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04-17-2021, 12:38 PM | #1 |
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Location: Sagle, Idaho
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20 or 30 Amp Fuse?
My 30 A Coup is 6 volt with alternator. No power accessories except turn signals. No LEDs. I have a main fuse at the starter which is currently 20 amp. Want to buy some spares. I have read on some threads to use a 30 amp. What's right for my car?
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04-17-2021, 12:47 PM | #2 |
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Re: 20 or 30 Amp Fuse?
If you are running halogen headlights I would go with 30. Otherwise 20 is sufficient. There is no original spec on this because fuses weren’t original to the car. All you will get is opinion. You don’t want to put in so big of a fuse to negate its purpose of protecting your wiring. On the other hand you don’t want to continuously change fuses. Look at your ammeter. How often do you go over 20 amps? Probably almost never.
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04-17-2021, 01:14 PM | #3 |
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Re: 20 or 30 Amp Fuse?
Fusing is to protect the wiring, the fuse size depends on the gauge of the wire from the alternator.
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04-17-2021, 02:16 PM | #4 | |
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Re: 20 or 30 Amp Fuse?
Quote:
What I mean is for the commonly used fuse (mounted at the starter), the fuse should be rated for the smallest gauge wire anywhere in the car (unless a specific circuit is protected by a separate fuse). 30a fuse is too large for the normal 14awg wiring used throughout the car. If you have a short, the wiring harness can burn before blowing the fuse. I had this happen when a headlight conduit connector shorted. It did lots of damage but never blew the fuse. If you have large loads and need 30 amps to the junction box, each individual circuit should be individually fused. My coupe is 6v with alternator, electrical system is in excellent condition. The only accessory is turn signals and a 20a fuse works fine.
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04-17-2021, 02:38 PM | #5 | |
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Re: 20 or 30 Amp Fuse?
Quote:
How many 20 amp fuses have you blown ? |
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04-17-2021, 07:24 PM | #6 |
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Re: 20 or 30 Amp Fuse?
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04-17-2021, 07:44 PM | #7 |
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Re: 20 or 30 Amp Fuse?
Some of the 20 amp fuse holders will not accept a 30 amp fuse. You can, however, put a 20 amp fuse in a 30 amp fuse holder.
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04-17-2021, 08:03 PM | #8 |
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Re: 20 or 30 Amp Fuse?
Use a 20 amp and if you need more look for the reason or problem!
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04-18-2021, 10:03 AM | #9 |
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Re: 20 or 30 Amp Fuse?
I have alternators in both of my Model A's, and I notice the following behavior:
The bottom line is, to cope with the peak starting current flow, try a 30A fuse or a 20A slow blow fuse.
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04-18-2021, 11:09 AM | #10 |
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Re: 20 or 30 Amp Fuse?
An explanation is in order of what the fuse is intended for.
The fuse is intended to protect the battery from discharging through a stuck cutout when the engine shuts down. It isn't in the path of anything else except the ammeter. As Stingray discovered it doesn't protect between generator and headlignts. Bob Bidonde raises an important consideration that an old weak battery may need more than 20 amps to recover from starter drain, but normal baterry charge current should not exceed 2 amps. It is wire size that determines fuse size for continuous current; 12 gauge wire will carry 20 amps. Where the wire is downsized for a branch circuit (lights, horn) each should be equipped with a separate fuse (10A continuous for #16 awg) if you want to protect these circuits. A 20 amp fuse for the battery will probably serve well, at least until your battery declines. 30 amps is normally too high for the 12 gauge wire it serves, but considering the surge is only of short duration, it should be ok. Keep your eye on the ammeter - you'll notice if the fuse blows. |
04-18-2021, 12:01 PM | #11 |
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Re: 20 or 30 Amp Fuse?
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04-18-2021, 08:38 PM | #12 |
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Re: 20 or 30 Amp Fuse?
Should you want to use a fuse that is mounted on the starter, the period looking ones the vendors sell are not always a rock solid connection after a while with intermittent continuity. It makes it hard to find the reason the car is acting up. As an alternative NAPA # BK782-3127 will also mount to the starter switch and use the newer ATC/ATO style fuse ,it has a weather enclosuer to prevent water and corrosion. It is not bad looking and the price is about $7.00
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