NATIONAL ELECTRIC CODE RULES - BRANCH CIRCUIT PROTECTION

422-10. Branch-Circuit Rating

This section specifies the ratings of branch circuits capable of carrying appliance current without overheating under the conditions specified.

(a) Individual Circuits. The rating of an individual branch circuit shall not be less than the marked rating of the appliance or the marked rating of an appliance having combined loads as provided in Section 422-62.

The rating of an individual branch-circuit for motor-operated appliances not having a marked rating shall be in accordance with Part B of Article 430.

The branch-circuit rating for an appliance that is continuously loaded, other than a motor-operated appliance, shall not be less than 125 percent of the marked rating; or not less than 100 percent of the marked rating if the branch-circuit device and its assembly are listed for continuous loading at 100 percent of its rating.

NOTE: The tankless water heater is not continuous!!

Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

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422-11. Overcurrent Protection

Appliances shall be protected against overcurrent in accordance with (a) through (g) and Section 422-10.

(a) Branch-Circuit Overcurrent Protection. Branch circuits shall be protected in accordance with Section 240-3.

If a protective device rating is marked on an appliance, the branch-circuit overcurrent device rating shall not exceed the protective device rating marked on the appliance.

(f) Electric Heating Appliances Employing Resistance-Type Heating Elements Rated More than 48 Amperes.

1. Electric heating appliances employing resistance-type heating elements rated more than 48 amperes, other than household appliances with surface heating elements covered by Section 422-11(b), and commercial-type heating appliances covered by Section 422-11(d), shall have the heating elements subdivided. Each subdivided load shall not exceed 48 amperes and shall be protected at not more than 60 amperes.

2. Commercial kitchen and cooking appliances

3. Water heaters and steam boilers employing resistance-type immersion electric heating elements contained in an ASME-rated and stamped vessel shall be permitted to be subdivided into circuits not exceeding 120 amperes and protected at not more than 150 amperes.

For example, the RA-28 with 7000w elements can draw 29 amperes at 240volts. Thus, under the 1999 NEC the heater requires 4 - 30 ampere circuits. That will change with the 2001 NEC.

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422-47. Water Heater Controls

All storage or instantaneous-type water heaters other than

(a) Storage water heaters that are identified as being suitable for use with supply water temperature of 82ºC (180ºF) or above and a capacity of 60 kW or above, or

(b) Instantaneous-type water heaters that are identified as being suitable for such use, with a capacity of 1 gal (3.785 L) or less shall be equipped with a temperature-limiting means in addition to its control thermostat to disconnect all ungrounded conductors, and such means shall be (1) installed to sense maximum water temperature and (2) be either a trip-free, manually reset type or a type having a replacement element. Such water heaters shall be marked to require the Tankless Water Heater Installation of a temperature and pressure relief valve.

FPN: See Relief Valves and Automatic Gas Shutoff Devices for Hot Water Supply Systems, ANSI Z21.22-1986.

NOTE: No pressure relief valve required according to the NEC

Table 310-16. Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60ºC Through 90ºC (140ºF Through 194ºF) Not More than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30ºC (86ºF)

See Temperature Rating of Conductor Table

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