If the Available Fault Current (AFC) at a panelboard is higher than the interrupting rating (AIC rating) of the circuit breakers installed the panelboard, one option to comply with NEC^® 110.9 might be to install a fused disconnect switch ahead of the panelboard. Fuses generally have an extremely high interrupting rating and if selected properly, several fuse types will be acceptable in a tested series combination. To verify the series combination, specific marking on the overcurrent devices is needed along with the series rating label or book inside the panelboard enclosure.

The image shows a fuse to breaker series rating chart provided with a new panelboard. The left side of the chart in blue represents the size of the fuse that will be ahead of the circuit breakers in the panelboard. The top of the chart in red shows the maximum AFC that the series combination is rated for. In grey is the fuse type or class. In yellow are the allowable circuit breaker types that have been tested as a combination along with the class of fuse directly above them in grey.

The green circles in the image show an example of a proper series combination. Based on the manufacturers chart and the green circles, if the AFC at the circuit breakers was as high as 100k (100,000 amperes), class T fuses rated not higher than 600 amperes could be installed ahead of the panelboard. The circuit breakers in the panelboard would only be allowed to be of the types circled in green under the class T fuse and the entire system including the fuses and the circuit breakers would have an overall rating of 100k.

It is critical to understand that a fuse to breaker or breaker to breaker series combination on a new installation can only be done if the series rating book provided by the manufacturer lists the combination. One cannot automatically assume that any fuse can be used to protect a circuit breaker. NEC^® 240.86(B) only permits “tested combinations”.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2014-2017 Code Language:

110.22(C) Tested Series Combination Systems. Equipment enclosures for circuit breakers or fuses applied in compliance with the series combination ratings marked on the equipment by the manufacturer in accordance with 240.86(B) shall be legibly marked in the field to indicate the equipment has been applied with a series combination rating. The marking shall meet the requirements in 110.21(B) and shall be readily visible and state the following:

CAUTION — SERIES COMBINATION SYSTEM RATED ____ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.