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Ohm's Law & Watt's Law Calculator with Examples

How to use the calculator:
  Enter any two known values and press Calculate to solve for the others. 
  Always press Reset before each new calculation.

Ohm's Law:
  States the relationships between current (amps), resistance (ohms) and voltage.
  Volts = Amps x Ohms
  Amps  = Volts / Ohms
  Ohms  = Volts / Amps

Watt's Law:
  States the relationships between power (watts), current (amps) and voltage.
  Watts = Volts x Amps
  Volts = Watts / Amps
  Amps  = Watts / Volts

Ohm's Law Example: Calculate Ohms from Voltage and Amps
- Rating Rotors by Resistance

You have a box of 27SI rotors and need to determine which are 12 volt and which are 24 volt.

You want to use an ohmmeter to test each rotor—but you don't know the resistance value (ohms) for each type of rotor coil. Delco-Remy published only field-current values (amps), not resistance (ohms).

Use the Delco-Remy 1G-188 chart to look up the voltage and amp draw of the 27SI rotor coils. From the chart you find that:
  • 12-volt rotors draw approximately 4.60 amps @ 12 volts
  • 24-volt rotors draw approximately 2.15 amps @ 24 volts
Enter 12 volts and 4.60 amps into the calculator and it will show the coil resistance as 2.61 ohms.

Enter 24 volts and 2.15 amps into the calculator and it will show the coil resistance as 11.16 ohms.

Now that you know the resistance value of each type of coil, you can quickly grade each rotor. (Don't forget to mark them!).

Helpful links:
Delco-Remy 1G-188 Alternator Test Specifications Guide
Watt's Law Example: Calculate Amps from Watts and Voltage
- Adding an Accessory

You are setting up a truck with a snow plow and want to add plowing lights.
You have selected bulbs that are rated 65 watts.

Two things you need to determine:
  • What amperage capacity relay to use.
  • What size wire to run from the relay to the headlights.
These are the known values:
  • Maximum operating voltage of the headlights: 14.5 volts
  • Wattage of each bulb: 65 watts (Since there are two bulbs, double the watts)
Enter 14.5 volts and 130 watts into the calculator. It will show the amperage as 8.97 amps.

Now you know that the amp draw is well within the range of a 40A Bosch-style mini-relay.

Use the ERA wiring-size chart to find the length of the wires from the relay to the headlights. For example, if the wire run is 8 feet and the amp draw is 10 amps or less, use 14 AWG wire.

Helpful links:
ERA wiring-size chart
ERA guide to using mini-relays (see Fix #4)
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