POWERBOATER’S GUIDE TO ELECTRICAL SYSTEMS
Fig. 1-8. The power formula. Like the Ohm’s law equation, the power equation can be used to determine the third value if two are known. Multiply and divide just as with Ohm’s law. This is the “pie formula,” which is useful for determining AC amperage. All UL-rated (Underwriters Laboratory) appliances must have either volts and watts or volts and amps indicated on an attached sticker. The sticker is useful for sizing circuit breakers and wire size.
Working with the Numbers A quick look at Ohm’s law and the pie formula, presented above, and some brief experimentation with a few actual numbers illustrate the interrelationship of all the elements we have discussed so far. As with any algebraic formula, we can move and substitute values so that we can use the same formula, written different ways, to determine any of our electrical values as follows:
V = A × R, or volts equal amps multiplied by ohms.
A = V ÷ R, or amps equal volts divided by ohms.
R = V ÷ A, or ohms equal volts divided by amps.
With the pie formula we get the following three variations:
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W = V × A, or watts equal volts multiplied by amps.
A = W ÷ V, or amps equal watts divided by volts.
V = W ÷ A, or volts equal watts divided by amps.
So, assuming a 13.5-volt constant (the normal voltage in a charged 12-volt battery), let’s see what happens to amperage when the resistance in a circuit changes from, say, 5.5 ohms to 7.8 ohms. By dividing 13.5 volts by 5.5 ohms we see that the amperage draw through this circuit will be 2.6 amps. By increasing the circuit resistance to 7.8 ohms, we will end up with 1.73 amps. If the resistance in a circuit is known or if it can be accurately measured, we can apply this simple formula as the first step in determining what size circuit breaker or fuse to use. (See chapter 4 for more detail on circuit protectors and how to select them.) As the resistance in a circuit goes up in value, the amperage goes down. Conversely, as resistance goes down, amperage goes up. This trade-off between amps and ohms is always valid, regardless of the amperage and resistance, as long as the voltage remains constant. As for a practical application of the pie formula, it’s quite useful when you’re adding AC appliances to your boat. All Underwriters Laboratories (UL) approved appliances must have a tag or label affixed to them with the operating voltage and wattage of the appliance clearly stated. By applying the pie formula, and dividing the wattage by the voltage, we can determine how many amps the appliance will require for operation. With DC appliances, determining the amperage used by the load is always the first step in determining the size of the fuse or circuit breaker we will need and the size of the wire we will use to supply the circuit.
Voltage Drop Voltage drop is simply the reduction of voltage in a circuit caused by amperage working to overcome resistance, and represents the conversion of electrical energy to some other form of energy. For example, when you turn on a bilge pump the current (amperage) used by the pump converts the power wattage to mechanical energy that turns the impeller of your pump and keeps your bilge dry. In another, less
