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. Is GFCI protection required in a dwelling unit garage for a dedicated receptacle for a fixed appliance like a freezer or in the ceiling for a garage door opener?

A1. GFCI protection is required for all 15A and 20A, 125V receptacles in garages, and in grade-level portions of accessory buildings used for storage or work areas of a dwelling unit [210.8(A)(2)].

Author’s Comment: See the definition of “Garage” in Article 100. A receptacle outlet is required in a dwelling unit attached garage [210.52(G)(1)], but a receptacle outlet isn’t required in an accessory building or a detached garage without power. If a 15A or 20A, 125V receptacle is installed in an accessory building, it must be GFCI protected[210.8(A)(2)].

Q2. How do we size the conductors and maximum overcurrent protection for an electric water heater? Please give an example.

A2. An electric water heater having a capacity of 120 gallons or less is considered a continuous load, for the purpose of sizing branch circuits [422.13].

Author’s Comment: Branch-circuit conductors must have a rating of at least 125 percent of the ampere rating of a continuous load [422.10] and the overcurrent protection is sized to 422.11(E)(3).

Branch-circuit conductors must have overcurrent protection in accordance with 240.4, and the overcurrent device rating must not exceed the rating marked on the appliance [422.11(A)].

A typical water heater is considered a single non-motor-operated appliance and many of these don't have a marked overcurrent protection size. For nonmotor appliances, the appliance overcurrent device must [422.11(E)]:

(1) Not exceed the rating marked on the appliance, if not marked.

(2) Not exceed 20A if the overcurrent device rating isn’t marked, and the appliance is rated 13.30A or less, or

(3) Not exceed 150 percent of the appliance rated current if the overcurrent device rating isn’t marked, and the appliance is rated over 13.30A. Where 150 percent of the appliance rating doesn’t correspond to a standard overcurrent device ampere rating listed in 240.6(A), the next higher standard rating is permitted.

Question: What’s the maximum size overcurrent protection device for a 4,500W, 240V water heater?

(a) 20A     (b) 30A     (c) 40A     (d) 50A

Answer: (b) 30A

Protection Size = 4,500W/240V

Protection Size = 18.75A x 1.50

Protection Size = 28A, next size up, 30A [240.6(A)]

Size the branch circuit conductors at 125% [422.10(A)]

Conductor size = 18.75A x 1.25

Conductor size = 23.4A, Table 310.15(B)(16), 10 AWG rated 30A at 60oC

Author's Comment: Section 240.4(D)(7) required a 30A protection device for 10 AWG. 110.14(C)(1)(a)(1) directs us to the 60oC column of Table 310.15(B)(16) to match conductor sizing to terminal ratings.

Q3. When is an on-site generator not a separately derived system?

A3. An alternate alternating-current power source such as an on-site generator isn’t a separately derived system if the neutral conductor is solidly interconnected to a service-supplied system neutral conductor. An example is a generator provided with a transfer switch that includes a neutral conductor that’s not switched [250.30 Note 1].

Author’s Comment:

• According to Article 100, a separately derived system is a wiring system whose power is derived from a source, other than a utility, where there’s no direct electrical connection to the supply conductors of another system, other than through grounding and bonding connections. Figure 100-separately derived sys 04
• Transformers are separately derived when the primary conductors have no direct electrical connection from circuit conductors of one system to circuit conductors of another system, other than connections through grounding and bonding connections.
• A generator having transfer equipment that switches the neutral conductor, or one that has no neutral conductor at all, is a separately derived system and must be grounded and bonded in accordance with 250.30(A).

For nonseparately derived systems, see 445.13 for the minimum size neutral conductors necessary to carry fault current [250.30 Note 2].

Q4. What are the rules in sizing a supply side bonding jumper sized for a single raceway containing service conductors?

A4. The supply-side bonding jumper (SSBJ) at a service ensures the required electrocal conductivity between metal parts required to be connected [Article 100 definition]. In single raceway installations, the supply-side bonding jumper is sized in accordance with Table 250.102(C)(1), based on the largest ungrounded conductor within the raceway [250.102(C)(1)].

Q5. What are the rules in sizing a supply side bonding jumper sized for parallel raceways?

A5. If the ungrounded supply conductors are paralleled in two or more raceways or cables, the size of the supply-side bonding jumper for each raceway or cable is sized in accordance with Table 250.102(C)(1), based on the size of the largest ungrounded conductors in each raceway or cable [250.102(C)(2)].

Question: What size single supply-side bonding jumper is required for three metal raceways, each containing 400 kcmil service conductors?

(a) 1 AWG     (b) 1/0 AWG     (c) 2/0 AWG     (d) 300 AWG

Answer: (b) 1/0 AWG [Table 250.102(C)(1)]

Q6. What are the rules in sizing an equipment bonding jumper sized for parallel feeder circuits in a raceway?

A6. Bonding jumpers on the load side of feeder and branch-circuit overcurrent devices are sized in accordance with 250.122, based on the rating of the circuit overcurrent device [250.102(D)].

Question: What size equipment bonding jumper is required for each metal raceway where the circuit conductors are protected by a 1,200A overcurrent device?

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

Answer: (d) 3/0 AWG [Table 250.122]

If a single bonding jumper is used to bond two or more raceways, it must be sized in accordance with 250.122, based on the rating of the largest circuit overcurrent device.

Q7. What are the rules in sizing an equipment bonding jumper sized for parallel feeder circuits in a cable?

A7. Bonding jumpers on the load side of feeder and branch-circuit overcurrent devices are sized in accordance with 250.122, based on the rating of the circuit overcurrent device [250.102(D)].

Question: What size equipment bonding jumper is required for each metal raceway where the circuit conductors are protected by a 1,200A overcurrent device?

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

Answer: (d) 3/0 AWG [Table 250.122]

If a single bonding jumper is used to bond two or more raceways, it must be sized in accordance with 250.122, based on the rating of the largest circuit overcurrent device.