Thursday, September 9, 2010

"Small Branch Circuit Rules"


            "How do you correctly size a 20 Amp tankless water heater (Insta-Hot)? shouldn't you have to follow the "80%" rule and place it on a 30 Amp circuit with a #10 wire?"

     Several code questions arose this week about an insta-hot water heater installation. They are becoming a much more common installations these days. In order to answer the question, we must look at what the requirements are for small branch circuit sizing. This device itself is also considered an "Appliance," so we must also include considerations for appliances. Our main issues here are, what are the percentages of permissible loads on the circuit, continuous load or non-continuous load classification, plug and cap connections, permanent wiring and disconnecting means, and ultimately, "Appliance" considerations.


     We use the 80% rule quite frequently in the field to size branch circuits. It has become a 'rule of thumb.' However, there is some confusion in its application. There is no "blanket" rule that requires EVERY 15A and 20A circuit to be restricted to 80%. Article 210.23 under 'permissible loads,' states that we may supply ANY load for that which it is rated. This article in one of our key controlling sections.
     There are three critical main installation specifics used to navigate and correctly apply Article 210.23. (1), The first is whether or not the equipment is "fixed in place." Is the equipment readily mobile? Does it have coaster wheels or the like, or is it small enough to be easily repositioned? (2), Next we examine what loads are supplied by that branch circuit. Is it a dedicated circuit, feeding that piece of equipment alone? Are there other loads, such as lighting or general purpose outlets, on the same circuit? (3), Finally, is the equipment connection anticipated to be a cord and plug type or will it be "hard wired?"

                         Rule #1:    A cord and cap connected piece of equipment that is not "fastened in place" is restricted to 80% of the maximum branch circuit rating.

     That rule applies no matter what else the circuit feeds. Whether it is a dedicated branch circuit is not a consideration.

                         Rule #2:     If the equipment IS fastened in place AND if the circuit ALSO supplies lighting and/or other cord & plug connected equipment that is not fastened in place, the load is restricted to 50% of the branch circuit rating. For example, a room air conditioner, placed on a 20A branch circuit, would NOT be allowed to exceed 50% - or 10 Amps. Waste disposal (in sink garbage disposal) and dishwasher (non-mobile) would be two other good examples.

continuous & NON continuous LOAD CLASSIFICATIONS

       As you can see, the rules are specific and narrow. If the equipment IS 'fastened in place,' and the circuit supplies nothing else, then there is NOT any de-rating or percentage restriction required. This does, however, raise the next major issue of "continuous Load and Non-continuous Load" classification.
      The statement above ignores continuous load considerations. Equipment that operates, or can be anticipated to operate, for 3 hours or more, shall be classified as a continuous load. Further, Article 422.13 specifically classifies storage type water heaters as continuous loads. Any tank type water heater that is 120 gallons or smaller, MUST be considered a continuous load. (NOTE: this excludes "tankless water heaters.")
      Article 210.20(A) clarifies that continuous loads must be calculated at 125% of their rating. This 125% is the sister reciprocal of our 80% branch circuit restriction. 80% deals with our branch circuit size (ie. a 20A breaker is DIVIDED by 80% = 16 Amps). 125% deals with the load size itself, which comes from the opposite direction than our breaker sizing step. A load, sized at 16 Amps MULTIPLIED (multiplication is the reciprocal operation of division) by 125% = 20 Amps! In other words, 80% is a divider, 125% is a multiplier. If you know the branch circuit size, divide by 80%. If you know the equipment size, multiply by 125%.
      Thus a 20 Amp tank type water heater would result in a 25 Amp breaker size, along with #10 AWG (CU) conductors. 20A X 1.25 = 25 Amps. Sized at a 25 A breaker per 210.3 as an "individual branch circuit," and 240.6(A), "Standard Ampere Ratings," and 240.4(D)(7) - the so called "Small Conductor Rule."

     We will examine the rest of the rules in Part II. Look for it this Saturday!

-Mitch Tolbert

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