Resistive loads could be described as static, which is to say that they stay the same throughout the operational cycle. The best example of this is a heater. The heater is either off or runs at 100° of its rating. Loads like this are easy to manage and wire. The circuit simply has to provide enough capacity to supply the needs of the load.
Motors and transformers are examples of inductive loads and these represent most of the loads in the average shop. The electrical characteristics of this type of load are constantly changing during use, and must be approached somewhat differently. When a motor is started, the in-rush current is approximately six times the full load amperage on the nameplate, and each element of the circuit must be properly rated in order to deal with this. When you start a motor, there is no resistance in the winding of the motor; essentially what you are doing is closing a switch to a short circuit. This is why there is a high in-rush current initially. It only lasts for a fraction of a second before the motor starts to spin. As the motor spins, magnetic fields are created that restrict the flow of electricity, and the current drops to a normal level.
When you cut a hard, dense wood on your table saw for example, the motor slows, the magnetic field starts to collapse, and the current is less restricted and starts to climb. To properly protect the windings of the motor, some form of over-current protection is included, either by the motor, or by the equipment manufacturer. This will interrupt the circuit if the current rises to a point where damage to the motor’s windings will occur.
A standard light switch is not the proper choice when switching motor loads, as they are not built to withstand these inrush currents. When the switch interrupts the current to the motor, the collapsing magnetic field will try to maintain the current flow. This causes arcing at the switch that will result in pitting and carbon build-up on the contacts and lead to heat build-up and unreliable operation.
One possible result is that you will not be able to turn the motor off. If you want to control a motor, use a motor-rated switch.
The Canadian Electrical Code (CEC) allows a circuit to be loaded to 80% of the rating of a circuit breaker. A circuit supplied by a fifteen amp breaker, feeding a receptacle rated at 15 amps, wired with #14 AWG (American Wire Gauge) wire is capable of delivering 12 amps to the load. The breaker is designed so that if the load exceeds 12 amps for more than 2 minutes an internal bimetal strip heats up and interrupts the circuit, tripping the breaker. This can also happen if the in-rush current for the motor is sustained too long. An example of this is if you try to start a machine under load and it has trouble coming up to speed quickly.
A breaker that has tripped many times will be less stable and more likely to trip when faced with current close to its rating. Also, breakers that have been used to switch loads on and off will face reliability problems unless they are marked for such use. If a circuit is already supplying some receptacles and lights, then the addition of a power tool to that circuit may push it over the limit, so it is a good idea to have dedicated circuits for the major equipment in your shop.
Placing dedicated receptacles close to each tool helps eliminate extension cords that can pose a tripping hazard. If you must use extension cords with your equipment, always use the largest size cord you can. Any extension cord for shop or construction use should be a minimum of #14 AWG.
If your cord is more than 80′ long, use a #12 AWG cord to prevent under voltage damage to the motor. A general rule of thumb is that to maintain the correct voltage drop at the load, you should increase one AWG size for every one hundred feet of distance. After spending a lot of money on tools, it makes sense to invest in some high quality extension cords to keep them in top form.
In the next article, we will look at some specific wiring situations in a workshop that will increase your productivity, enjoyment, and safety.