Review of Rooftop Condensing Unit Circuits After a Roof Curb Fire, North Texas (2020)

In 2020 the owner of a multifamily property in North Texas asked us to review the wiring to its rooftop air conditioning condensers after a fire in a roof curb enclosure. Those enclosures carry the condenser circuits across the roof. The property and its owner are not named.

Two questions on one roof

The owner needed to know what started the fire and whether the rest of the installation was safe to keep running. Those are different questions, and they were not ready to be answered at the same time. When we wrote our letter, the cause of the fire was still undetermined. The condition of the installation could be judged right away, by looking at it and comparing it with the plans the city had approved.

The installed work against the permitted design

We inspected the rooftop circuits and curb enclosures in person and reviewed the approved permit drawings for the same work. The installation differed from the plans. By our count, about 165 condensing units were fed with #12 conductors on two-pole 25 A breakers. That arrangement may meet the minimum code requirements, but it leaves less margin than the design the engineer of record had specified.

The nameplate of an air conditioning condenser gives a minimum circuit ampacity and a maximum overcurrent device rating, and the electrical code lets the conductor and breaker be sized from those values instead of the general limits for building wire. That is how an installation can comply and still be leaner than the engineer intended. Only the engineer of record can say whether a field change is acceptable to the design, so that question had to go back to the engineer of record.

What we recommended

First, obtain written and sealed confirmation from the engineer of record that the installation as built had been reviewed in person, found compliant and accepted as a modification of the design. Second, although the code in force at construction did not require it, have an engineer design the replacement of the condensing units on #12 circuits with units that leave a wider margin on the installed wire. Until then, the breakers for those units could be removed so they would not run. We presented the second step as a reasonable response to a fire whose cause was still unknown, not as a code requirement.

Cause and condition are separate

An undetermined fire does not leave an owner with nothing to do. The condition of the rest of the system can be acted on while the question of cause stays open. More on this kind of work is on our page for electrical failure analysis. Fires in apartment and commercial buildings are covered under commercial fire investigation.