20 or 30 Amp Fuse?

[Oldgearz]

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?

[Bill G]

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.

[JSeery]

Fusing is to protect the wiring, the fuse size depends on the gauge of the wire from the alternator.

[Stingray70]

Originally Posted by JSeery

Fusing is to protect the wiring, the fuse size depends on the gauge of the wire from the alternator.

Actually, the fuse is protecting everything "downstream" of wherever it is placed. The wire size from the alternator to the fuse is irrelevant.

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.

[Patrick L.]

Originally Posted by Oldgearz

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?

How many 20 amp fuses have you blown ?

[Oldgearz]

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Thanks all. I'll stick with 20 amp fuses. I only fried one and that was because I shorted a wire.

[CarlG]

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.

[McMimmcs]

Use a 20 amp and if you need more look for the reason or problem!

[Bob Bidonde]

I have alternators in both of my Model A's, and I notice the following behavior:

• When starting the engine, significant current is drawn from the battery leaving it in a condition that the alternator's regulator senses as discharged;
• The alternator goes to its rated output and within a minute or so, its output current reduces to a few amps as it satisfies the battery.

The output current spike immediately after starting the engine will exceed a 20A fuse. Because of the short duration of the current spike and durability of your fuse, the 20A may survive for awhile. However, as the battery ages, its internal resistance increases, so it excites the alternator to produce a higher starting current flow. Same condition happens in cold weather.


The bottom line is, to cope with the peak starting current flow, try a 30A fuse or a 20A slow blow fuse.

[Badpuppy]

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.

[Patrick L.]

Originally Posted by Oldgearz

Thanks all. I'll stick with 20 amp fuses. I only fried one and that was because I shorted a wire.

Well, now you know it works as intended !

I keep spares in the vehicle, just in case.

[mleder]

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