Ampere Interrupting Capacity

[john37]

First, please forgive my ignorance...Could someone please explain to me on how I figure out the AIC rating for a distribution board? Also say for example the distribution AIC is rated at 65,000AIC but I don't understand how to calculate the rating for the panelboards down stream. Thanks.

[augie47]

One method is to use engineered or factory data for a "series rated" system which allows breakers with recuced AIC ratings to be used in combination with breakers rated at or above the available AIC. For example on a system with an available AIC of 50k, you might have a 65k rated breaker in the main panel feeding a subpanel with 22k "series-rated" breakers
This is covered in 110.22

(B) Engineered Series Combination Systems. Where circuit breakers or fuses are applied in compliance with series combination ratings selected under engineering supervision and marked on the equipment as directed by the engineer, the equipment enclosure(s) shall be legibly marked in the field to indicate the equipment has been applied with a series combination rating. The marking shall be readily visible and state the following:
CAUTION — ENGINEERED SERIES COMBINATION SYSTEM RATED _______ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.
FPN: See 240.86(A) for engineered series combination systems.
(C) Tested Series Combination Systems. Where circuit breakers or fuses are applied in compliance with the series combination ratings marked on the equipment by the manufacturer, the equipment enclosure(s) shall be legibly marked in the field to indicate the equipment has been applied with a series combination rating. The marking shall be readily visible and state the following:
CAUTION — SERIES COMBINATION SYSTEM RATED ____ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED.
FPN: See 240.86(B) for tested series combination systems.

[jim dungar]

First, please forgive my ignorance...Could someone please explain to me on how I figure out the AIC rating for a distribution board? Also say for example the distribution AIC is rated at 65,000AIC but I don't understand how to calculate the rating for the panelboards down stream. Thanks.

You always chose an AIC rating that is higher than your available fault current at the line side of the breaker. This means you must be able to perform short circuit current (fault) calculations.

In most situations the available bolted fault current at the service entrance is provided by the utility (they often provide their 'design' value). You must then determine the impedance of the circuit from one panelboard to the next. Use the circuit impedance and the beginning fault current to determine the fault current at the downstream panelboard. Unless you are very experienced, it is not worth trying to consider the current liming effect of any items except the conductors (as Augie pointed out series combinations of devices are not easy to determine).