MIL-DTL-23928G
3.4.3.3 Polarity and phase identification.
3.4.3.3.1 Polarity. For dc, like polarities shall be similarly disposed. Facing the panel, the left-hand bus shall be
negative and the right-hand bus shall be positive, upper contacts of all circuit breakers (horizontally mounted) shall be
of positive polarity. Polarity identification shall be clearly marked (see 3.4.3.4).
3.4.3.3.2 Phase sequence. For ac, like phases shall be similarly disposed. Facing the front of the panel, the
phase rotation shall be C, B, and A respectively, from left to right, bottom to top or from back to front. Phase
identification shall be clearly marked (see 3.4.3.4).
3.4.3.3.3 Wiring diagram. When the panel contains two or more circuit breakers, circuit breakers with internal
auxiliary devices, or when indicator lamps or other components are furnished, panels shall have a wiring connection
diagram attached on the inside of the cover or hinged door. The diagram shall be protected and attached by means
of a standard size mounting plate in accordance with MIL-HDBK-2036.
3.4.3.4 Bus bars. Rectangular copper bus bars, with rounded corners and square or round edges, hard tempered
in accordance with ASTM B187/B187M, shall be used in panels furnished to this specification. Bus bars may have
square edges when there are no bends. The following requirements apply:
a.
Bus bar sizes shall be standardized as specified in table II. In no case shall the bus bar or connection bar
sizes be smaller for a given current than the corresponding sizes specified in table II. A reduction of the
ratings specified in table II shall be made if spacing between bus bars is not large enough to neglect mutual
inductance between phases of interference in heat radiation.
b.
Bus bars shall be clearly marked in a visible location with their polarity (plus and minus signs) or phase
Bus bars shall be supported firmly in place with an insulating material (see 3.3.4.1), observing creepage
c.
and clearance requirements (see 3.3.4.2). The design, number, spacing, and mounting of the bus bar
supports shall be sufficient to prevent contact of live parts with each other or grounding during the short
d.
Bus bars shall extend not less than 1.75 inches (44.45 mm) beyond the outermost circuit breaker bases at
the top and bottom of the panel to allow for main feeder lug terminal installation (see 3.3.8). Unless
otherwise specified (see 6.2), bus bar extensions shall be left blank when lug terminals are not provided.
TABLE II. Ampere rating of rectangular bus bars and connection bars. 1/
Bus or connection
Cross-sectional area
AC ampere rating
DC ampere rating
bar size
(square inches)
(60 and 400 Hz)
(inches)
Bus bars
1.00 x .250
.250
480
490
1.50 x .250
.375
675
685
Connection bars
0.75 x .125
.094
250
250
1.00 x .125
.125
330
330
1.50 x .125
.188
500
500
1.50 x .1875
.281
570
580
1/ Based on 50°C ambient and 50°C rise.
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