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3 Ways to Find Amps. An ampere, shortened frequently to amp, is a unit of measure that is used for electrical current. Current is the measure of electrons that flow through a given circuit. This information can be very useful in the event you are trying to connect a tool or appliance to the mains, which is the term used to describe the AC current that flows directly from an electric company’s generating station to your household
Divide the watts of a given electrical item by the total number of volts available from the electric outlet to calculate amperage draw. The amount of current flowing through the wire is measured in amperes, or amps. The equivalent of available electricity at the power source is voltage, or volts. Finally, the power produced by the electricity is measured in watts. All of these measurements are interrelated when calculating electricity usage.
#1. Converting Watts to Amps
- Apply the conversion formula for DC electricity. Electric current, represented by I, which is measured in amps (A), can be found by dividing power in watts (W) by the volts (V) of voltage. This is represented by the following formula:
- I(A) = P(W) / V(V)
Or, more simply: Amps = Watts / Volts
- Understand power factor (PF) for AC electricity problems. Power factor is a ratio representing the real power used to do work and the apparent power supplied to an alternating current circuit, a value ranging from 0 to 1. Therefore, power factor is your real power P, in watts, divided by your apparent power S, measured in Volt-amper (VA), or:
- Calculate apparent power to find your power factor. Apparent power can be calculated byS = Vrms x Irmswhere S is the apparent power in Volt-amper (VA), Vrms is your root mean square voltage and Irms is your root mean square current, both which can be found by solving the following:
- Vrms = Vpeak / √2 in volts (V)
- Irms = Ipeak / √2 in amperes (A)
- Use the power factor for single phase AC electricity. Your single phase current will be represented by I and measured in amps (A), and can be calculated by dividing the real power (P) measured in watts (W) divided by a power factor (PF) multiplied by the root mean square (RMS) voltage as measured in volts (V). This is represented by:
- I(A) = P(W) / (PF x V(V)
Or, more simply: Amps = watts / (PF x Volts)
#2. Measuring DC Amperage with an Ammeter
- Make sure your current is DC. DC electricity, or direct current electricity, is electrical current that flows in a single direction. If your circuit is powered by a battery, the current used will be DC.
- In most countries, the electricity provided by utility mains is AC current (also called alternating current). AC current can be converted to DC current, but only through the use of a transformer, a rectifier, and a filter
- Determine the path of electricity. To take the reading of the amperage of your circuit, you will need to tie your ammeter into your circuit. Follow the positive and negative ends of your battery and the connecting wires to find the circuit path.
- Test your circuit. If there is a break in the circuit or a flaw with your battery, your ammeter will likely not be able to gauge (or will not gauge accurately) the current of your circuit. Turn on your circuit to see if it is functioning normally. Find Amps
- Switch off your circuit. For some simple circuits, this might require removing the battery completely. With more powerful batteries, there is a possibility that you could be shocked, so take care to make sure the circuit is off. If you are unsure, use insulated rubber gloves to prevent getting shocked.
- Tie in the positive end of your ammeter. You ammeter should have come with two leads: one red and one black. The red lead is your positive end (+) and the black your negative (-). Take the wire leading from the positive end of your battery and tie the end leading away from your battery to the positive end of your ammeter.
- The ammeter will not interrupt the flow of electricity, but as current flows through the meter, it will measure the current, causing a reading to display.
- Complete the circuit with the negative lead of your ammeter. Take the black (-) lead from your ammeter and use it to complete the circuit you have just broken. Clamp the lead onto the location where the wire you have tied to your positive lead would have fed into its destination in the circuit
- Turn on your circuit. This might simply mean reinstalling your battery, but when you do so, your device should turn on and your ammeter should read the current in either amps (A) or milliamps (mA) for smaller current devices Find Amps
#3. Calculating Amperage with Ohm’s Law
- Familiarize yourself with the concept of Ohm’s Law. Ohm’s Law is an electrical principle that establishes a relationship between the voltage and current of a conductor. Ohm’s law is represented by the formulas V = I x R, R = V/I, and I = V/R, with the letter terms defined as:
- V = the potential difference between two points
- R = the resistance
- I = the current flowing through the resistance
- Determine the voltage of your circuit. If your circuit runs on a 9-volt battery, you already have part of the equation. You can find the specific voltage of the battery you are using by checking the packaging in which it came or doing a quick online search. Find Amps
- Most common cylindrical batteries (AAA through D) provide approximately 1.5 volts when fresh
- Find the resistors in your circuit. You will need to know what kind of resistor is part of your circuit and how much resistance it is creating to the electricity flowing through it. Since each circuit will be different (some simple circuits may not even have resistors), you will have to investigate your circuit and locate the resistors for your unique case and their resistance in Ohms (Ω).
- The wiring your electricity flows through will also have resistance. This will likely be negligible, unless the wiring is very poorly manufactured, damaged, or your circuit conducts electricity over a long distance. Find Amps
- The formula for resistivity is as follows: Resistance = (resistivity x length)/area
- Apply Ohm’s Law. Due to the fact that battery voltage is applied to the circuit entirely, to approximate the current of your circuit you will need to divide the total voltage by each resistors resistance, with resistance being measured in Ohms (Ω). Your resulting answer will be the current (I) in amps (A), solved with the following calculation:
Calculating from Watts and Volts (Find Amps)
Find the wattage load of an device that requires electricity. Any device that draws energy is called a load. Examples of loads include a light bulb and a microwave. The wattage is often printed on the device itself, but if you can’t locate the number, you might need to check the owner’s manual.
Determine the voltage of your power source. In the United States, most household outlets run at 120 volts, although some, such as those for electric stoves or dryers, often run at 220 volts. If your power source is a battery, you will need to look up the voltage. Larger batteries are often 9 or 12 volts, while smaller closed cell batteries, such as C, AA or AAA, run between 1 and 3 volts, depending on size and composition. Find Amps
Divide the wattage rating by the voltage from your power source. For example, if you have a 100-watt light bulb in a lamp that is plugged into a 120-volt outlet, it will draw 0.83 amps.
Calculating from Ohms and Volts (Find Amps)
The electricity flowing through the wires in your house is often compared to water running through a hose. You can observe the size of the hose, the amount of water flowing through it, the water pressure and the result of the water spraying out. For electricity, the flow of the current is limited by the resistance to flow, measured in Ohms.
Use Ohm’s law to calculate amps using resistance. Many appliances have a listed resistance. The wire connecting the circuit also has a variable resistance. In the same sense, you can fit less water through a garden hose than a fire hose. You don’t need to include this resistance unless you have a lot of wire or need to be very accurate.
Find the voltage of your power source as you would when calculating from watts and volts.
Ohm’s law states that the voltage equals the amperage times the resistance, so if you divide the voltage of your power source by the resistance of the load, you will find the amps. For example, if you plug a 40-Ohm dryer into a 220-volt outlet, the appliance will draw 5.5 amps. Find Amps
Things You’ll Need
- Object or owner’s manual
- Specifications for the electrical system
- The calculations described are for a single load. When calculating amperage over multiple loads you can simply add wattage ratings together, but resistance can change depending on how the circuit is configured.
- Use caution when working with electrical energy, and have your calculations double checked by a trained professional if you are calculating amps for a home electrical system.