Electrical Systems Design-Conductors and Overcurrent Protection

Name the classifications for electrical conductors.

Bare, covered, and insulated.

Define a bare conductor.

In a bare conductor, there is no outer covering or insulation.

Define a covered conductor.

A covered conductor has material with no special insulation rating covering it.

What is an insulated conductor?

In an insulated conductor, the material covering the conductor is recognized by the NEC as an insulating material. 

What are materials that can be used for electrical conductors?

Copper, aluminum, and copper-clad aluminum

Name two types of conductors.

Solid and stranded

Name four uses for electrical conductors.

EGC, GEC, grounded conductor under some conditions, and ungrounded conductor

What’s a common characteristic of all types of electrical insulation?

They provide an opposition or resistance to the flow of electricity.

What information must the markings on conductors and cable include?

It is required that all conductors and cable be marked to identify the maximum rated voltage at which the conductor is listed, the letter or letters necessary to identify the type of wire or cable, the manufacturer’s name or trademark or other identifiable markings, and the AWG or circular-mil area size of the conductor.

What contributes to insulation degradation?

These factors are listed in Informational Note 1 to 310.14(A)(3). They are the ambient temperature in which the conductor insulation shall operate, the internal heat created in the conductor as a result of current flow, the dissipation rate of the heat into the surrounding environment, and the heat generated by adjacent current-carrying conductors. All these factors must be considered before we select the type of insulation and determine a conductor’s allowable ampacity.

What does Section 110.5 require?

Section 110.5 requires that all conductors used to carry current be copper, aluminum, or copper-clad aluminum, unless otherwise specified by the NEC®.

What is a primary disadvantage of copper?

Its cost.

True/false: copper conductors installed in vertical raceways do not require support at shorter intervals than that required for aluminum conductors.

False

Which section lists the required intervals and methods for vertical supports to ensure that the weight of the conductors isn’t passed on to the conductor terminations?

Section 300.19(A)

How can we directly splice aluminum to copper?

There are listed twist-on wire connectors for this purpose.

How should we not terminate conductors with dissimilar materials?

. Conductors of different materials, like copper and aluminum, should not be terminated in a manner that causes the dissimilar metals to come in direct contact with each other unless the termination or splicing device is identified for the purpose and conditions of use.

How are conductors designed for direct physical contact between aluminum and copper labeled?

AL-CU

Where can we use conductors designed for direct physical contact between aluminum and copper?

Only in dry locations

What sections reference the color code requirements for grounded, equipment grounding, and ungrounded conductors?

310.6. Subpart (A) references 200.6, containing the identification requirements for grounding conductors. Subpart (B) covers the identification requirements for the EGC located in 250.119. 310.6(C) lists the identification requirements for ungrounded conductors. 

What colors are permissible for the identification of branch AC conductors?

Any color other than white, gray, green, or any combination thereof with distinguishing markings.

Where do we find identification information for branch circuits?

210.5(C)(1) and (2)

Where do we find identification information for feeders?

215.12(C)(1) and (2)

Where else do we put identification information for color-coded ungrounded conductors?

We put them at the branch circuit panelboard.

When must we identify branch-circuit and feeder conductors?

When more than one nominal voltage system supplies a premises

How do we identify conductors in existing installations when we install a different voltage?

Only the new system conductors need identification, but we must place labels stating that only one voltage system has been marked at each voltage system distribution equipment. 

How shall we identify conductors where AC branch circuits are supplied from more than one nominal voltage system?

Each ungrounded conductor shall be identified by phase and system at all terminations, connections, and splice points. Each identification means must be readily available and posted using a durable label at each panelboard or branch circuit distribution equipment. Typically, brown, orange, and yellow are used as color identification for 480Y/277 V ungrounded conductors and black, red, and blue are used for 120/240 V and 208Y/120 V ungrounded conductors.

What does Section 250.119(A) permit?

Section 250.119(A) permits the reidentification of EGCs, 4 AWG and larger, at the time of installation by permanent means such as stripping the insulation or covering from the exposed conductor, coloring the exposed insulation or covering green, or marking the exposed covering or insulation with green tape or green labels.

What does Section 250.119(B) permit?

Section 250.119(B) permits similar methods of reidentification for multiconductor cable, provided the conditions of maintenance ensure that only qualified personnel service the installation.

What are parallel conductors?

Parallel conductors are two or more conductors that are electrically connected at both ends to form a single conductor.

What kinds of conductors can be installed in parallel?

Copper, aluminum, and copper-clad aluminum can be installed in parallel.

What are the conditions required for parallel conductors?

The conductors must be of the same length, the same material, the same size, the same insulation type, and have the same termination per 310.10(G)((2).

How do we derate parallel conductors?

We derate them per Table 310.15(C)(1). EGCs have to comply with 310.10(G)(5) where installed in parallel, and they must be sized per 250.122.

What does Section 310.10(G)(6) require?

It requires that parallel bonding jumpers or supply-side bonding jumpers installed in raceways be sized in accordance with 250.102.

What information does Table 310.16 contain?

It lists conductor ampacities for insulated conductors under 2000 V, installed in a raceway, cable, or the earth. With these values, it is assumed that the ambient temperature is 30°C (86°F) and not more than three current-carrying conductors are in a raceway or cable.

What are the three basic classifications of insulation?

60 degrees Celsius (140 degrees Fahrenheit), 75 degrees Celsius (167 degrees Fahrenheit), and 90 degrees Celsius (194 degrees Fahrenheit)

What is an overcurrent?

An overcurrent is any amount of current flow in excess of that for which a conductor or equipment is rated.

Can any equipment handle overcurrents?

Some equipment is designed to accommodate overcurrent for a certain time period depending upon the load. Temporary surge or start-up overcurrents are common for some electrical equipment, such as motors, and generally have no long-term, harmful effects. When overcurrents occur more frequently or persist for longer periods of time, damage to the electrical system may occur.

What causes overcurrents?

Overcurrent can result from overloads, short circuits, or ground faults.

When do we select overcurrent protection?

We select overcurrent protection once the allowable ampacity is determined.

What do we do when the calculated ampacity doesn’t correspond to a standard-size fuse or circuit breaker?

The next higher standard device rating may be used in these cases per 240.4(B), provided the conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug connected loads and the next higher standard device rating does not exceed 800 A. In cases where the cord-and-plug-connected portable loads are supplied by multioutlet receptacle circuits, and the ampacity of the conductor does not correspond to a standard rating for a fuse or circuit breaker, the next lower standard device rating shall be selected.

What do we do when the calculated conductor ampacity doesn’t correspond to a standard size rating for fuses and circuit breakers and the rating of the overcurrent device is over 800A?

The next lower standard-size fuses or circuit breakers shall be selected per 240.4(C).

What does the grounded conductor do in most systems?

In the majority of electrical systems, the grounded conductor is the neutral for the system and carries the return circuit current.

When do we give grounded conductors overcurrent protection?

We don’t usually give them overcurrent protections, but there are two exceptions. The first is for installations in which all of the circuit conductors are opened or disconnected from the supply simultaneously and the design is such that no pole can operate independently of another. The second is for motor overload protection as per 430.36 or 430.37. Some systems, like a 3-phase, 3-wire, corner-grounded delta, operate with a grounded conductor that’s a normal current-carrying conductor. 

What does the 10’ tap rule do?

The 10′ tap rule permits tap conductors to be supplied from a feeder, without overcurrent protection at the point where they receive their supply, provided five conditions are met.

What lays out the five conditions allowing the 10’ tap rule to apply?

Section 240.21(B)(1) sets these conditions.

What is the 25’ tap rule?

The 25′ tap rule permits tap conductors to be supplied from a feeder, without overcurrent protection at the point where they receive their supply, provided four conditions are met.

What is the major advantage of the 25’ tap rule over the 10’ tap rule?

The additional length that the conductors can run without overcurrent protection.

What are the 5 conditions allowing the 10’ tap rule?

The tap conductors cannot be longer than 10’. The tap conductor’s ampacity cannot be less than the combined load and cannot be less than the rating of equipment containing an ocpd supplied by the tap conductors or OCPD at termination of the tap conductors. The tap conductors cannot extend beyond the panelboard. The tap conductors must be protected in the raceway. The line-side OCPD cannot exceed 10x the tap conductor’s ampacity.

What are the 4 conditions allowing use of the 25’ tap rule?

The tap conductors cannot be longer than 25 feet. The tap conductors’ ampacity cannot be less than 1/8 of the rating of the OCPD protecting the feeders. The tap conductors must terminate in a single OCPD which limits the load to the ampacity of the tap conductors. The tap conductors must be protected from physical damage.

What does the 25’ transformer tap rule do?

The 25′ transformer tap rule permits tap conductors to be installed without overcurrent protection at the point where they receive their supply, provided five conditions are met.

What are the conditions under which the 25’ transformer tap rule can apply?

The primary conductor’s ampacity must be at least 1/3 of the rating of the feeder OCPD. The ampacity of the secondary conductors x the ratio of primary-to-secondary voltage is at least 1/3 the rating of the feeder OCPD. The primary and secondary are limited to a 25’ total length. The tap conductors must be protected from physical damage. Secondary conductors must terminate in a single OCPD which limits the load to the ampacity of the conductors.  

What does the over 25’ tap rule permit?

The over 25′ tap rule permits tap conductors to be supplied from a feeder, without overcurrent protection at the point where they receive their supply, provided nine conditions are met.

Where are the conditions for the over 25’ tap rule laid out?

240.21(B)(4)

What does the outside feeder tap rule do?

The outside feeder tap rule permits tap conductors to be supplied from a feeder, without overcurrent protection at the point where they receive their supply, provided five conditions are met.

What are the conditions for the outside feeder tap rule?

The tap connectors are installed outdoors except where terminated. The conductors are protected from physical damage. The tap conductors terminate in a single OCPD which limits the load to the ampacity of the conductors. The OCPD is an integral part of disconnecting means or located adjacent. The disconnecting means is in a readily accessible location near a point of entry.

Where do we put OCPDs?

Circuit breakers and switches containing fuses shall be installed in a readily accessible location, free from physical damage, and in a location that ensures that all occupants of a building or structure have access to the overcurrent devices protecting the supply conductors for their premises. They shall not be installed near easily ignitable materials or in bathrooms. We also don’t put them in clothes closets.

What OCPD protection is required for ungrounded conductors?

A fuse or circuit breaker is installed in every ungrounded conductor.

What OCPD is required for low-voltage, 1-phase circuits of 120V or less and DC circuits?

One OCPD is required for such circuits.

What OCPD protection does the neutral conductor in AC circuits provide?

These do not require an OCPD.

What OCPD protection does the negative polarity in DC circuits require?

They do not require an OCPD. 

What kind of OCPD protection is required for 1-phase, high-voltage circuits (208V, 230V, 240V)

2 OCPDs are required for these.

What kind of OCPDs are required for 3-phase?

3 OCPDs are required for all 3-phase circuits, regardless of voltage. All three ungrounded conductors must be protected by an OCPD.

Where should plug fuses not be used?

Plug fuses shall not be installed in circuits operating at over 150 V to ground.

How do we identify a plug fuse’s ampere rating?

The plug fuse ampere rating is identified by the shape of the window on the fuse. Plug fuses with ampere ratings of 15A or less have a hexagonal window. Plug fuses with ratings greater than 15A have a round window.

What are the two basic types of plug fuse?

Edison-base fuse and Type S fuse

What are the configurations for cartridge fuses?

They’re designed with either a ferrule or knife-blade configuration. 

How do cartridge fuses work?

They’re inserted into fuseholders, which make the connection from the line to the load.

How do non-time delay fuses work?

Non-time delay fuses (NTDFs) contain a fusible link that melts and opens a circuit at a set overcurrent.

How do time-delay fuses work?

Time delay fuses (TDFs) can detect and remove a short circuit almost instantly, but they allow small overloads to exist for a short period.

When do we use time-delay fuses?

Time delay fuses are designed to be used with utilization equipment, like motors, that are subject to temporary start-up or surge currents. Time delay fuses allow overcurrents to exist for short periods of time to avoid nuisance tripping due to equipment start-up characteristics.

How do thermal-magnetic breakers work?

Thermal-magnetic CBs contain both a bimetal strip and a magnetic element to sense overloads and faults. Thermal CBs contain a spring-loaded electrical contact which opens the circuit. The spring is used to open and close the contacts with a fast snap action. A handle is added to the contact assembly so the contacts may be manually opened and closed. The contacts are automatically opened on an overcurrent by a bimetal strip and/or an electromagnetic tripping device. The contacts have one stationary contact and one movable contact. The movable contact is attached to a spring that provides a fast snap action when tripped.

What configurations do inverse-time CBs come in? 

Single-, two-, and three-pole configurations

How shall circuit breakers be marked?

They need to be marked to show if they’re in the open position (OFF) or the closed position (ON). On vertically mounted switches, the ON position shall be up. They also need to indicate the ampere rating, the interrupting rating, and the voltage rating of the device. If they’re to be used as switches, they need to be marked indicating that they’re okay for such use.