This week's question is a continuation of last week's topic regarding overcurrent devices that do not have an adequate interrupting rating.

When the calculated short circuit current is greater than the device's short circuit interrupting or withstand rating, the device is often referred to as "overdutied". (X/R can play a role too but we'll save that for another day)

This week's question:

Do you/client presently have any overdutied devices?
Yes
No
Not Sure
Rather Not Say

Yes. I am in the middle of modeling 103 different buildings on a college campus and find this problem in maybe 25% of the buildings. It is mostly confined to being just over 10,000 amps on 10KAIC rated breakers in 208Y/120V panelboards. Not always but mostly. We have many buildings with 208V services and transformers 500 kva and larger so it is easy to get over 10K.

The first thing I do is look to see if a series combination applies. Sometimes it does, sometimes it doesn't. I have become very familiar with most of the manufacturer's published breaker series combo ratings and the series charts for fuses and breakers.

When that doesn't work, we usually change out the breakers or maybe add a fused disconnect ahead of the under rated panel if there is a published series rating for the existing breakers and the added fuses if that is cheaper and easier than changing out all the breakers in the panel.

If a series rating is achieved one way or the other I print labels and affix them to the panel. See attached example. I print these with the office laser printer/copier machine on polyester label stock, the same as I print the arc flash labels. Labels available from onlinelabels.com #OL475LP.

One other approach we've taken in a couple of spots where the switchboard was under rated was to either artificially increase the feeder length or reduce the feeder size or both and add or change out fuses. Example. Existing 800A switchboard is over dutied/under rated for available fault current. Metering data shows that the maximum load on the switchboard since 1999 was 210 amps. Switchboard is fed with 2 parallel sets of 500's. Disconnecting one set of 500's and adding a fused disconnect at the transformer fused at 400A reduced the fault current enough to get under the rating due to "halving" the conductor size and the current limiting 400A fuses reduced the incident energy. We labeled the old 800A main accordingly since it is now really worthless as far as OCP goes as it does not coordinate with the 400A fuses at all.

I will also add, in some of our problem areas it is because instead of the usual green oil filled pad mounted transformer with 5%-ish impedence feeding the building we have in a few instances 3 single phase oil filled cans wired for 3 phase and the cans only have 1.6-ish% impedence allowing higher fault currents on the secondary.

I voted NO because I do not know of any overdutied devices but I also have 5 plants that I have not completed a study for and most likely will have some there. We fix all we find. Finding the time to find the devices is proving the hardest part.