Alternator mega fuse and general wiring questions

[egchewy79]

What size mega fuse should I be using on my alternator? Couldn't find any casting numbers on my donor alternator, but assume it's a fairly run of the mill 3G one. I've seen 100 and 175A listed in various places. Also, how are others managing EFI wires? Can they lay on the intake manifold and be ok temp- wise?

[CraigS]

I protected my wires w/ a split loom
https://www.autozone.com/starting-charging-and-miscellaneous-electrical/wire-sleeve/dorman-conduct-tite-black-3-8in-x-10ft-flex-split-wire-conduit/64473_0_0
It comes in various sizes. I will let others comment on the mega fuse.

[DadofThree]

I am using a 100A MegaFuse for my 3G Alternator and laying my efi wires, and alternator wires on the intake protected by a split loom.

[CFranks]

I am doing same thing as others regarding split loom on the intake, I have a 100 amp (1-wire) alternator and use the same mega rating. Breeze has a nice little fused cable kit that provides all the pieces you need if you don’t already have all the pieces or feel like shopping for all of them. I just installed it last weekend, works great.

[egchewy79]

I ended up going w/ a larger 175a mega fuse. Not sure what my amperage output on the alternator is since it's a donor and I can't seem to find any casting/serial numbers. Looks like common ford 3G alts are either 150 or 225 amps.

[sread]

the AAW Highway 22 harness that I am installing in my 33 Hotrod comes with 175 amp mega fuses stating that it is intended for use with modern alternators

[NAZ]

Using split loom for routing wires is a common and tidy way to organize wiring, but be advised that the typical plastic loom melts at a much lower temperature than the insulation on the wire. The takeaway here is that plastic split loom is not a good way to protect wiring from heat exposure and if you are successfully operating with plastic split loom then the temp is far below what would constitute a problem for un-shielded wiring. Want to know more about wire insulation thermal ratings click this link: https://www.awcwire.com/faq-automotive-wire

As for the mega fuse rating, first look at why you need a fuse in that position. To help you understand how to size fuses, follow this link as it does a fair job of explaining it: https://www.12voltplanet.co.uk/fuses-guide-uses.html

[JohnK]

Hey NAZ, thanks for posting that link to the fusing guide. There's some interesting information in that article. There's a paragraph in there that talks about "fusing exceptions" and why you don't want a fuse on the cable between the battery and the starter. That paragraph includes this:

"The other reason for not fusing the starter circuit is that if the battery is disconnected from the alternator whilst it is turning (as would be the case if a fuse blew) the diodes in the alternator's rectifier can be damaged."

Does that same logic apply to fusing the alternator output with a mega fuse, i.e., if the mega fuse were to blow and disconnect the alternator from the charging circuit it can damage the rectifier?

-John

[egchewy79]

Using split loom for routing wires is a common and tidy way to organize wiring, but be advised that the typical plastic loom melts at a much lower temperature than the insulation on the wire. The takeaway here is that plastic split loom is not a good way to protect wiring from heat exposure and if you are successfully operating with plastic split loom then the temp is far below what would constitute a problem for un-shielded wiring. Want to know more about wire insulation thermal ratings click this link: https://www.awcwire.com/faq-automotive-wire

As for the mega fuse rating, first look at why you need a fuse in that position. To help you understand how to size fuses, follow this link as it does a fair job of explaining it: https://www.12voltplanet.co.uk/fuses-guide-uses.html

would you think temps on the surface of the intake would be hot enough to melt plastic looms?

[NAZ]

Hey NAZ, thanks for posting that link to the fusing guide. There's some interesting information in that article. There's a paragraph in there that talks about "fusing exceptions" and why you don't want a fuse on the cable between the battery and the starter. That paragraph includes this:

"The other reason for not fusing the starter circuit is that if the battery is disconnected from the alternator whilst it is turning (as would be the case if a fuse blew) the diodes in the alternator's rectifier can be damaged."

Does that same logic apply to fusing the alternator output with a mega fuse, i.e., if the mega fuse were to blow and disconnect the alternator from the charging circuit it can damage the rectifier?

-John

Interesting question but I honestly don't know. I know that high current can kill the diodes and rectifiers but believe that is due to the excessive heat caused. Heat seems to be the most common explanation for diode and rectifier failures. But my experience has been limited to diagnosing the failures and not in finding the root cause. Perhaps someone here has more in depth electrical experience and can weigh-in.

What I do know (from experience) is that changing an alternator (or in the old days, R&R bad diodes) is much easier than replacing melted wires, so regardless of the answer I will likely still be adding fuse protection for the alternator hot -- some of these late model alternators produce enough current you can literally weld with them.

[NAZ]

would you think temps on the surface of the intake would be hot enough to melt plastic looms?

Yes! But that depends on the intake design. The old-school carburetor intakes used to have a heat riser built in that used exhaust gas to heat the plenum. Also, some used exhaust gas to operate the automatic choke. These areas on the intake manifold get hot enough to melt plastic split loom. I can't recall any OEM EFI manifolds that I've worked on that would get hot enough to melt plastic split loom and never seen a high performance high-rise style manifold that got that hot.

Temp range: https://www.cabletiesandmore.com/black-wire-loom-tubing

[JohnK]

FWIW - Del City (and I'm sure others) carries a high-temp split loom with a temp range up to 300F vs 200F for the standard loom:

https://www.delcity.net/store/3:8%22-High!Temp-Loom/p_811352.h_811361