Breakers, Wires, Fuses, and Overloads
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In this class, Bryan teaches the Kalos techs about breakers, wires, fuses, and overloads. The practices associated with these components can be a bit controversial in the HVAC industry, so we look at these through the lens of NFPA 70 (National Electrical Code/NEC).
The wire sizing guide can be found in NEC 310.15 Table B-16, and NEC 440 is just for air conditioning. Most of our disagreements with electricians come from the separation of these two sections of the code, as electricians may not pay attention to NEC 440. (We also can’t agree on whether the circuit breaker’s main purpose is to protect the conductor at all costs or not.)
A circuit breaker allows you to close a circuit manually; it is resettable. A circuit breaker has three settings: on, tripped, and off. A breaker that is ON will have its lever at the very top. An OFF breaker will have its lever at the very bottom. A tripped breaker will have its lever in the middle, so you can tell the difference between an off breaker and a tripped breaker. A breaker may trip due to excess current in the case of a short. If the current is only slightly over the breaker’s rating, the breaker may take a long time to trip. Arc fault breakers can get quite hot, but their high temperature doesn’t mean that they’re malfunctioning.
Fuses come in many varieties, but each one has a current rating (and usually a voltage rating). Amperage will blow a fuse, but excess voltage could potentially cause arcing. Some fuses will blow slowly, and others will blow quickly, but the amount of overcurrent also dictates how quickly a fuse will blow. Fuses must be able to withstand a motor’s higher current upon startup, so we typically prefer slow-blow fuses for those. Fast-blow fuses need to protect electronics.
Overload means that you’ve placed too much load on something. So, an overload is a device that breaks a circuit when there is too much load. When a motor’s physical resistance increases, its electrical resistance decreases. The electrical resistance decreases because the motor doesn’t spin as quickly and produces less back EMF.
Your compressor can overload after coming out of defrost due to high suction pressure. The refrigerant is very dense in the suction line, so the compressor current must increase to deal with the load.
A ground fault is an important electrical term; it is the momentary connection between a current and ground. A ground fault is similar to a short, and you can prevent a ground fault with a GFCI (ground fault circuit interrupter). Condensers will soon need GFCIs according to code.
Overcurrent protection prevents excessive current; these components typically guard against instantaneous spikes of current. A circuit breaker is an example of overcurrent protection.
Overload protection protects a system against a running overcurrent that causes overheating; these components typically protect from electrical problems and other thermal overload issues.
A conductor is basically a wire. When sizing a conductor, we must consider circuit amperage, ambient temperature, insulation temperature rating, box/conduit fill, and voltage drop. The voltage drop must merely be acceptable; the NEC doesn’t require specific voltage drop parameters for sizing, for the most part. (Conductor length affects voltage drop, and you want to measure voltage under load.) Luckily, many manufacturers make it easy to access the data you need for proper conductor sizing on their products.
Information on conductor temperature ratings can be found in NEC Table 310.104(A). (Be sure to check if you’re using copper, aluminum, or copper-plated aluminum.) We often use the left column on that table. Tightened connectors also have torque ratings that you must account for, not just ambient temperature. Also, make sure your termination points can accept the wires you plan to use. You can’t connect Romex to an outdoor coil because it would operate under damp conditions, which is a code violation.
The insulation rating is just as important as sizing. Insulation doesn’t affect voltage drop, but more insulation will help prevent overheating. Voltage drop also does NOT indicate a hot circuit.
Starting current is when the voltage is first applied to the windings, so there is no back EMF; starting current is always HIGH at first. Running current is the current level when the motor is up to speed (and back EMF is in play, providing resistance). Locked rotor amps (LRA) indicate the current that a motor will draw if the motor stays locked (5-6x running amps).
Remember, mechanical problems often cause electrical problems. Make sure you give your connections good metal-to-metal contact. Also, only check connections with thermal imaging when the system is under load.
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