Overloaded heating circuit caused a thermostat to combust

Incident Investigation

Overloaded heating circuit caused a thermostat to combust

February 12, 2020


Reference Number:

II-979265-2020 (#16604)

Location: Duncan

Regulated industry sector: Electrical - Low voltage electrical system (30V to 750V)



Qty injuries: 0

Injury description: NA

Injury rating: None


Damage description: Electrical wires, and thermostat damaged.

Damage rating: Minor

Incident rating: Minor

Incident overview

Overloaded heating circuit caused a thermostat to combust.

Investigation Conclusions

Site, system and components

Baseboard Heating Circuit: Electrical cables transmit electricity through and overcurrent device (Circuit Breaker), and a thermostat to resistive heating elements.

Failure scenario(s)

Home owner changed out baseboard heaters and a thermostat. The total heating load added 4750 watts. The programmable thermostat was only rated for 4008 watts or 16.7 amps. This caused the thermostat to overheat and combust. The circuit breaker protecting the circuit was a 20 amp Federal Pioneer Breaker, these breakers are 80% rated and are approved to run continuously at 16 amps. The total load on the breaker was 19.8 amps (overloaded by 124%).

Facts and evidence

Photos and witness statements.

Thermostat overloaded as determined during the investigation on February 12, 2020.

Causes and contributing factors

It is highly probable that the thermostat caught fire because it was overloaded due to poor wiring practices by an unqualified individual.

Picture #1, Damaged Outlet Box.


Picture #2, 20 amp FPE breaker.

Picture #3, Damaged thermostat.

Picture #4, Specification Sheet of Thermostat.