You're right to question this. Here's a breakdown of why a 20A breaker might be insufficient, and why a 25A breaker (or possibly even a 30A) might be necessary

Understanding the Code and Safety
NEC 210.21(B)(3) and 210.23

These sections of the National Electrical Code (NEC) are crucial. They deal with branch circuit ratings and the loads permitted on those circuits. The general rule is that the rating of the overcurrent protection device (breaker) must not exceed the ampacity of the conductors.

Continuous Load

A space heater is almost certainly considered a continuous load. The NEC defines a continuous load as a load that is expected to operate for 3 hours or more at a time. For continuous loads, the overcurrent protection device must be rated for at least 125% of the continuous load current.

Calculation


Continuous Load Calculation

16.7 amps (heater) 1.25 = 20.875 amps.
2.

Minimum Breaker Size

Based on the continuous load calculation, you need a breaker rated for at least 20.875 amps. A 20 amp breaker is too small because it will trip.
3.

Standard Breaker Sizes

Common breaker sizes are 15A, 20A, 25A, 30A, etc. Since you need at least 20.875 amps, the next standard size up is likely a 25A breaker.

Additional Considerations



Wire Size

The breaker size MUST be matched to the wire gauge used for the circuit. A 25A breaker requires a wire gauge that is rated to carry 25A continuously. Typically, this is 12 AWG copper wire. If the wire is smaller (e.g., 14 AWG), you CANNOT use a larger breaker, even if the heater requires it. Using a larger breaker than the wire is rated for is a SERIOUS FIRE HAZARD.

Dedicated Circuit

The heater MUST be on a dedicated circuit (meaning it's the ONLY thing powered by that breaker). You cannot add other outlets or lights to the same circuit.

Local Codes

Always consult and comply with your local electrical codes, as they may have stricter requirements than the NEC.

In summary, the engineer is incorrect. A 20A breaker is insufficient for a 16.7A continuous load. You need a breaker rated for at least 20.875 amps, and typically the next size available is 25A. It is very important to verify that the wire gauge is appropriate for a 25A breaker, and that the circuit is dedicated. If the wire is insufficient you will have to run a new wire.

Recommendation


Consult a Qualified Electrician

This is crucial! Do not attempt to change the breaker yourself if you are not a licensed electrician.2.

Verify Wire Gauge

The electrician will need to determine the wire gauge of the circuit supplying the heater. This is usually printed on the wire's sheathing.3.

Proper Breaker and Wiring

The electrician will install the correct size breaker (most likely 25A, but potentially 30A if the wire is heavy enough), ensuring the wire is properly rated.4.

Dedicated Circuit Confirmation: They will also confirm that the heater is on a dedicated circuit.Ignoring these electrical codes can lead to overheating, fire, and serious injury. Flag for review