NEC Article 210: Branch Circuits—Part 2

Jerry WalchStarred Page By Jerry Walch, 28th Feb 2014 | Follow this author | RSS Feed | Short URL http://nut.bz/vq2jmwec/
Posted in Wikinut>Guides>DIY>Electrical

As you will recall from part 1 of this series, the current rating of a general-purpose branch circuit is determined by the rating of the final Overcurrent Protection Device or OCPD. Overcurrent Protection Devices found in the home are of three basic types—Edison Base Fuses, Standard ThermalCircuit Breakers or CBs, Arc Fault Circuit Interrupter CBs, or Ground Fault Current Interrupter CBs.

Edison Base Fuses

The Edison Base Fuse was invented and patented by Thomas Edison in 1890 and can still is found in the fuse panels of older homes today. The Edison Based Fuse incorporates a link of metal that is designed to melt and open the circuit when the current flow through it exceeds it rating. Excess current flow causes the temperature of the strip to exceed its melting point and the strip melts stopping all current flow until the fuse is replaced. The problem with Edison Based Fuses is that many people screwed in a higher amperage fuse to keep them from blowing out repeatedly from a circuit overload condition and that caused the wiring to overheat and a fire often resulted. In addition, even worse danger was people bypassing the fuse altogether by placing a penny underneath them in the socket, leaving the circuit without any overcurrent protection at all.

Standard Circuit Breakers

The standard circuit breaker is nothing more than a thermally operated, spring-loaded switch. The circuit breaker is held in the closed position by a bimetallic strip of metal through which the electric current flows. When the current flowing through the bimetallic strips exceeds the breaker’s amperage rating, the bimetallic strip heats up and bends, releasing the catch and the spring opens the circuit. The heat causes the bimetallic strip to bend because the two different metals sandwiched together expand at different rates. These simple thermal circuit breakers are the ones found in most homes today but are slowly being replace by Ground Fault Current Interrupter (GFCI) CBs and Arc Fault Current Interrupter (AFCI) CBs in newer homes.

GFCI CBs

The GFCI circuit breaker was first introduced in the National Electrical Code in 1968 to protect human being against electrical shocks from ground fault conditions. Regular fuses and circuit breakers are designed to protect equipment and property against circuit overloads and short circuit and not to protect people against electrical shocks. Ground faults occur when there is a break in the neutral conductor and the human body becomes the return path to ground for the electric current. Ground Fault Current Interrupters operate by sensing the difference in current flow between the hot wire and the neutral wire, when they are not equal, a solenoid trips open the circuit. The GFCI breaker also contains a thermal trip mechanism that protects against circuit overloads and short circuits.

Arc Fault Current Interrupters

An arc fault occurs when a high resistance short occurs between a hot wire and a neutral wire or between a hot wire and some other earth ground. The resistance is high enough to keep it from appearing as a short circuit and tripping a standard breaker open but the heat generated by the arc can set the surrounding structural material on fire. Since most arc faults occurs inside the walls of a residence, they go undetected until after a fire occurs. Arc Fault Current Interrupters contains circuitry similar to that found in a GFCI breaker to detect the fault condition and open the circuit before a fire can start. Today the NEC requires AFCI protection on all circuit terminating in or passing through habitable rooms.

What Breaker to Use for New Circuits?

Depending on where the circuit is being installed and its purpose, the Code requires that you use either a GFCI or an AFCI breaker. If you are replacing a defective standard thermal circuit breaker, you can replace it with a standard breaker if cost is a real issue, but it strongly recommended that you replace it with either a GFCI or an AFCI breaker.
References:
NEC & NEMA Standards
Related Articles:
NEC Article 210: Branch Circuits—Part 1
In part three of this series, I will discuss branch circuits supplying fixed in place appliances.

Tags

Ampacity, Article 210, Branch Circuits, Circuit Breakers, Conductors, Fuses, National Electrical Code, Nec, Ocpd, Overcurrent Protection, Overcurrent Protection Devices

Meet the author

author avatar Jerry Walch
Jerry Walch is a 71 year old freelance writer for hire living in Colorado Springs, Colorado. He has been writing since the late 1970s, and writes for both the print and online media. He specializes in

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Comments

author avatar Songbird B
28th Feb 2014 (#)

Another great Star page, packed with informative and easy to understand electrical advice Jerry..Another must read for the husband too! \0/x

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author avatar Jerry Walch
28th Feb 2014 (#)

Thanks Bev. Stay tuned to the same Bat Channel, many more to come.

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author avatar Fern Mc Costigan
28th Feb 2014 (#)

Interesting post!

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author avatar Jerry Walch
28th Feb 2014 (#)

Thanks Fern.

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