By Mike Holt for EC&M Magazine

Try your best to answer these questions without looking at the answers. You are welcome to use these questions as a quiz. Feel free to print and email to your peers.

Q1. What are the rules for sizing service and feeder conductors to a single dwelling unit?

A1. For one-family dwellings and individual dwelling units of two-family and multifamily dwellings, service and feeder conductors supplied by a single phase, 120/240V system can be sized using 310.15(B)(7)(1) through (4).

(1) Service conductors supplying the entire load of a one-family dwelling or an individual dwelling unit in a two-family or multifamily dwelling can have an ampacity of 83 percent of the service rating.

 

Question: What size service conductors are required if the calculated load for a dwelling unit equals 195A, and the service disconnect is rated 200A?

(a) 1/0 AWG     (b) 2/0 AWG     (c) 3/0 AWG     (d) 4/0 AWG

Answer: (b) 2/0 AWG

Service Conductor: 2/0 AWG rated 175A at 75°C [Table 310.15(B)(16)] 
(200A rated circuit breaker multiplied by 83% =166A).

 

Author’s Comment:

310.15(B)(7) can’t be used for service conductors for two-family or multifamily dwelling buildings.

(2) For a feeder rated 100A through 400A, feeder conductors supplying a one-family dwelling, or an individual dwelling unit in a two-family or multifamily dwelling, can have an ampacity of 83 percent of the feeder rating, but only if the feeder supplies the entire load of the dwelling.

 

Author’s Comment:

310.15(B)(7)(2) can’t be used to size feeder conductors where a feeder doesn’t carry the entire load of the dwelling unit, except as permitted in 310.15(B)(7)(3).

Question: What size feeder conductors are required if the calculated load for a dwelling unit equals 195A, the service disconnect is rated 200A, and the feeder conductors carry the entire load of the dwelling unit?

(a) 1/0 AWG     (b) 2/0 AWG     (c) 3/0 AWG     (d) 4/0 AWG

Answer: (b) 2/0 AWG

Feeder Conductor: 2/0 AWG rated 175A at 75°C [Table 310.15(B)(16)] (200A rated circuit breaker multiplied by 83% =166A).

 

WARNING: 310.15(B)(7) doesn’t apply to 3-wire service or feeder conductors connected to a three-phase, 120/208V system, because the neutral conductor in these systems always carries neutral current, even when the load on the phases is balanced [310.15(B)(5)(b)]. For more information on this topic, see 220.61(C)(1).

(3) Feeders for an individual dwelling unit are never required to be larger than the conductors in 310.15(B)(7)(1) or (2).

(4) Neutral conductors are sized using 220.61 and 230.42 for services and 220.61 and 215.2 for feeders.

 

CAUTION: Because the service neutral conductor is required to serve as the effective ground-fault current path, it must be sized so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the neutral conductor in accordance with Table 250.102(C), based on the area of the largest ungrounded service conductor [250.24(C)(1)].

 

Question: What size neutral conductor is required if the calculated load for a dwelling unit equals 195A, the maximum unbalanced load is 100A, and the service disconnect is rated 200A with 2/0 AWG conductors?

(a) 6 AWG     (b) 3 AWG     (c) 2 AWG     (d) 1 AWG

Answer: (b) 3 AWG

Neutral Conductor: 3 AWG is rated 100A at 75°C in accordance with Table 310.15(B)(16), and 310.15(B)(7)(3) doesn’t allow for the 83 percent deduction for neutral conductors. In addition, 250.24(C) requires the neutral conductor to be sized no smaller than 4 AWG based on 2/0 AWG service conductors in accordance with Table 250.102(C).

Q2. What are the requirements for a building service disconnecting means?

A2. The service disconnecting means must open all service-entrance conductors from the building premises wiring [230.70].

The service disconnecting means must be placed at a readily accessible location either outside the building, or inside nearest the point of service conductor entry [230.70(A)(1)].

 

WARNING: Because service-entrance conductors don’t have short-circuit or ground-fault protection, they must be limited in length when installed inside a building. Some local jurisdictions have a specific requirement as to the maximum length permitted within a building.

The service disconnecting means isn’t permitted to be installed in a bathroom [230.70(A)(2)].

 

Author’s Comment:

Overcurrent devices must not be located in the bathrooms of dwelling units, or guest rooms or guest suites of hotels or motels [240.24(E)].

If a remote-control device (such as a pushbutton for a shunt-trip breaker) is used to actuate the service disconnecting means, the service disconnecting means must still be at a readily accessible location either outside the building, or nearest the point of entry of the service conductors as required by 230.70(A)(1) [230.70(A)(3)].

 

Author’s Comment:

See the definition of “Remote Control” in Article 100.

The service disconnecting means must consist of a manually operated switch, a power-operated switch, or a circuit breaker that’s also capable of being operated manually [230.76].

Each service disconnecting means must be permanently marked to identify it as part of the service disconnecting means [230.70(B)].

 

Author’s Comment:

When a building has multiple services and/or feeders, a plaque is required at each service or feeder disconnect location to show the location of the other service or feeder disconnect locations. See 230.2(E).

Each service disconnecting means must be suitable for the prevailing conditions [230.70(C)].

There must be no more than six service disconnects for each service permitted by 230.2, or each set of service-entrance conductors permitted by 230.40 Ex 1, 3, 4, or 5. [230.71(A)].

The service disconnecting means can consist of up to six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard, or in switchgear.

 

CAUTION: The rule is six disconnecting means for each service permitted, not for each building. If the building has two services, then there can be a total of 12 service disconnects (six disconnects per service).

The up to six service disconnecting means for each service must be grouped [230.72(A)]. .

To minimize the possibility of simultaneous interruption of power, the disconnecting means for fire pumps [Article 695], emergency [Article 700], legally required standby [Article 701], or optional standby [Article 702] systems must be located remote from the one to six service disconnects for normal service [230.72(B)]. .

 

Author’s Comment:

Because emergency systems are just as important as fire pumps and standby systems, they need to have the same safety precautions to prevent unintended interruption of the supply of electricity.

In a multiple-occupancy building, each occupant must have access to their service disconnecting means [230.72(C)]. .

Ex: In multiple-occupancy buildings where electrical maintenance is provided by continuous building management, the service disconnecting means can be accessible only to building management personnel.

The service disconnecting means can consist of [230.76]:

(1) A manually operable switch or circuit breaker equipped with a handle or other suitable operating means.

(2) A power-operated switch or circuit breaker, provided it can be opened by hand in the event of a power supply failure. The service disconnecting means must indicate whether it’s in the off (open) or (closed) position [230.77]. .

The service disconnecting means for a building must have an ampere rating of not less than the calculated load according to Article 220, and in no case less than [230.79]:

(A) For installations consisting of a single branch circuit, the disconnecting means must have a rating not less than 15A.

(B) For installations consisting of two 2-wire branch circuits, the disconnecting means must have a rating not less than 30A.

(C) For a one-family dwelling, the disconnecting means must have a rating not less than 100A, 3-wire.

(D) For all other installations, the disconnecting means must have a rating not less than 60A.

 

Author’s Comment:

A shunt-trip button doesn’t qualify as a service disconnect because it doesn’t meet any of the above requirements.

Q3. What are the disconnecting means requirements for a building supplied by a feeder?

A3. A disconnect is required for all conductors that enter a building [225.31].

The disconnecting means for a building must be installed at a readily accessible location either outside or inside nearest the point of entrance of the conductors [225.32].

Supply conductors are considered outside of a building or other structure where they’re encased or installed under not less than 2 in. of concrete or brick [230.6].

Ex 1: If documented safe switching procedures are established and maintained, the building disconnecting means can be located elsewhere on the premises, if monitored by qualified persons.

 

Author’s Comment:

A “Qualified Person” is one who has skills and knowledge related to the construction and operation of the electrical equipment and installation, and has received safety training to recognize and avoid the hazards involved with electrical systems [Article 100].

Ex 3: A disconnecting means isn’t required within sight of poles that support luminaires.

 

Author’s Comment:

According to Article 100, within sight means that it’s visible and not more than 50 ft from one to the other.

Ex 4: The disconnecting means for a sign must be controlled by an externally operable switch or circuit breaker that opens all ungrounded conductors to the sign. The sign disconnecting means must be within sight of the sign, or the disconnecting means must be capable of being locked in the open position [600.6(A)].

The building disconnecting means can consist of no more than six switches or six circuit breakers in a single enclosure, or separate enclosures for each supply grouped in one location as permitted by 225.30 [225.33(A)].

The building disconnecting means must be grouped in one location, and they must be marked to indicate the loads they serve [110.22 and 225.34(A)].

To minimize the possibility of accidental interruption of the critical power systems, the disconnecting means for a fire pump or for standby power must be located remotely away from the normal power disconnect [225.34(B)].

In a multiple-occupancy building, each occupant must have access to the disconnecting means for their occupancy [225.35].

Ex: The occupant’s disconnecting means can be accessible to only building management, if electrical maintenance under continuous supervision is provided by the building management.

The building disconnecting means can be comprised of a circuit breaker, molded case switch, general-use switch, snap switch, or other approved means. If an existing building uses the neutral conductor for the bonding of metal parts [250.32(B) Ex], the disconnect must be listed as suitable for use as service equipment [225.36].

 

Author’s Comment:

“Suitable for use as service equipment” means, among other things, that the service disconnecting means is supplied with a main bonding jumper so a neutral-to-case connection can be made, as required in 250.24(C) and 250.142(A).

Q4. Please explain the ten foot tap rules?

A4. Except as permitted by (A) through (H), overcurrent devices must be placed at the point where the branch-circuit or feeder conductors receive their power. Taps and transformer secondary conductors aren’t permitted to supply another conductor (tapping a tap isn’t permitted) [240.21].

Branch-circuit taps are permitted in accordance with 210.19 [240.21(A)].

Conductors can be tapped to a feeder as specified in 240.21(B)(1) through (B)(5). The “next size up protection rule” of 240.4(B) isn’t permitted for tap conductors [240.21(B)].

Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following [240.21(B)(1)]:

(1) The ampacity of the tap conductor must not be less than [240.21(B)(1)(1)]:

a. The calculated load in accordance with Article 220, and

b. The rating of the overcurrent device supplied by the tap conductors.

Ex: Listed equipment, such as a surge protection device, can have their conductors sized in accordance with the manufacturer’s instructions.

The tap conductors must not extend beyond the equipment they supply [240.21(B)(1)(2)].

The tap conductors are installed in a raceway when they leave the enclosure [240.21(B)(1)(3)].

The tap conductors must have an ampacity not less than 10 percent of the rating of the overcurrent device that protects the feeder [240.21(B)(1)(4)].

Note: See 408.36 for the overcurrent protection requirements for panelboards.

Ten-Foot Tap Rule

Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeder that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for these taps?

• 200A: 3/0 AWG is rated 200A at 75°C, and is greater than 10 percent of the rating of the overcurrent device (400A).
• 150A: 1/0 AWG is rated 150A at 75°C, and is greater than 10 percent of the rating of the overcurrent device (400A).
• 30A: 8 AWG rated 40A at 60°C. The tap conductors from the 400A feeder to the 30A overcurrent device can’t be less than 40A (10 percent of the rating of the 400A feeder overcurrent device.

Q5. Please explain the twenty-five foot tap rules?

A5. Feeder tap conductors up to 25 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following [240.21(B)(2)]:

The ampacity of the tap conductors must not be less than one-third the rating of the overcurrent device that protects the feeder [240.21(B)(2)(1)].

The tap conductors terminate in an overcurrent device rated no more than the tap conductor ampacity in accordance with 310.15 [240.21(B)(2)(2)].