Electrical fuse for 130 amp upgrade?
- Thread starter Alley Oop
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Fuse based on the wire size and lenght of run.
Did you use 60 or 90c wire? How long of a run?
Did you use 60 or 90c wire? How long of a run?
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Blown88GT
Founding Member
Keep in mind that circuit breakers must be de-rated from a fuse.
http://bdbreakers.com/breakersizing.php
You're only trying to protect the 4AWG wire from a short circuit in the alternator, placing a direct short across the battery.
This wire will not handle 500A for very long.
With no protection, a short circuit will melt the insulation & the copper & probably start a fire.
I'd stick with the fuse, 150A for the 130A 3G.
I wouldn't go 150. Not in this application. 125 is already pushing it.
One thing that's never taken into consideration is ambient temp. Most wire load charts (such as in the NEC) are at 85*F, but underhood temps can reach much higher.
Just going up to 100 degrees you are at 90% of the rating with 90c wire and 80% with 60c. Go up even more and it keeps dropping off. Most audio grade 4g wire I've seen is the cheaper 60c wire. Get up to 120F and now you are at 60% rating with that stuff.
The amp rating charts on the net are all over the place as well. The NEC considers max ampacity of 4g 90c wire to be 95 amps at 85F. It's a tad conservative
One thing that's never taken into consideration is ambient temp. Most wire load charts (such as in the NEC) are at 85*F, but underhood temps can reach much higher.
Just going up to 100 degrees you are at 90% of the rating with 90c wire and 80% with 60c. Go up even more and it keeps dropping off. Most audio grade 4g wire I've seen is the cheaper 60c wire. Get up to 120F and now you are at 60% rating with that stuff.
The amp rating charts on the net are all over the place as well. The NEC considers max ampacity of 4g 90c wire to be 95 amps at 85F. It's a tad conservative
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Blown88GT
Founding Member
You didn't read your own chart, degC, NOT degF.
That table is not applicable to DC current in engine spaces .
It's for AC conductors in a raceway with all kinds of caveats in the notes.
https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity
4AWG is rated for 136A.
As a former rocket scientist, I can tell you this is what you do & what works. I designed all kinds of DC power supply systems for DoD-DARPA.
That table is not applicable to DC current in engine spaces .
It's for AC conductors in a raceway with all kinds of caveats in the notes.
https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity
4AWG is rated for 136A.
As a former rocket scientist, I can tell you this is what you do & what works. I designed all kinds of DC power supply systems for DoD-DARPA.
You didn't read your own chart, degC, NOT degF.
That table is not applicable to DC current in engine spaces .
It's for AC conductors in a raceway with all kinds of caveats in the notes.
https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity
4AWG is rated for 136A.
As a former rocket scientist, I can tell you this is what you do & what works. I designed all kinds of DC power supply systems for DoD-DARPA.
You are right and I was incorrectly applying AC logic as that's what I'm most familiar with.
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I'm big on safety. I'll go with a quality 4 gauge wire and a quality 125 amp fuse. Thanks again.
Blown88GT
Founding Member
If you can find marine grade wire, it's usually the best. Some stereo shops use it, too.
I also want to point out that the 130A is not a continuous rating, but a peak or intermittent rating.
Just like gasoline engine horsepower can supply full power for less than 1 minute. Diesels somewhat closer to their rating.
Jet engines & rocket engines can put out full rated power for as long as there is fuel.
Blown88GT
Founding Member
The chart is an excerpt from the standards for marine construction, American Boat & Yacht Council (ABYC), Std. E-11, Table IV-A, the marine equivalent of the NEC (National Electrical Code). The 160A is for 105degC rated insulation, which my marine grade wire is. It's printed on the insulation & the lugs were factory soldered.
https://www.paneltronics.com/atimo_s/news/E11Excerpts.pdf
I have
I have never worked on rockets or boats; only aircraft and automobiles, so those are boundaries of my experience with things that move under their own power.
Just curious your routing that required a 7' cable. Mine was like that when I ran it over the the radiator support on a previous installation. I later re-routed the cable running it underneath along K member. The new cable including the fuse block and extension from it was less the 4' in length. The shorter cable will perform better with less amp loss at the load side.
John, it was just a guesstimate based on what I had read. When I get to the point of installation, I will try to make the route as short as possible.
Blown88GT
Founding Member
My battery is relocated to the passenger side because the SC is on the driver's side. This necessitates a longer cable to the starter relay but a much shorter one from the alternator to the battery. Replacement battery was changed from group 58 to group 59, which is a bit larger. Battery to fuse to alternator cables & fuse had to be re-routed. One cable now goes on top of the battery instead of the side.
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