Hotel Guestroom Fault: Limiting to Under 10,000 Amps

Summary: Save money on costly high AIC rated breakers by adding length to panel feeders.

Hotels with 208/120 volt services can have very high fault currents near the service entrance.  It is tempting to locate some of the branch panels in the electrical room to serve nearby loads, including the guestrooms in that vicinity.  However, this can result in excessive fault current exposure to the guestroom circuits.

One simple way to reduce the fault at nearby guestrooms is to route the feeders down and back the corridor with sufficient extra circuit length to bring the fault current below 10,000 amps.  The impedance of small circuits is quite high and has the effect of reducing fault current effectively with relatively short runs. 

As an example, suppose you have a 100 amp guestroom panel in the room adjacent the service electrical room.  The feeder size is probably #1 copper conductors and the length of the feeder to the panel is 25 feet including conductors within the switchgear.  Assume the fault at the switchboard is 65,000 amps.  The fault at the panel would be 24,000 amps symmetrical.  By simply routing the feeder 30 feet down and back the corridor to extend the feeder length to 75 feet the fault is reduced to about 9,000 amp symmetrical.  This added feeder length is only required for the first few rooms until the feeder length exceeds 75 feet anyway.

 

Below is the Design Master fault printout with the guestroom panel feeder only 25 feet long based on shortest route from switchboard to panel.  Fault is above 10,000 amps and the lowest cost panels would not be adequate to handle this fault.

 

Below is the same system in Design Master but showing the fault below 10,000 amps with the feeder length extended to 75 feet.

 

Leave a Reply