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Available Fault Current values at the service equipment can be obtained from the electric utility company.

#1) Available Fault Current.

Typical letter from the utility company providing the AFC at a given point.

#2) Available Fault Current and Characteristics of the...

The interrupting rating is the highest current at rated voltage that a device is identified to interrupt under standard test conditions.

#3) Article 100 Definitions. Interrupting Rating.

This is a passive component and provides no overcurrent protection. It has a Short-Circuit Current Rating of 5kA or 5000 amps.

#4) Article 100 Definitions. Short Circuit Current Rat...

The time clock is a passive device that cannot protect itself against any fault occurring on the load that it controls.

#5) 110.9 Interrupting Rating of Defensive and Passive...

Overcurrent devices (top) must have an interrupting rating at least equal to the current that is available at the line terminals. Passive devices like the lighting contactor on the left require an interrupt rating at least equal to the current they are interrupting.

#6) 110.9 Interrupting Rating.

Circuit breakers are generally marked somewhere with their interrupting rating. If not marked, then they have a default value of 5,000 amperes.

#7) 240.83(C) Circuit Breaker Marking. Interrupting Ra...

The electrician must understand resistance, impedance and other circuit characteristics.

#8) 110.10 Circuit Impedance, Short-Circuit Current Ra...

This label provides the AFC and the date of the calculation.

#9) 110.24(A) Available Fault Current. Field Marking.

Changing the length or size of the service entrance conductors will also change the amount of available fault current at the service equipment.

#10) 110.24(B) Available Fault Current. Modifications.

In a fully rated system, all OCPD’s have an AIC rating equal to or more than the AFC at their line terminals.

#11) Fully Rated Systems.

Series ratings can be between OCPD’s in the same enclosure or in separate enclosures.

#12) Series Rated Systems.

Often, a spare space in an older MDP is used if a new subpanel is added. For this one, the engineer has it easy since each switch shown is fused with common fuse sizes. Many modern series combinations will have a fuse at the MDP supplying a panelboard with breakers downstream. This series combination can be verified by the series rating book that comes with the new subpanel.

#13) 240.86(A) Series Rated Systems. Selected Under Eng...

Specific labeling is required for engineered series rated systems.

#14) 110.22(B) Engineered Series Combination Systems.

Unfortunately, the series rating book is usually one of the first things tossed into the trash by the electrician after the panelboard is installed. This one was fished out of the trash and placed at the switchgear after the inspector red tagged it during the final inspection.

#15) 240.86(B) Series Rated Systems. Tested Combination...

Specific labeling is required for tested series rated systems.

#16) 110.22(C) Tested Series Combination Systems.

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