Sizing Short-Circuit & Ground-Fault for Motors
Electrical contractors tend to have fewer insurance claims compared with other contractors. This is because every circuit that an electrician installs has protection designed to prevent accidents and damage. If a loose conductor falls out of its terminal while energized and hits the frame or enclosure of a piece of equipment, there will be a ground-fault and a breaker will trip. This keeps the metal parts of the equipment from becoming energized. If two energized conductors of different phases, or an energized conductor and neutral conductor, somehow make contact with one another it creates a short-circuit; this will also trip a breaker. Because of the difference of potential between the two conductors, completing the circuit before the load causes an increase in amperage. This increase is what trips the breaker. Similarly, if a piece of equipment pulls more amps than the breaker is rated for, it overloads and causes breaker trips. This overload protection is there to protect the conductors from melting due to an ampacity that is too high.
I’m sure every plumber in the world would love it if the water turned off before a house or building got flooded. Or, if somehow a roofer could have some sort of protection that stopped a leak before it could cause damage. Well, I guess electricians can call themselves lucky! Our homes and businesses are safer because of overload, ground-fault and short-circuit protection.
In the National Electrical Code, Section 210.18 states that circuit rating is done based off of the overcurrent protection device (breaker or fuse), and not the conductors. However, this is not true for all types of loads or circuits. Loads or circuits such as motors, air-conditioning, electric welders, and others are sized differently. You can find a list of these different circuits in Table 240.4(G) of the National Electrical Code book. In this article we will be going over how to properly protect a motor: its short-circuit and ground-fault protection.
Motor protection is different from other types of circuits because of what a motor is and how it works. Motors are made with coils, and operate primarily off of electrical induction. Motors can also have a lot of torque and get moving very fast, depending on the motor and the application that it is used for. Because of this, the coils in the motor can become hot. In order to protect the coils and the motor, overload protection is separate from the ground-fault and short-circuit protection.
One of the differences of a motor from other types of loads is what is called ‘inrush current’, or locked rotor, which is a spike in current that happens during the start-up of the motor. This inrush current can be up to six times higher than the motor’s operating current. Since a protection device opens when current gets higher than the rating of the device, we can’t size the protection off of the operating current — it has to be able to handle the inrush current as well. Accordingly, Section 430.52(B) says that the branch-circuit, short-circuit and ground-fault protective device must be able to carry the starting current. It sounds obvious, but motors are such a huge part of the world we live in that it needs to be explicitly stated in the code book. This is so that electricians and technicians everywhere will understand the importance of sizing the overcurrent protection of a motor correctly.
Short-circuit and ground-fault protection can be established with a breaker or a fuse. It really depends on the motor and the application in which it is being used. Table 430.52 is where we go to size the short-circuit and ground-fault protection devices. But first, we need to go to the tables found at the end of Article 430. Table 430.248 is for single- phase motors, Table 430.249 is for two-phase motors (not a very common system) and Table 430.250 is for three-phase motors. These tables contain the FLC (full load current) for each of these types of motor, and that is what we use to size the short-circuit and ground-fault protection. Use information found on the nameplate of the motor with this table.
For example, if we look at the nameplate of a motor and see that it says three-phase, then we go to Table 430.250. Then we look to see what voltage it is, and the horse power. Lets say its a five horse power, 208 volt, three-phase motor. The FLC found in Table 430.250 is 16.7 amps, so we take that 16.7 amps over to Table 430.52. We will see multiple types of short-circuit and ground-fault protection devices on this table: non-time delay fuses, dual element fuses, instantaneous trip breakers, and inverse time breakers. On the left hand side of the table there are different types of motors. There are different percentages for each of these types of devices that you would multiply your FLC by, depending on what fuse or breaker you decide to use.
Let’s use an inverse time breaker. The highest percentage on the table that we can multiply 16.7 amps by is 250 percent, for an AC polyphase motor. This ends up at 41.75 amps. But there is not a breaker rating at that amperage. Section 430.52(C)(1) Ex(1) says that we are allowed to go to the next size up. Table 240.6 gives us the standard rating for overcurrent protection devices. In this case 45 amps is the next size up, so we can protect this motor from a short-circuit or ground-fault with a 45 amp breaker even if the motor only pulls 16.7 amps.
This might be hard for you or anyone else to comprehend. But if you take the time to understand that the motor is protected from pulling too many amps, and all we are doing with this protection is guarding against short-circuit and ground fault, then it might start making sense. Also know that the the values given in Table 430.52 are maximums, and you can go smaller with this protection as long as the motor starts. The reason we can go bigger is because of the locked rotor, or inrush current, but we have still protected the motor with separate overload protection.
I hope this helps with any questions you have or had with motor short-circuit and ground-fault protection.