Electrical 101 - Read This First


The intent of this topic is to provide a starting point for those wishing to tackle changes to their electrical systems. This can be as simple as adding or removing some functionality all the way up to, and including, a full rewiring job with custom harness.

Many folks view electrics as difficult or as black magic, but I assure you it's a lot simpler than you'd think. There are a few basic ideas and by combining these ideas in different ways, you can end up with different results.

Here's a quick list of the tools any DIYer should have in their toolbox:
  • Multimeter - Should read in both AC and DC and be able to measure resistance. Ideally it should be able to read current as well as voltage.
  • Wire Strippers - Needs to handle anything from 10 gauge to 20 gauge
  • Crimpers - Used for adding crimped connectors to the ends of wires. Don't get a cheap set.
  • Soldering Iron - For soldering wires together. Again, don't get a cheap one. 60W or better.
  • Heat Shrink Tubing & Heat Gun - Possibly an optional item, but goes a long way toward making your wiring jobs look more professional.

Ohm's Law
Ohm's Law isn't something you'll be using constantly, but it is important to know what it is and how it applies. Ohm's law is represented as
. I stands for current (or amperes), V is voltage, and R is resistance (measured in Ohms and also signified by the Greek letter, Omega: Ω).

What this equation means is that you can change one variable to affect the others and Ohm's law is why your bike blows fuses. When voltage remains at a constant (12V, usually) and resistance drops to near zero (such as when a short occurs), then current can get very high. Flowing too much current through too small of a wire results in a lot of heat, often enough to melt the wire and even ignite the insulation. This is why fuses are put into place. If a fused circuit pulls too much current, then the fuse blows (technically, it melts) and prevents damage to other portions of the circuit.

A Watt is shorthand for one joule second. It's the measurement used for electrical power (not to be confused with current or voltage). Wattage is to amperes and voltage as horsepower is to torque and RPMs. In a circuit in your motorcycle it is often useful to think of Watts as being consumed. The alternator will have a maximum output, but your bike will only use so much of it. Your manual will state how much Wattage your bike can generate, but it's up to you to figure out how much is being used. Luckily, that's an easy calculation.

Current times Voltage equals Wattage (or I x V = W, if you prefer). 99.9% of the stuff on most of our bikes will be running at 12V (or 6V for smaller and older bikes) as so we only need to figure out current in order to calculate wattage. This can be done by measuring the resistance across each electrical component (coils, headlight, tail light, etc) and then using Ohm's Law to calculate current and then Wattage. Many multimeters will be able to measure current or Wattage directly, too.

If the Wattage draw of your components exceeds the Wattage being created by your alternator, then your battery is being drained. Your battery will only charge when the Wattage being produced exceeds the Wattage being consumed.

Common ways to reduce Wattage consumption on your bike are to replace incandescent bulbs with LEDs (lower current at same voltage means less Watts!), change circuits to switch off unwanted components (e.g. Rewiring your headlight on a switch so it's not always on), or even removing some components all together (indicator lights, gauge bulbs, etc).

Wiring Basics
Wiring is actually a simple process, but there are a few things to keep in mind.

First up, always use stranded copper wire. Solid wire may eventually fail due to the vibrations of your bike and aluminum wire tends to corrode faster than copper.

Next, all stressed connections should be crimped and not soldered. I know people will argue with me on this one, but I stand by this statement. Wires that are under tension (which is almost all the wires on the bike) may eventually break soldered connections. I've never seen it happen, personally, but I've heard second hand accounts. I prefer crimped connections, anyway, because they're easier to undo in case you need to make future changes.

Also, when wiring a circuit, make sure you use the correct gauge wire. Going too small on the wire adds heat into the circuit, which is not good when so many wires are bundled together. This increases the potential for failure of the connectors and/or insulation as well as causing increased Wattage draw (hot wires have greater resistance and will use more power when conducting current). Going too large on the wires increases both weight and cost. Use this chart as a rule of thumb:
Gauge Max Current

If you need bigger than ten gauge, you may want to rethink what you're doing. Also, don't use smaller than 20 gauge as it just gets too flimsy and is prone to breakage.

Series VS Parallel
There are two basic ways to run a circuit; series or parallel. Your bike is several parallel circuits, each made up of additional parallel circuits and/or series circuits. All parallel circuits can be broken down into a combination of series circuits.

Series circuits are the easiest to understand. Electricity follows one path from point A to point B (generally thought of as coming from Positive to Negative). An example of a series circuit on your motorcycle would be a wiring run from the battery, to the ignition, to the kill switch, to the coils, and then to ground.

In reality, this is a parallel circuit. The ignition switch usually represents a junction where several different circuits branch off any follow their own series, only one of which is the kill switch and coil circuit.

In the attached image, this is made clearer. A solid red wire runs from the battery to the ignition switch. When it reaches the ignition switch, it is split into black, brown, and brown/white wires. Each of these is a new branch in a parallel circuit.

The important thing to remember about parallel circuits is that each branch does not change the voltage; it changes the current. Each branch will still be running at around 12V, but each branch will have differing amps. The trunk of the branch will have current that is the sum of each of the branches. So if you have three branches, each at three amps, the trunk will have nine amps. Plan your wiring accordingly.

Fuse sizes should be based on the smallest wire in a circuit. If you're choosing to run a single fuse at the trunk of your circuit and your circuit contains 20 gauge wire, this fuse needs to be 3A. If the smallest wiring you're running is 18ga, you can use a 5A fuse. It is often better to run several different fuses, one for each branch, than it is to run a single fuse. If a fuse gets blown this will help you isolate the problem circuit. Also, your current requirements as a whole may exceed the rating of your smallest wire and so a single fuse will not be feasible.


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As a recent purchaser of a basket case bike I wish I could understand all this well enough to ditch the old harness and wire the bike new. I cannot read the wiring diagram well enough to do it though.

I get some of the circuits, the simple ones like lights. I can't figure out the charging, regulator, rectifier bit though. What's the switch(?) right next to the headlight switch that has yellow and yellow/white wires? Is this part of the headlight switch too? The Radio Shack mod? I'd love to see a similar diagram of the Kohler mod-alternator-charging circuit.

Times like this I miss my dad who was an electrician by trade. I need a 'Bike wiring for dummies' book.

sourjon said:
need a 'Bike wiring for dummies' book.


You found it. Keep reading. :) The nice thing about the current release is that you can ask it questions, and only about one out of three times will it be a wiseass.
Rich Ard said:
You found it. Keep reading. :)

Yeah I've been lurking and devouring threads for about a year feeling really inadequate and when I finally bought this thing I joined up. Hope I can contribute something sooner or later and not just be asking for help.

Very cool, will probably have to read through it a few times before I fully comprehend what you're talking about.

Cheers mate!
This still seems like black magic to me, how do you know what currents are running to and from different components and what wires to use.
Qawls said:
This still seems like black magic to me, how do you know what currents are running to and from different components and what wires to use.

Current, Voltage, and Resistance are all related due to Ohm's law.

Current = Voltage / Resistance.

You know voltage is set at 12V (or in some cases 6v if your bike is old enough), so measuring the resistance across a component will tell you how much current will be drawn.
Add to Your excellent post Sonreir:

Watts = Amps x Volts

Amps = Watts/Volts

Volts = Watts/Amps

So to determine the Amps a 60 Watt headlight bulb needs:

60/12 = 5 Amps

Your ignition coil is 5 Ohms (Stock)
Using Ohm's Law, 12V/5 = 2.4 Amps

Now, put one of those fancy 3 Ohm Coils in:

12V/3 = 4.0 Amps.

Better yet, remember, the alternator runs the bike at 14.5 volts when running properly.

14.5/3 = 4.8 Amps -

Wonder why your CB350 with a 110 Watt alternator is losing its charge?

110 Watts at 12V = 9.2 Amps
110 Watts at 14.5 Volts = 7.6 amps

So what happens with a 60 Watt / 5 amp Lamp and a 3 amp Ignition? 8 amps leaving 1.2 amps for the other lights and charging the battery.

Now do 60 Watt lamp and 3 ohm Coils: 5 amps and 4 amp = 9 amps...nothing left from the alternator to charge the battery.

Keep in mind, the resistance of bad connections, wire length, all contribute to current use, and these numbers are based on a NEW CB350, not a 40 year old one.

Another note, the ignition does not draw the full amps continuously. It has about a 25% duty cycle, so 75% of the time, it is drawing nothing.

However, your 40 year old alternator probably isn't producing its full wattage either.

Some food for thought.
I have a '75 Honda cb125 that needs a new ignition coil. It has proven rather difficult to find a '75 honda cb125 coil. There has to be something else I could use, right?
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