RES Technical Corner by Brett Eliasz

Technical Corner

This month’s topic will address the difference between the short circuit current rating for an actual panelboard itself in addition to the short circuit current ratings for overcurrent protective devices such as circuit breakers and fuses that reside within the panel.

In a fully rated panelboard, all the overcurrent protection devices must have a short circuit current rating above the anticipated available short circuit current that can be present at that point in the electrical system.

If each overcurrent protection device has different short circuit current ratings the device with the lowest rating within that panel is the “weakest link” and thus creates the rating of that panel even though many of the other breakers will have larger short circuit current ratings.

I’ve always wondered if there was an actual rating for the panelboard itself. I recently ran across an existing, older style panelboard which had a sticker on the front which indicated that the panelboard itself had a short circuit current rating. I thought to myself, aren’t they talking about the breakers or fuses within the panel..?

Turns out panelboards in fact do have an actual short circuit current rating. However, this rating, called Bus bracing is an entirely different rating and should not be confused with the theory presented above regarding breakers and fuses.

Bus bracing is related to the ability to withstand the mechanical forces that will be created by the extreme magnetic field repulsion between bus bars under fault current conditions. What does this mean..?

Square D has attested that they qualify the busses in their panelboards at 100kA. This ensures there is no movement that is mechanically or electrically detrimental to the bus or insulating components at values less than 100kA. Regardless of the bus bracing, the bus rating will be labeled as the AIC rating, based on the lowest AIC rated breaker or fuse.

Also, did you know most panel manufacturers offer a study and services to upgrade/increase existing bus bracing to compensate for an increase in short circuit currents rather than replace the panel all together?

Hopefully this article finds you well and can be used as a reference for your project needs.

If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director

res - technical corner DECEMBER 2020 The ROCHESTER ENGINEER |

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