Isolated phase duct is moderately special as compared to the other duct systems termed and needs much more space. It is also considerably more costly to buy and install, but considered a unique system for particular applications and, in specific, as a generator duct feed. Generally, rigid conductor type Segregated Phase Bus Duct length is computed from centre line to centre line, and the tools to which it is attached, to form overall duct-length footage. Bar extensions within a termination cubicle are regarded as part of the termination tool. For cable duct, the same measures are used for establishing the length.
The only exemption is 60 inches of duct length should be added for creating overall length dimensions, because of the cable extension leads vital at every end. Such cable lead extensions are measured part of the duct length, somewhat than part of the termination equipment.
Segregated duct that comprises of cable duct basically has the same constant current ratings, from 600A to 6000A. Duct of higher ratings is delivered up to 12000A; nevertheless this is for special applications needing special design, fabricating and installation considerations. Capacities are usually based on 40°C ambient where conductors have silver plated joints, and the permissible temperature rise of the conductor is 65°C. Noncurrent carrying parts that can be voluntarily touched are allowed a 40°C rise. Ambient temperature range limits are -30°C to +40°C, and height is up to 1000 metres (3300 feet). Other ambient situations require special consideration and may perhaps contribute to de-rating.
Voltage range for these kinds of duct is chiefly from above 600V to 34.5kV to ANSI but to 46kV to CSA. In everyday terminologies, the maximum rating is 38kV with a BIL rating of 150kV. It comprises of cable duct, even though cable at supposedly 38kV can have a higher BIL rating and can undoubtedly have higher insulating voltage class, up to 33 per cent insulation on higher voltage ratings.