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Sir Sparks
Sir Sparks, President/Owner/ Master Electrician
Category: Electrical
Satisfied Customers: 2532
Experience:  US State Certified & UK Electrical Contractor for all electrical fields/disciplines
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I recently purchased a Milbank Meter Main/Breaker Panel combo

Resolved Question:

I recently purchased a Milbank Meter Main/Breaker Panel combo to replace an aged outdoor meter and to supply juice to a new barn. The main breaker in the combo main/breaker panel is a Siemens 200 amp QN2200RH (22 kAIC) for which they say on the label that branch breakers should be Siemen's type QP or QPH. The box doesn't have pass through lugs and I'd rather not use tap connectors on the load side conductors going to the house. Also, I like the idea of being able to shut off the underground juice to the barn between my utility pole (near the house) and the barn 240' away in case I need to do some digging in the yard. I found a very good price on a Murray QP 200 amp double pole branch breaker (10 kAIC) that looks like it would plug right in to the breaker panel using 4 of the 8 available spaces (2 adjacent and 2 opposite). I understand that Murray and Siemens breakers are identical except for branding and everything I have found on-line would support that notion - the literature I've found on the Siemens QN2200RH and Murray MPD2200R both share identical (I mean identical) layout/style, type font, images and the graphs of time-current characteristic curves for the 2 breakers is also identical. In fact, the Murray literature (including the TCC graph) is copyrighted by Siemens. Can anyone imagine my having any problem using the Murray breaker in the combo panel to feed my barn even though it apparently isn't series rated for the Siemens? Also, am I correct in assuming that there should be no problem in using the Murray branch breaker w/ only a 10 kAIC circuit interupting rating downstream from the 22 kAIC Siemens main breaker? Thanks
Submitted: 6 years ago.
Category: Electrical
Expert:  Sir Sparks replied 6 years ago.

Sir Sparks :

A Master Electrician here to help. I am a double qualified; USA & UK electrician. Please give me a moment to formulate a reply.

Expert:  Sir Sparks replied 6 years ago.

Unless you have documentation to show that Siemens states these Murray breakers are an acceptable alternative to their own both as a breaker and for series rating then any electrical Inspector caching it is likely to red tag the inspection.


I suggest you contact your local inspector to ask his view.

Edited by Sir Sparks on 11/30/2010 at 2:11 AM EST
Expert:  Sir Sparks replied 6 years ago.
Another expert informs me that in his area Siemens/Murray crossovers are permitted without documentation so your only safe way is to check with your inspector.
Customer: replied 6 years ago.
Code status aside (I don't expect an inspection to be the problem), would you perceive a problem w/ it working properly based upon the main and branch breaker having identical time-current characteristic curves and the branch breaker having a 10 kAIC circuit interupting rating downstream from the 22 kAIC rated Siemens main breaker?
Expert:  Sir Sparks replied 6 years ago.
Bearing in mind the additional fact that Siemens owns Murray and that therefore they most likely are compatible I would foresee serious problems.

Edited by Sir Sparks on 12/1/2010 at 4:32 AM EST
Expert:  Sir Sparks replied 6 years ago.

One other point here;


Your barn being 240 ft away and with a 200 amp feed would require 4/0 copper which has a resistance of 0.0608 ohms per 1,000 ft. With these values then there would be an available fault current at your barn of 8,218 amps.


So even if we thought of the AIC as being infinite before your 10 KAIC breaker then still only 8.218 kAIC is available at your barn.



If I include an AIC not exceeding 22 kAIC as a feed from the main for this circuit then the final available fault current at the barn will not exceed = 5.985 kAIC



Now this figure is "at the barn" of course, if there were to be a fault in the 240 ft of wiring to that barn then the fault current would increase, but the most likely location of a fault would surely be at the barn unless later excavation work is done damaging that feed.


I hope this helps.

Edited by Sir Sparks on 12/1/2010 at 4:07 PM EST
Sir Sparks and 2 other Electrical Specialists are ready to help you
Customer: replied 6 years ago.
Somebody will have to interpret all that for me but thanks.
Expert:  Sir Sparks replied 6 years ago.

I hope this is simpler;


Even IF series rating does not apply then in any case;


Your main breaker is rated at 22 kAIC and your barn breaker is rated at 10 kAIC. When In fact your barn service AFTER the 240 ft run only needs a minimum of about 6 kAIC.


This doesn't make it right because a fault could occur between the barn breaker and the 240 ft of wire at a point where there might be MORE than 10 kAIC. But it makes it better.


(Higher the kAIC rating the better)


You CAN use my calculations to show that no matter what the case ALL breakers in the barn panel (s) will only need to be 10 kAIC (the minimum normally available).

Edited by Sir Sparks on 12/1/2010 at 10:16 PM EST