Fuses

I have a Farmall A converted to 8 volt battery. What is your recommendation for fusing in the light switch. The old fuse is almost unreadable under igh magnification but it looks like----BUSS SAE 20. Thanks

15 was original IIRC Jim

Fuse won't care if its 12V, 8V or 6V. They are all the same - 20 A = 20 A. (My $0.02 worth jal-SD)

Henry,

I believe that the current in the circuit, consequently, the current through the fuse, will increase by 1/3 by powering the lights off the 8 volt battery instead of a 6 volt battery. One of the derivations of Ohm's law is: Current (I) = voltage (E) divided by resistance (R).

Unless you have installed a bulb that is specifically for an 8 volt application, or, unless you have added a small resistor in series with bulb in the circuit, you are now dividing the resistance of the light bulb into 8 volts instead of 6 volts. I believe that this will increase the current by about 33% .

But then, others might see it differently.

Tom in TN

It does not matter what the draw will be, the fuse needs to protect the wiring, replace with same amps as original.

All bulbs are marked either 6 or 8 volt, no such thing as a 6 volt or an 8 volt bulb.

Not quite. Bulbs are rated in Watts (power)

P(watts)= I x E or current times voltage.

Solving for current gives us: I = P/E

If we assume a 40 watt bulb, on 6 volt circuit, 40/6 = 6.67 amps.

Assuming the same 40 watt bulb on an 8 volt circuit, 40/8 = 5 amps.

So, a given wattage of lamp will draw fewer amps as the voltage increases.

The 20 amp is the fuse for that system. Your bulbs are rated for 6-8 DC so theres now worry there. That chargine system will keep everything going going. Ive did other repair on some of those tractors using an 8v batt batt was fully charged hy reading was up to par so you should be OK. By the way i dont have any "flopers" for the exhaust systems last spare went to Florida. I dont want to sell one from a complete pressure part.
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My 1977 CUB has a 10amp slow blow. It is a 12 volt system. Two headlites and a rear tail also. A good rule of thumb is as voltage goes up the amps. go down. For 6 volt you may have a 20 amp but twelve might just get by with a 15 or even 10 amp. What you can do is measure the amp. load from one bulb with your meter and then multiply by how many bulbs you have. Also add about 1/4 to that figure. It should work for you. Radio Shack has several types of slow blow fuses in the cabinets. Not all car places have them anymore!

A 20 amp fuse will be adequate, no problem unless you have more than three lights with greater than factory wattage. As far as bulbs being marked 6 & 8 volts, well a normal 6 volt system operates at 7.2 volts when everything is perfect , thus you need a bulb capable of handling more than 6 volts in a 6 volt system therefore all 6 volts bulbs are marked 6 & 8 volt. A normal recommended voltage setting for an 8 volt system is 9.2 -9.6 to keep a battery fully charged. Gets tough on 6 volt bulbs, more current through points if battery ignition and just plain screws up the whole system. My two cents after less than stellar results of converting customers to 8 volt systems years ago.

I guess you better look up the specs on the bulbs used on the old I-H even says on the box 6-8v.

The purpose of a fuse is to protect the wiring. When I was an electrical design engineer (long since retired) the approach was FIRST compute the load,,,,,,,,,SECOND size the wire to safely carry the current,,,,,,,,,,THIRD size the overcurrent protection device (fuse or breaker) to protect the wire.

Basically size the fuse to protect what wire gauge is used and wire gauge is sized to handle the load current.

John T

John,

Are you and I the only ones getting this?

NO Dave and John I get it. Back in the early 70s I was studying to be an electrical engineer. By the second year of community college I got so POed with the instructors cause they only knew the book and had no practical. I was grading their test papers for them and designing some of the tests for the class! I quit and started working at a sound studio for a while. I sould have moved to a real college but so it goes.
Now most lite circuts are 20 or 18 gauge? If you turn on the lites in a modern car after about twenty minutes you can feel the wires are a tiny bit warm! Just a tad. Right on the edge of that knife blade. Old tractors are much more forgiving with wire size. Nothing like a little wire corrosion in the fuse holder!

I agree. My take on this original question is he is going with the wiring that is on tractor and had been operating as a 6 volt system. I doubt he is going to get out his calculator and start figuring watts, amps, volts, ohms or anything else. All those six volt systems came with a 20 amp fuse and when they went to 12 volts they put in a 10 amp fuse. If he puts in a 20 amp fuse it will protect his system just fine and if it keeps blowing the fuse he can easily jump up 5 amps or so.

(quoted from post at 16:25:45 05/06/15) Not quite. Bulbs are rated in Watts (power)
P(watts)= I x E or current times voltage.
Solving for current gives us: I = P/E
If we assume a 40 watt bulb, on 6 volt circuit, 40/6 = 6.67 amps.
Assuming the same 40 watt bulb on an 8 volt circuit, 40/8 = 5 amps.
So, a given wattage of lamp will draw fewer amps as the voltage increases.

Click to expand...

Wrong,
Better stick to reading the comics.

Thanks Jeff

From a fellow EE.

That is what I said.

Fawteen,

It's been a lot longer for me than it has for you since we attended ET"A" school, so you're probably right.

However, in my weak, old mind, I would see the situation differently from you. A 40 watt bulb is only a 40 watt bulb at a specified voltage due to the resistance of the filament and the consequent current in the circuit. The resistance of that bulb is not going to change. I agree that P=I*E, however, in this case, I believe that the current is going to increase, and the voltage is certainly going to increase, therefore, the power consumption is going to increase as well. The bulb will burn brighter and consume more power.

But then again, I graduated ET"A" school 52 years ago, so I could be all wrong.

Tom in TN

Hmmmmmm.

I withdraw my opinion.

I'm sure there are others who "get it" As I've often noted being on here over 15 years is, no topic gets more opinions then electrical or legal, everyone and their brother in law and Billy Bob and Bubba crawl out of the woodwork and all are experts. Its just a certain way I was educated and then practiced for years its old hat to us. I remember as a pimpled kid who knew nothing about overcurrent protection if a circuit blew a fuse we installed a bigger fuse lol that fixed it huh. The only way to correctly answer a fuse size question, is as I stated: Calculate the load, calculate the needed wire gauge, size the fuse to PROTECT THE WIRE.

Fun chattin with ya

John T

Don't know why I'm pitching my hat in this ring buttttttt.

Protecting the wire IS the task (as stated) and is a confusing task as numerous things come into account: Load current, environmental (ambient) temperature where the wire is routed, forced or natural convection movement of the air adjacent to the insulation, wire cross sectional area, wire material resistance, insulation temperature rating, wire bundling or lack thereof, raceway confinement or lack thereof........

I think I'd go with the "replace with the one that you had".....specified value for that tractor. If it blows then worry about it. Course just because it is an 8v lamp doesn't necessarily mean it will draw a proportionally larger current than a 6V. Depends on wattage consumption when at full luminance for a given applied voltage. If the 6v was a 35 W rated bulb at 6v and the 8v is a 30w rated bulb, you haven't increased your current much (17%) so questioning your fuse size is not an issue.

Fuse applications are usually designed with a safety margin to allow for impulse currents, like motor starting, "lamp ratings" where the resistance of the wire cold is like 1/10 what it is when the filament is glowing. Course you have different types of fuses also, like ultra fast blow for sensitive electronic circuits, normal blow which means a couple of things, slow blow, and breakers with predetermined load/time ratings.

Other thing on a fuse is the overload. Speed and value of the overload work hand and hand. A short high current spike could get by without blowing the fuse if the temperature it generates fits the envelope of the fusing material. A slight overload over a long period of time can cause it to blow if it exceeds that envelope.

On an SAE and AWG type, to name a couple, normal blow, glass envelope fuse, it's easy to see what kind of overload you experienced: If the wire is melted, like solder looks as it is changing from the solid to molten state, you had a long term slight overload. If there is a big gap between the pieces of wire and splattering on the glass, you had a high, short duration overload, like something shorted directly to ground (power return).

Littlefuse is a major mfgr. of fusing devices and their www has a lot of helpful info.

Can't recommend the 8V battery . It's a bandaid for the symptom not a solution for the problem.
System voltage to properly charge A 8volt battery is 9.2 Volts.