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Topic Title: Grouping factors in ladders
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Created On: 22 October 2013 02:53 PM
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 22 October 2013 02:53 PM
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SreejithK

Posts: 6
Joined: 14 November 2012

Hi
Working on a project where we have more than 20 cables on a single ladder. According to the regs and Amtech the grouping factor for the maximum number of cables is 0.78. But if there are more than 20 cables what is the grouping factor that is to be used.
On calculating in Amtech if the number of cables are more than 20 it automatically selects 3 ladders, and the derating factor changes. Has anyone come across this before. And this simultaneously increases the cable size from 35mm to 50 mm, which we do not want to implement. Any ideas or anyone looked into it.

Cheers
 22 October 2013 03:30 PM
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AJJewsbury

Posts: 10952
Joined: 13 August 2003

According to the regs and Amtech the grouping factor for the maximum number of cables is 0.78. But if there are more than 20 cables what is the grouping factor that is to be used.

As they're in a flat layer, once each cable has a couple of neighbours, further cables don't make much difference (heat loss will be perpendicular to the ladder only) - looking at table 4C1 - 0.78 applies for anything over 8 cables (up to 20) - so it's a fair guess that it won't be much different beyond 20.

Physically getting 20+ cables on a ladder might be another issue.

- Andy.
 30 October 2013 06:10 PM
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HarryJMacdonald

Posts: 254
Joined: 15 May 2002

I suspect that whether the cables are in one layer, whether they are spaced or touching and the general ventilation of the space are far more important. Another consideration is how many of the cables are likely to be fully loaded, often some are sized on volt drop and so don't contribute much to mutual heating.
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