Techtalk - Basic Electricity

March 2001 Telltale Page 14 - by Tom Winlow and Marcel Laroche.

We have all studied basic electricity in high school, but because it is essentially invisible, it is a difficult subject to understand and remember.

In an operating electrical system, there are always three things present: Voltage (E). Resistance (R). Amperage (I).

Voltage (volts) is the electrical pressure supplied by a battery, generator or alternator.

Resistance (ohms) is the opposition to the current flow caused by wires and electrical "gizmos".

Amperage (amperes) is the current flow through the wires and "gizmos".

To visualize how an electrical system operates, let's compare it to a garden hose connected to a house tap.

With the tap fully opened, there is good water pressure (ELECTRICAL PRESSURE/VOLTS), and a substantial water flow (CURRENT FLOW/AMPERAGE) through the hose (WIRE + GIZMOS). If you squeeze the hose (INCREASE THE RESISTANCE), the water flow (CURRENT FLOW) will decrease. Notice that we did not change the water pressure, we reduced the water flow, simply by increasing the resistance to the water flow. Now let's leave the hose alone and turn the tap halfway towards the closed position. The water pressure in the hose will decrease and the water flow in the hose will decrease without changing the resistance.

A basic electrical system works exactly the same as the garden hose example.

For a constant voltage, increasing the resistance will decrease the current flow (amperage). Conversely, decreasing the resistance will increase the current flow. (You are doing this when you use a dimmer switch.)

This basic formula will help you solve common electrical problems.

E = I x R or



I * R


I - Amperage (CURRENT FLOW)


If you change one of the three components of the equation, you will affect only one other component. IE: Increase the voltage without changing the resistance, the amperage will increase. Lower the resistance without changing the voltage, the amperage will increase, NOTE: You can't change the amperage by itself, because it is a function of voltage over resistance. E / R = I


That is to say, that if you reduce the resistance to a very low value, there will be a very large current flow. As an example, if a large wrench touches both battery terminals, the current flow through the wrench would be very high, because of the low electrical resistance of the wrench.

Now for a bit of mathematics with a 12 volt system…

E = I R or



I * R

To find the amperage when voltage and resistance are known, put your finger over the I and you have: E/R (Voltage divided by resistance). If voltage is 12 volts and resistance is one ohm, you have 12/1=12 AMPS current flow. If the resistance is increased to 12 Ohms, you have 12/12=1 AMP current flow. To find the resistance of the circuit when voltage and amperage are known, put your finger over the R and you have: E/I (Voltage divided by amperage). If voltage is 12 Volts and amperage is 3 amps, you have 12/3=4 ohms circuit resistance.


If a fuse blows, replace it with a fuse of the same rating. If it blows gain, do not replace it again until the cause of the failure has been corrected. When a circuit breaker trips, reset it once. If it trips again, do not reset it until the cause has been rectified.

When a fuse blows or a circuit breaker trips, it is telling you that the circuit/gizmo down stream of it is drawing too much current. If the one amp fuse in your instrument blows, but the 10 or 15 amp panel circuit breaker does not trip, the problem lies with the instrument. If on the other hand, the 10 or 15 amp panel circuit breaker trips, but the instrument fuse does not blow, the problem is not in the instrument but in the circuit feeding the instrument or with another gizmo connected to the circuit.

Bypassing fuses with aluminum foil or installing higher rated fuses is dangerous, because it could cause an electrical fire. When you examine a blown fuse, you will often see a blackening of the glass inside the fuse. This is because the filament in the fuse melted, due to the heat generated by the over-current flow. That is safe because the heat was contained within the fuse. Now, imagine if you had a short circuit in the wiring leading to the instrument. If the fuse is of the proper rating, the short circuit (lower resistance) will cause the current flow to increase to a point where the fuse will blow or the C.B. will trip at the main panel.

When a fuse is by-passed with metallic foil or a higher rated fuse is used, the safety feature of the fuse is lost. A short circuit in a compromised or unprotected circuit could result in an electrical fire. When a short circuit occurs, the resistance of the circuit is greatly reduced, and since the "amperage is equal to the demand", the amperage increases considerably. This will cause the wires to heat up (toaster), soon followed by melting insulation and in turn melting of the insulation of adjacent wires, and possibly a fire. If this happens, turn the main battery switch

OFF immediately before investigating anything.


When discussing electricity, we often hear the term "wattage" or "watts". Ontario Hydro sells us electricity by the kilowatt-hour (kWh). We use 60 and 100-watt light bulbs and baseboard heater rated at 1500 watts.

A watt is a unity of electrical power (P). A one-volt circuit with a one-ampere current flow has one watt of electrical power.

The formula is

P = E x I (Prince Edward Island)



E * I

Some more basic mathematics… When a 12-volt circuit drawing 10 amps, it is consuming 120 WATTS of electrical power. 12 Volts X 10 Amps = 120 WATTS.

When the ammeter on your engine instrument panel is indicating a discharge rate of 50 AMPS when starting, the starter motor is drawing 600 watts (12 X 50 = 600). So what? Well that's almost one horsepower. Then after starting, the ammeter is indicating a charge rate of 30 amps, it means that your alternator is putting out 360 watts or almost ½ horsepower. (H.P. = 745.7 WATTS).

Light bulbs are rated in watts; the higher the wattage, the brighter they are. In order to shine brighter, it has to draw more current (AMPS); the same is true in your house. A 60 WATT bulb draws ½ AMP in a 120 Volt system. To find the current draw, put your finger over the I in the power formula and you have P/E or 60/120 = .5 AMP. A 100 WATT bulb would draw almost 1 amp (100 / 120 = .83AMP). A baseboard heater of 1500 watts would draw 12.5AMPS (1500 / 120 - 12.5 AMPS). A toaster rated at 600 Watts draws about 5 AMPS. A kettle rated at 1500 WATTS draws 12.5 AMPS. The two together draw 17.5 AMPS. That is why the circuit breaker trips because it is rated usually at 15 AMPS.

Hope that this has refreshed your memory about basic electricity. Next month we will look at various electrical devices in your boat.

Next month we will look at various electrical devices in your boat.