Watts to Amps conversions for aspiring Electricians.

The job of the electrician is to install and maintain electrical wiring in a safe manner that protects people and property from electrical shock and potential fire risk, and the only known way to do that is to be able to make accurate calculations at the time of the installation.

Before we can do that we must understand what is meant by safe as it pertains electrical wiring and what are the standards and guidelines for safe practice.

Load Size

The first thing an electrician needs to know about a circuit is its load size.

When we know the load size we can then determine what size wire will be suitable for this circuit. The way to determine load size in any circuit is to examine what devices or equipment will be plugged into the circuit. If for example the circuit is to supply a microwave oven in a kitchen then the rated power requirements of the device in watts will be stamped on the specifications tag on the appliance.

As an example the power requirement of my microwave oven is 1550 watts. From the wattage we can determine the load in amps that this device will use during operation.

From the basic power triangle below we know that Power (P) = Volts (V) x Amps (I)

It may be slightly confusing that the symbol “I” is often used for amps instead of “A” but when used, the symbol “I” stands for intensity of the current in amps.

To apply this or any formula we need to know two values of the equation in order to find the third value.

• The appliance specifications tag will give us the first value in watts.

• The second value is also a known quantity as we are aware that the standard line voltage in the United States for domestic appliances is 120 volts unless told otherwise.

To apply the formula to this circumstance, 

• P = V x I 

• P = 1550 watts 

• V = 120 volts 

• I = ? 

To use the power formula to find a missing value we cover the value that we do not have, in this case amps. 

What remains when we cover I is P / V 

P = 1550 watts

V = 120 

In all formula’s what is below the line is divided into what is above the line, 

1550 divided by 120 = 12.91 amps

In conclusion, my microwave oven will pull a maximum current of 12.91 amps when operating.  

As a second example we will take a look at my dishwashers specifications tag.

The tag on my dishwasher tells me that my dishwasher uses 1.5 kW

Kilo comes from the Greek word meaning thousand, so 1.5 kilo is one and a half thousand or another way to say is fifteen hundred written as 1500.

To apply the formula to this circumstance, 

• P = V x I 

• P = 1500 watts 

• V = 120 volts 

• I = P divided by V = 1500/120 = 12.5

My dishwasher will pull a current of 12.5 amps when running.

It is a useful exercise for all aspiring electricians to do a total load calculation at home to become familiar with the type of loads encountered in a typical residential installation.

Take a look inside all your appliances and you will see a specifications tag giving technical information.

If you would like to get a really good understanding of electricity in the home then please complete this exercise. Check the specification tag on all these appliances in your home and calculate the load in amps of each appliance based on its stated wattage.

Microwave oven

Vacuum Cleaner

Toaster Oven

Cell phone charger

Washing Machine

Electric Mixer

Dishwasher

Television

Gaming Console

Laptop Computer

Hair Dryer

Wire sizing.

As a starting point for an aspiring electrician one of the first and most important things to be learned is wiring sizes and ampacity charts. The electrician needs to know what the load requirements are for a particular circuit and only when he knows that can he decide what size wire to use and what size circuit breaker will protect that wiring.

As it pertains to general purpose wiring for electrical outlets and lighting circuits, it is permissible by the NEC to use #12 gauge wire with a 20 amp breaker or #14 gauge wire if the circuit is protected by a 15 amp circuit breaker, but one should always check the local electrical codes as there are regional differences from the NEC in various localities and so the electrician should always consult the local Authority having jurisdiction. It is important to realize that the job of a circuit breaker is to protect the wiring being supplied by said breaker. As an example of the critical nature of this we should understand that if we used a 20 amp breaker on a circuit wired in #14 gauge wire then the wire would not be protected from overcurrent. If we plugged a device or a number of devices up to 2.4Kw (2400 watts) into this outlet then the current draw would be 20 amps. The circuit breaker would not trip as the current was not exceeding its capacity but the wiring would be carrying 5 amps over its rated capacity and would likely melt and become a fire hazard even though the breaker never saw a problem.

This is also the risk of using 15 amp rated extension chords on circuits protected by 20 amp circuit breakers. The electrician needs to know with any size wire what its current carrying capacity is, and what load the circuit will be expected to carry. In the case of #12 wire its rated capacity is 20 amps. The primary purpose of the circuit breaker in any circuit is to protect the wiring from overcurrent damage and fire risk so it is of critical importance that the electrician be aware of the ampacity capabilities of any electrical wiring that he may install.

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