Stumped by the Code? NEC Requirements for Sizing Conductors for an Electric Water Heater
All questions and answers are based on the 2014 NEC.
Q. How do we size the conductors and maximum overcurrent protection for an electric water heater? Please give an example.
A. An electric water heater having a capacity of 120 gal or less is considered a continuous load, for the purpose of sizing branch circuits [422.13]. Branch circuit conductors must have a rating of at least 125% of the ampere rating of a continuous load [422.10] and the overcurrent protection is sized to the requirements in Sec. 422.11(E)(3).
Branch circuit conductors must have overcurrent protection in accordance with Sec. 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 non-motor 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% of the appliance rated current if the overcurrent device rating isn’t marked, and the appliance is rated over 13.30A. Where 150% of the appliance rating doesn’t correspond to a standard overcurrent device ampere rating listed in Sec. 240.6(A), the next higher standard rating is permitted.
Let’s run through an example now to drive these points home.
What’s the maximum size overcurrent protection device for a 4,500W, 240V water heater (Fig. 1)?
Protection Size = 4,500W ÷ 240V = 18.75A × 1.50 = 28A
Looking to the next size up, we would choose a 30A device [240.6(A)].
Size the branch circuit conductors at 125% [422.10(A)].
Conductor Size = 18.75A × 1.25 = 23.4A
Referring to Table 310.15(B)(16), we see a 10 AWG conductor is rated 30A at 60°C.
Section 240.4(D)(7) required a 30A protection device for 10 AWG. Section 110.14(C)(1)(a)(1) directs us to the 60°C column of Table 310.15(B)(16) to match conductor sizing to terminal ratings.
Q. When is an on-site generator not a separately derived system?
A. An 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], as shown in Fig. 2.
According to Art. 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.
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 Sec. 250.30(A).
For non-separately derived systems, see Sec. 445.13 for the minimum size neutral conductors necessary to carry fault current [250.30 Note 2].
Q. What are the rules for sizing an equipment bonding jumper sized for parallel feeder circuits in a raceway?
A. Bonding jumpers on the load side of feeder and branch circuit overcurrent devices are sized in accordance with Sec. 250.122, based on the rating of the circuit overcurrent device [250.102(D)]. For example, let’s say you’re working with metal raceway where the circuit conductors are protected by a 1,200A overcurrent device. A quick check of Table 250.122 reveals the minimum size bonding jumper needed is 3/0 AWG. It’s also important to note that if a single bonding jumper is used to bond two or more raceways, it must be sized in accordance with Sec. 250.122, based on the rating of the largest circuit overcurrent device.
