Protective Device Coordination Study for a Small Multi-Unit Residential Building With Standby Power, Denver (2020)

In 2020 we prepared a protective device coordination study for a small multi-unit residential building in Denver, Colorado, while its electrical system was being built. The goal was a set of breaker and fuse settings under which the device nearest a fault opens first and the rest of the building stays on.

A small system with fixed trips

The building takes one utility feed through a 300 kVA transformer at 208 V. An 800 A main fuse protects the service, and a 400 A fuse protects the meter stack, which feeds eight 125 A load centers, two on each of four floors. Three 250 A breakers feed two house panels and an automatic transfer switch, and the elevator circuit has a 90 A breaker and a 90 A shunt trip breaker. A 60 kW standby diesel generator connects through the transfer switch.

Most of these breakers are thermal magnetic with fixed trips, so there is little to adjust. On a system like this, coordination comes from the few devices that do have settings, here the adjustable magnetic trips on the three 250 A breakers, and from checking that each fuse and breaker curve sits in the right place against the devices above and below it.

Built in ETAP from the one-line diagrams

We modeled the system in ETAP from the one-line diagrams supplied for the project and worked out the coordination of the breakers and fuses directly, device by device. The report carries a device settings document and time-current curves for the sections of the system. Each breaker is to be set exactly as the settings document shows and checked against it in the field.

What was not known yet

Some inputs did not exist when the study was done, and the report says so. The utility had not reported the available fault current at the service, so the settings are to be reviewed for short circuit duty once that number is known. The data came from the drawings and was not verified on site, so the study is to be confirmed once the installation is complete. The generator output breaker was also unknown when the first issue went out. A revision issued the same day added a 200 A generator breaker to the model.

Related services

What a coordination study covers is described on our page for protection and coordination studies.