battery with the appropriate heavier inline amperage fuse. And make sure that inline fuse is as close to the battery terminal as possible and also use the appropriate heavier gauge wire to power those fog lights.
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Steve83

You & your friend don't seem to understand how fuse blocks are wired, or how Ford (& most other automakers) designs wiring harnesses.

There's not a wire capable of passing ONLY 25A supplying a 25A fuse. For the always-hot circuits, there's a wire capable of passing ~130A feeding a thick metal bar (buss bar) that goes to each fuse position. When the vehicle's circuits are designed, they're assigned to fuse positions, and the appropriate-size fuse goes there. But it can be virtually any size from 1A to 30A (depending on fuse type & build era - that's the range for APM blade fuses) as this diagram shows:


(phone app link)

The wire on the OTHER SIDE (the wire going to the load) is the one that's sized for that load. So if Ford designed the circuit to draw 20A, and chose a 25A fuse to protect it, that wire would be sized for ~30A (read the first sentence of that caption above for more info). If you then pulled the 25A fuse, and added that tap, the original hot-side fuse block terminal & the hot side of tap would have to pass the new combined load (35A in your example) which is still within the safety range of the fuse block terminal. Read the fine print of the tap instructions to find out what its limit is (if the retailer or mfr. has the guts to disclose it).

But the truck's original load circuit wire would still only carry the original 20A. The feed wire TO the fuse block would carry the extra load, but it can easily handle an extra 15A (assuming no OTHER increases on the truck). If you push that big wire over its limit, there still wouldn't be a fire because Ford designs for that scenario. It would just blow the fusible link wire (or fuse) closer to the battery (often on the starter relay). Read the long article below the line in that caption for all the details.

For more details about adding a fuse to an unused position in the stock fuse block, read the captions in this photo album:


(phone app link)

SnowHater

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Well not really. Oh, he may not but I am well aware that a fuse box with say 50 fuses in it does not have 50 little wires running to it.
What we were looking for was some one to answer just as you did. With a great amount of quality information for all to read. With all the lurkers and members that almost never post, you gave them a good read. Useful info that some of them may have picked up and will remember.
Good job.

Steve83

It would have at least 49 small (fuse-sized) wires, and at least one bigger wire.

SnowHater

Well if it did then wouldn't we be right back to adding a second fuse and over loading that one wire?

Steve83

We're not talking about adding fuses now - we're talking about an original fuse block's wiring.

But no - you still wouldn't overload any of the original small wires by adding a new small wire to that tap. Nor would that tap alone overload the one big wire.

SnowHater

Okay.