7.3.2: Fuses and Breakers
General
Panelboards and Loadcenters
Panelboard- A single panel that includes automatic overcurrent devices used for the protection of light, heat, and power circuits.
Loadcenter- A type of panelboard that contains the main disconnecting means for the residential service entrance as well as the fuses or circuit breakers used to protect circuits and equipment like water heaters, ranges, dryers, and lighting.
Safety Switches- A safety switch is used as a disconnecting means for larger electrical equipment. It is typically mounted on the surface of or near the equipment and is operated with an external handle. Safety switches can simply be an On/Off device or can have overload protection devices incorporated in their design. Safety switches can be found in both cartridge fuse or breaker configurations.
Fuses
Plug fuses- These fuses “screw” into a socket device, either an Edison base model or a Type S model. These devices are seldom used as circuit breakers can be reset and are considered more reliable and tamper resistant. Some codes may restrict the use of plug fuses in building electrical systems.
Cartridge fuses- Cartridge fuses are available as a ferrule model or a blade-type model. Fuses must be plainly marked, either by printing on the fuse barrel or by a label attached to the barrel showing the amperage and voltage ratings. Often used in equipment safety switches.
Circuit Breakers
Circuit breakers- are available as a single-pole device for 120-volt applications and as a two-pole device for 240-volt applications. They also come as a twin or dual device that fits in the space of a regular single-pole breaker. Circuit breakers are designed so that any fault must be cleared before the circuit breaker can be reset. Even if the handle is held in the “ON” position, the circuit breaker will remain tripped as long as there is a trip-rated fault on the circuit. In some cases, time is required for the breaker to cool before it can be reset.
- Most branch circuits are 120-volt circuits. These are wired with 14 AWG or 12 AWG copper conductors and require 15 or 20 amp single-pole circuit breakers. A single-pole circuit breaker takes up one space on a panelboard.
- Many branch circuits serve appliances like electric water heaters, air conditioners, and electric heating units. These loads require 240 volts to operate properly. since it is a 240-volt circuit, it needs a two-pole circuit breaker. A two-pole circuit breaker takes up two spaces on the panelboard.
It is important to note the manufacturer and style of a breaker when replacing it. Different manufacturers produce propitiatory designs that can only be used in their own panels and are not compatible with others.
240-Volt Branch Circuit Requirements
• 15-amp circuit breaker when wired with 14 AWG wire
• 20-amp circuit breaker when wired with 12 AWG wire
• 30-amp circuit breaker when wired with 10 AWG wire
Appliance Circuits- There may be a need for 120/240 volts to be supplied to appliances such as electric clothes dryers and electric ranges. This installation requires a two-pole circuit breaker, just like the 240-volt-only application. The difference is that a three-wire cable with a grounding conductor is used.
Ground Fault Circuit Interrupter (GFCI)- Although GFCIs look very similar to regular circuit breaker, there are two very evident differences: A GFCI breaker has a white pigtail attached to it that is wired to the neutral bar in the panel; and a GFCI breaker has a “Push to Test” button located on the front. GFCIs are also available as receptacle devices that can be placed in-line with standard breakers.
Arc Fault Circuit Interrupter (AFCI)- AFCI devices are designed to trip when they sense rapid fluctuations in the current flow that are typical of arcing conditions. AFCI protection is provided with AFCI circuit breakers and new codes require that all residences be constructed with them. AFCI circuit breakers look very similar to GFCI circuit breakers. The “Push-to-Test” button is typically a different color than that of a GFCI breaker.
Common Branch Circuits
General Branch Circuits
- 14 AWG copper conductor and protected with a 15-ampere fuse or circuit breaker.
- 12 AWG copper conductor protected with a 20-ampere fuse or circuit breaker.
Small Appliance Branch- 12 AWG copper conductors. Larger size wire may be used to compensate for voltage drop when the distance back to the electrical panel is very long.
- Washer- 120 Volt 20 Amp
- Garbage Disposal- 120 Volt 15 Amp
- Dishwasher- 120 Volt 15 Amp
Range Branch- Uses an 8/3 copper cable with ground protected by a 40-ampere circuit breaker, or a 6/3 copper cable with ground protected by a 50-ampere circuit breaker.
Clothes Dryer Branch- Usually a 30-amp circuit wired with 10/3 cable. Usually connected to the electrical system in a house through a cord-and-plug type connection.
Water Heater Branch- Electric water heaters used in homes normally operate on 240 volts. They normally require a 10 AWG conductor with a 30-ampere overcurrent protection device. Some smaller single element electric water heaters may require 120 volts and will be wired with a dedicated branch circuit with 12 AWG conductors and a 20-ampere overcurrent protection device.
Circuit Breaker Replacement
Always turn off electrical power at the main service breaker when working in an energized main breaker panel.
• The LOAD side of the panel will be disconnected, but the LINE side will still be energized.
• If you are working in an energized subpanel, find the circuit breaker in the service panel, turn it off, and lock it in the OFF position.
Test the panel you are working on with a voltage tester to verify that the electrical power is off.
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NEVER assume the panel is de-energized
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Circuit breakers are installed by attaching them to the main bus bar assembly in the panel. The bus bar assembly is connected to the incoming service entrance conductors and distributes the electrical power to each of the circuit breakers located in the panel. In the case of a subpanels, bus bars are connected to the incoming feeder conductors.
Circuit breakers are attached to the bus bar by contacts in the breakers being snapped onto the bus bar at specific locations, commonly called stabs.
• A single-pole circuit breaker has one stab contact.
• A two-pole circuit breaker has two stab contacts.