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.

Series Ratings NEC 240.86

The National Electrical Code article 240.86 addresses series ratings with the following:

Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in (A) or (B), and (C).

(A) Selected Under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment. For calculated applications, the engineer shall ensure that the downstream circuit breaker(s) that are part of the series combination remain passive during the interruption period of the line side fully rated, current-limiting device.

During the early 1980’s there were some problems with series ratings and tested combinations were introduced. Series rated test standards in accordance with U.L were developed and circuit breaker manufacturers begin to provide tables with their listed series ratings like we see today. 240.87(B) states

(B) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.
Informational Note to (A) and (B): See 110.22 for marking of series combination systems.

240.87 (C) addresses the situation where short circuit contribution from motor’s a.k.a. “motor contribution” may occur between the line side and load side devices that make up a series rating. The consideration is as follows:

(C) Motor Contribution. Series ratings shall not be used where (1) Motors are connected on the load side of the higher rated overcurrent device and on the line side of the lower-rated overcurrent device, and
(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.

An example of a series rating may be a breaker that normally has an interrupting rating of 22,000 Amps but when protected with upstream fuses of a specific size and class, may have a series rating of 100,000 Amps. This must be either a listed combination or determined under engineering supervision as stated above in 240.86 (A)

Those that consider using series ratings should thoroughly review the requirements of 240.87.

With all of that information, here is this week’s question.

Does your facility/client’s facility use series ratings?
– Yes
– No
– Multiple sites – some do
– Don’t know
– Doesn’t apply to me

CLICK to answer and review discussion.


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