Ch 28 Overcurrent Protection-Fuses and Circuit Breakers

When to use 2-pole circuit breaker

- On 3-wire 120/240-volt branch circuits (two "hots" and one neutral) for loads such as electric ranges, electric ovens, and electric clothes dryers. - For straight 240-volt branch circuits (two "hots") for such loads as electric water heaters, electric furnaces, electric baseboard heaters, electric AC, and heat pumps.

NEC 230.79(C)

100 amperes is the minimum size service requirement for one-family dwelling

What is a tap conductor?

A conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in NEC 240.4

Cartridge fuses

Are available with the same three basic types of time-current characteristics as plug fuses. They are available as 250-volt and 600-volt ratings with ampere ratings from 0 to 600.

What article does the NEC cover overcorrect protection of conductors?

Article 240

How are fuses and circuit breakers sized?

By matching their ampere ratings to conductor ampacities and connected load currents.

Time-delay loaded link fuse

Has one link (fusible element) that is "loaded" with a heat sink next to the "necked down" portion. This "load" absorbs a considerable amount of heat before the "necked down" portion melts open. This heat sinks provides a time delay.

Non-time delay fuse

Has one link (fusible element). One part of the link is "necked down" so when excessive current flows it will open in the weakest part of the link - the necked down portion. They're not the best for motor circuits due to the high starting inrush current of motors.

Time-delay dual-element fuse

One fuse element opens quickly when a short circuit, heavy overload, or ground fault occurs. The other element in series opens slowly on overload conditions. Excellent choice for motor circuits because they will not open needlessly on momentary overloads.

NEC 240.24(F)

Overcurrent devices are not permitted to be over steps of a stairway

NEC 240.22

Overcurrent protection devices are generally not permitted in the grounded conductor. Exceptions are: (1) if the overcurrent protection device opens all conductors of the circuit at the same time, and (2) if the overcurrent device is used for motor overload protection

NEC 240.24(A)

Overcurrent protective devices are required to be accessible

What is time-current?

Refers to how long it will take a fuse or circuit breaker to open under different current values

NEC 230.70(A)(2)

Service disconnects are not permitted to be located in bathrooms

NEC 210.24 Summary of Branch-Circuit Requirements

This table shows the maximum overcorrect protection for branch-circuit conductors

What is selective coordination?

Under overload, short-circuit, or ground-fault conditions, only the overcurrent device near the fault opens. The main fuses or circuit breaker remain closed.

What is nonselective coordination?

Under overload, short-circuit, or ground-fault conditions, the branch breaker or fuses might open, the main breaker or fuses might open, or both might open.

Ground fault circuit condition

When an ungrounded (hot) conductor or ungrounded conductor comes in contact with a grounded surface, such as a grounded metal raceway, metal water pipe, sheet metal, etc. The current flows outside the "intended path."

Overload circuit condition

When the current flowing is more than the circuit and/or connected equipment is designed to safely carry. The current still flows in the "intended path.

Normal circuit condition

When the current flowing is within the capability of the circuit and/or connected equipment

Short circuit condition

When two or more normally insulated circuit conductors come in contact with one another, resulting in a current flow that bypasses the connected load. The current flows outside the "intended path."

NEC 240.4(B)

If the standard ampere rating of fuses or circuit breaker does not match the ampacity of the conductor, the next higher ampere rating is permitted. This is permitted if the branch circuit does not supply more than one receptacle for cord-and-plug connected portable loads and for overcurrent devices not exceeding 800 amperes

NEC 240.21

Overcurrent protection is generally required at the point where the conductor receives its supply. The tap rules act like exceptions to the general requirement.

NEC 240.20(A)

Overcurrent protection is required to be provided for each ungrounded (hot) conductor

NEC 240.4

Overcurrent protection is sized according to the ampacity of a conductor

NEC 240.24(E)

Overcurrent protective devices are not permitted located in bathrooms, unless used for supplementary overcurrent protection

NEC 240.24(D)

Overcurrent protective devices are not permitted to be near easily ignitable material, such as in clothes closets

NEC 230.90(A)

Overload protection is required for each ungrounded (hot) service-entrance conductor. This is accomplished by the overcurrent protective device(s) in the service-entrance main panelboard

Plug fuses 15 amperes and less will have what shape?

Plug fuses 15 amperes and less will have a hexagon shape in the window or the fuse body of the fuse will have the hexagon shape

How Much Load Can a Circuit Breaker Safely Carry?

The NEC permits 100% continuous loading if the device is listed for 100% loading. At the moment no device is listed at 100% continuous loading. A maximum of 80% continuous loading is good practice - less is even better.

What is a 22/10 series rated panelboard?

The main circuit breaker has an interrupting rating of 22,000 amperes and the branch circuit breakers have an interrupting rating of 10,000 amperes. This is adequate for typical residential main panelboards.

NEC 230.91

The main service overcurrent device is usually an integral part of the service disconnecting means. For large services, the service disconnecting means could be a separate disconnect switch.

NEC 240.4(D)

The overcurrent protection of small conductors

NEC 240.6(A)

The standard ampere rating for fuses and breakers

NEC 310.15(B)(7)

This section allows the ampacity of the service-entrance conductors and specific feeders for 3-wire, single-phase, 120/240-volt or 120/208-volt (two ungrounded legs and the neutral of a 208Y/120-volt, 3-phase service) supplies for dwellings to be rated 83% of the overcurrent device. This rule applies to services and specific feeders rated 100 through 400 amperes.

Open circuit condition

Where the circuit is not closed somewhere in the circuit and no current can flow

NEC 240.21(B) "tap rules"

− Taps Not Over 3 m (10 ft.) Long - Conductor must be rated minimum 10% of the ampacity of the feeder conductor − Taps Not Over 7.5 m (25 ft.) Long - Conductor must be rated a minimum of 33% of the ampacity of a feeder conductor − Taps Supplying a Transformer [Primary Plus Secondary Not over 7.5 m (25 ft.) Long] − Taps over 7.5 m (25 ft.) Long − Outside Taps of Unlimited Length