Temperature of buss bars

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electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
I was installing some CT's around some buss bars in a switchboard. Had a project manager tell me today that some CT's aren't rated to be installed around buss bars. I checked with the manufacturer and the supplier said no problem. Anyway, it got me thinking about temp rise in conductors vs buss bars. What are the design limits for switchboards and other equipment and what standard covers this?
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Electrofelon...

Temperature-rise limit of bus bar is similar to that applied to wire of the same cross sectional area! Usually, however, the main concern is its ability to withstand mechanical stress caused by short-circuit current! It, generally, produces a size that precludes the detrimental effect of temperature!

That said, I believe you will find temperature-rise limits in the NEMA standards!

Regards, Phil Corso
 
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gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
171208-1040 EST

electrofelon:

I would agree with Phil.

From some time long ago I remember there was some ballpark criteria for amperes per some unit of area for transformer wire. But I don't remember the value.

From the ITT (not AT&T) "Radio Engineers Handbook" fourth edition p 55 the fusing current of wire is I = K*d^(3/2) where d is wire diameter in inches and K = 10,244 for copper. Note d is not squared, but is taken to the 3/2 exponent..

Note: fusing below means where the wire melts. Not how you would select a fuse for the wire. Wire surface area increases linearly with radius or diameter, while cross sectional area increases as the square of radius or diameter.

For #12 wire fusing (melting) is listed as I = 235 A. Using this I we get I^2 = 55,225, but for *12 it is usually rated at 20 A, or 20^2 = 400. Thus, the power ratio is 138 to 1 for melting to nominal rating.

My Sq-D 200 A main panel has bus bars about 0.25" * 1" or 0.25 square inches.. This is about 0.8 A per 1/1000 sq-in.

A #12 copper wire is nominally rated at 20 A. The area of a #12 wire is Pi*r^2 . r = 0.08/2 . #12 area = 0.005 sq-in, p 51 of ITT. Or about 4 A per 1/1000 sq-in. Thus, 5 times my bus bar value.

I do not heavily load my panel. Bus bars read 78 and 77 F, and panel box 74 F. All the breakers seem to read about 78 F. I believe the bus bar heat is mostly from the breakers conducted to the bus bars.

.
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
ElectroFelon...

No! Not my usual finding! Even, if Epoxy coated!

Unfortunately, there's no "One-Fits-All" solution! The usual formula for current-carrying area of most electrical apparatus is:

A = K1 * K2 * K3 * K4 * Isc * Sqrt(t) where:

A = the area of the conducting element.

K1 is a constant related to the type of material,i.e., aluminum, copper.

K2 is a constant related to voltage, i.e., LV, MV, HV, EHV!

K3 is a constant related to the application, i.e., cable, bushings, CTs, Busbar.

K4 is a constant related to units conversion, i.e, square inches, square millimeters.

t is the current-carrying duration, i.e., seconds.

Of all the ""types" bus-design is very, very, complicated! I always recommend such apparatus should be obtained from a well qualified manufacturer!

Phil










BTW CT's have rated tePhil
 
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