Easy way to get a series combo rating – sometimes??? Maybe?

Hi all. As I continue to slog through my list of 104 buildings on campus I need to do studies on I discovered yet another over-dutied situation yesterday. Common scenario, 208Y/120V “service” with 15-ish kA available at the main panel and a 100A MLO branch panel right next to it (very short feeder) with 14-ish kA available. Square D QOB (10KAIC) branch breakers. No main breaker. The main panel is Cutler Hammer so no series ratings exist between CH and SQ D.

I’ve come across this several times over the course of the first 50 buildings’ studies and have been changing the breakers out to 22KAIC rated, QOB-VH (or whatever, brand specific) breakers to solve it if no series ratings exist already (existing or easily replaceable feeder breaker or fuse in combo with the branch panel breakers).

I was looking at the Square D series chart and it appears to me there is a series rating of 22KAIC available with a QOB-VH main and QOB branches.

Hmmm. Just move the feeder conductors from the main lugs over to a new 3P80 or 3P90 or 3P100 branch mounted QOB-VH breaker. Simple and easy enough if the feeder reaches. If not, it is only about 4′ long to start with so it is easy to replace.

Not sure why it never occurred to me before now to look for a branch mounted main solution for situation like this, Square D or Siemens or GE or Cutler Hammer or otherwise.

Probably commonly done by others on here but if not, now you have another weapon in your arsenal to consider in certain situations. If you have, say, an existing 42 ckt panel with 30+ circuits in-use to deal with adding a branch mounted main like this should be a lot easier and cheaper with less down time than replacing all the breakers. READ MORE

Using the series rating charts – real example

Don’t mean to beat this topic to death. Well, yeah, I guess I do, but since I had another example of being over dutied/under rated come up today I thought I would share it.

Scenario: Fairly low impedance (2.9%) 300 kva dry transformer originally feeding a large dimmer rack. Over the years someone added a panelboard off the secondary of this transformer plus a cam-lock quick hookup for a mobile TV broadcast truck.

The branch panel is a Siemens S1 with a 125A type ED4 main breaker and a bunch of 1P20A type BL branch breakers. Voltage is 208Y/120V.

SKM model tells me I have a little over 10KA available at this panel. The BL breakers are only good for 10KAIC. Hmmm. What to do.

1. Replace the breakers with BLH’s or other 22KAIC rated breakers.
2. Artificially increase the feeder length until we get below 10KA available.
3. See if there is a tested series combination rated high enough.

See attached 3 page PDF for the solution. CLICK to get PDF and read more.

Existing Overdutied Equipment

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 READ MORE

Overdutied Protective Devices and Catastrophic Failures

People seem to love to watch videos of electrical explosions. One type of failure is when a protective device is applied beyond its interrupting rating and catastrophically fails (explodes) when it has to interrupt a short circuit. Although this does not happen for every overdutied device, it could happen. That is the main reason for performing a short circuit study – to identify these types of deficiencies so they can be corrected.

So with that intro, here is this week’s question.

Have you/clients ever had an overcurrent device catastrophically fail when it interrupted a fault?
Yes
No

READ MORE