You are correct to question the engineer's selection of a 20A breaker for a 1200VA, 120V boiler. Let's break down the NEC requirements and see why a 20A breaker is likely excessive.1. Calculating the Full Load Amps (FLA)

VA = Volts x Amps Amps = VA / Volts Amps = 1200 VA / 120V = 10 Amps

2. NEC 2017 Applicable Sections



NEC 430 - Motors, Motor Circuits, and Controllers

While the boiler itself might not be a motor, it likely has a motor-driven pump or fan as part of its operation. Even if it's just a small circulator pump,

430.22(A) states: "The feeder conductor ampacity shall be not less than 125 percent of the motor full-load current rating or not less than the feeder minimum ampacity." This is a crucial point. NEC 430.52 - Motor Short-Circuit, Ground-Fault, and Overload Protection

This section governs the maximum size of the breaker or fuse protecting the motor circuit. The allowable breaker/fuse size depends on the motor's code letter designation (if available) and type of motor (e.g., single-phase, three-phase).

3. Calculation with 125% of FLA

10 Amps x 1.25 = 12.5 Amps

4. Standard Breaker Sizes (NEC 240.6)

The common, standard breaker sizes are: 15A, 20A, 25A, 30A, etc.

5. Feeder Breaker Sizing Considerations



Continuous Load (NEC 210.19(A)(1))

If the boiler is expected to operate for 3 hours or more continuously, the branch circuit rating (and therefore the overcurrent device, i.e., breaker) must be sized at 125% of the continuous load. In this case, if you don't apply 430 requirements, this would be the controlling factor, resulting in a minimum 12.5A requirement.

Smallest Size that Accommodates the Load

You want the smallest breaker that can safely handle the load and any inrush current. Oversizing can reduce the effectiveness of the overcurrent protection.

Why the 20A Breaker is Likely Too Large



Without Motor Considerations (Unlikely)

If the boiler truly has NO motor components (highly improbable), a 15A breaker would be appropriate since 12.5A< 15A. The 20A breaker does not provide as much protection against overloads.

With Motor Considerations (More Likely)

Assuming a small motor, a 15A breaker may be suitable, but without knowing the motor specifics (HP, code letter, etc.), it's difficult to say definitively. A 20A may be okay if it is the lowest standard breaker that is able to handle the inrush current.

What to Do


Obtain the Boiler's Electrical Specifications

Specifically, get the following:

Full Load Amps (FLA) of the boiler (or the primary load if the boiler itself doesn't have an FLA rating). Motor Horsepower (HP) and Code Letter (if any) of any motor-driven components (pump, fan, etc.). This is critical for properly sizing the breaker according to NEC 430.52.
2. Re-evaluate with NEC 430

Use the motor information and NEC 430 to determine the maximum allowable breaker size. This might still be 20A, but you need to confirm based on the NEC tables and calculations.3.

Consider Inrush Current

Boilers and their associated equipment can have a momentary inrush current when starting up. The breaker must be able to handle this inrush without tripping. Some breakers have a higher instantaneous trip setting to accommodate motor inrush.4.

Consult the Engineer

Ask the engineer why they specified a 20A breaker. There might be a specific reason based on the equipment characteristics or local code interpretations.

In summary, while the 20A breaker might work, it's likely oversized based on the information provided. You need more data about the boiler and its components, especially any motors, to determine the correct breaker size according to the NEC. Prioritize getting the motor information and consulting NEC 430. Flag for review