Main fuse blowing

[6T5VAIR]

There is a 30 amp fuse on the main battery line in my '65. It frequently blows, but never while driving. I have had it happen exactly as I turn off the motor or on other occasions at some unknown time while shut down and sitting in the garage. No other circuits ever blow and there are no electrical issues I'm aware of. Most of the wiring in the car is updated/replaced. Does anyone have any thoughts as to what could be causing this fuse to blow when no other electrical issues seem to be present? I know that's a hard question without knowing what was done on the car but if you could give me some ideas where to look I'd be very appreciative.

[Frank DuVal]

30 amp is low if this is in the line off the positive battery terminal. Do you have a. Non-stock alternator that can put out 60+ amps if it sees the need? Even the smallest stock alternator was 37 amp. Factory was no fuse there, just the HRPT screw terminal. A common replacement for the HRPT is a 40 to 60 amp breaker, like found on convertible top motors.

Now why it blows on turn off I do not know, but I have read this before.

HRPT is horrid red plastic thingy.

[cnicol]

Agree, 30-amps is low and subject to nuisance failures.

I would cautiously install a 60 and see what happens. If there's a problem, it will still blow a 60. I sell main fuse kits with 60s but I'm thinking of switching to 80 or 100 to reduce voltage loss across the fuse.

FYI, a short in the main battery wire to the front draws 10 to 100 times the fuse value.
Also FYI, the initial battery inrush current after a cold start is 30 amps or so for the first few seconds.

[6T5VAIR]

Frank yes it is first in line right off the battery. I'm not sure about the alternator I'll have to do some research on that, I believe it was a rebuilt stock unit.

I've ordered a 40-amp circuit breaker that plugs into the glass fuse receptacle, will let you know how that goes.

The 30 amp fuse was added inline during the build up but the car was never run on it until now, so it's probably just a poor selection of amperage in the application as you indicate rather than being indicative of a new problem.

Thanks guys

[cnicol]

Even 40-amp is a little light. The 10ga main battery feed to the car is rated to handle 55-amps. Hopefully your circuit breaker is a manual reset. Automatic reset breakers keep applying power to the short circuit until your battery is dead.

You don't really need to research your alternator. Current is determined by loads not supply. Battery resistance limits inrush to 30 amps and that's only for a few seconds so it's more informative to look at loads such as high wattage headlights or a high wattage stereo.

I'd go with 60-80 amps and by the way, this is my business; I design and manufacture fuse panels and wiring systems for custom vehicles.

[6T5VAIR]

The way I understand this one is it stays broken until power is removed so I'll pull it and re-insert it to reset it as needed.

https://www.eaton.com/ca/en-gb/skuPage. ... tions.html

If this doesn't work I'll move up to a higher amperage glass fuse.

[66vairguy]

Good comments. The wire to the fuse box will handle up to 55Amps before it starts to overheat the insulation.

Craig made a good point about voltage loss across a fuse. The lower the fuse amperage rating the higher it's resistance. Some of that can be negated by moving the alternator sense wire to after the fuse supplying main power to the fuse box. The sense wire is small and it will fail/open if there is short.

Unless you install a "SLO-BLO" fuse, surges can blow a fuse quickly. I use 60Amp fast blow fuses without issue. One fuse installation I did saved a Greenbrier when the owner was having an "option" installed and the installer shorted the main supply wire to ground. The fuse prevented a fire. As the owner told me "There was a bright flash, then nothing worked".

If you don't like fuses then you can install the "fusible links". The small wire from the battery cable to the "horrid red plastic thingy" terminal should be a fusible link, but often is replaced by a regular copper wire. If you look at the 67 wiring harness in the shop manual you'll see Chevy FINALLY added more fusible links to protect the wiring, battery, AND alternator.

[cnicol]

Hopefully it will work out for you; 40-amps might just be enough. Yes, that style breaker is manual reset. FYI, I'd be surprised to see fuses or circuit breakers over 40 amps in that style.

Maxi-fuses are the next step up and as I recall Maxi-fuses are available in 20-100 amp ratings. Maxi fuse holders are available with inline or screw-on fuse holders.
https://www.racersrods.com/category-s/1822.htm

[County98]

FWIW, I changed out my alternator to a 63a and got rid of the little red tie point going into cabin. I used a 60a maxi fuse and haven't had any issues.

Maxi Fuse (1).jpg (57.68 KiB) Viewed 93 times

[6T5VAIR]

Hopefully it will work out for you; 40-amps might just be enough. Yes, that style breaker is manual reset. FYI, I'd be surprised to see fuses or circuit breakers over 40 amps in that style.

Maxi-fuses are the next step up and as I recall Maxi-fuses are available in 20-100 amp ratings. Maxi fuse holders are available with inline or screw-on fuse holders.
https://www.racersrods.com/category-s/1822.htm

You are correct 40 is the highest in that style.

I haven't seen anything that would go into the same holder, was hoping not do do any wiring but I can if needed. Let's see how the 40 does anyway. :-)

[Lane66Monza]

Craig is accurate. He evaluated my 66 More Door loads and recommended an 80 amp protection for the car. I have added loads with halogen headlights, electric fuel pump and hi-flow fan, but reduce loads a small amount using LED park/stop/tail lamps, courtesy, done & instrument lamps. 7" display radio was a wash compared to OEM radio, i think. I have a 65 amp internally regulated alternator.

I have installed an 80 amp Buss circuit breaker to replace that HRT the power wires were connected to.