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# Ductbank ampacity calc - does length matter?

## Ductbank ampacity calc - does length matter?

(OP)
I just did a ductbank ampacity calculation in ETAP. There's nowhere to input ductbank length so I'm guessing it doesn't matter.

Anyone know if that is true?

### RE: Ductbank ampacity calc - does length matter?

The calculation usually follows the IEEE 835/1994 based on Neher and McGrath publication[The Calculation of the Temperature Rise and Load Capability of Cable Systems] or IEC 60287.
The calculation includes conductor losses and heat evacuation through different materials as insulation, sheet ,conduit [ducts] , concrete and earth. The length taken in consideration is one unit length [m, feet and so on].

### RE: Ductbank ampacity calc - does length matter?

Well, capacity isn't affected by length. The voltage drop is, and has to be confirmed if the duct is long. If it's a short run then it shouldn't be a concern.

### RE: Ductbank ampacity calc - does length matter?

This calculation assumes that the duct back is sufficiently long such that assuming it is 'infinite' is reasonable. An infinitely long duct bank only has radial heat transfer. There are no ends, so no heat dissipates out the ends of duct bank. When a duct bank ends in free air, the last bit of the duct bank will run a bit cooler. When the duct bank ends in a vertical 90 degree bend, the corner of duct bank may be a bit hotter.

An example of a very short duct bank is a cable passing from one side of a building firewall to the other side. In this case, the insulated firewall penetration prevents most of the radial heat transfer. Instead, most of the heat travels axially through the conductor past the face of the wall and then the heat dissipates into the room. Typically ampacity analysis switches from assuming 'infinitely' short duct banks to 'infinitely' long duct banks at around 2 feet length.

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