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Topic Title: Generator Supply using HO7 Singles cables
Topic Summary: Current carrying capacity of HO7 cables
Created On: 07 January 2013 01:50 PM
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 07 January 2013 01:50 PM
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I'm looking at using 2 x 240mm HO7 singles for the TP&N supply between a generator and the DB. The cables will be run on a tray and the ambient temperature is 50c. For the current carrying capacity I have used Table 4F2A, the three phase column which is 487A. But my colleague says that I should use the single phase column allowing us to use 150mm cables. Does anyone know what is the correct column to use and why?
 07 January 2013 03:32 PM
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I don't have the tables to hand to check, but off the top of my head, if you used the single-phase rating, you'd have to adjust downwards for grouping (as there'd be three loaded conductors in place than two - in terms of heat generation & dissipation, similar to one-and-a-half single phase circuits), which I suspect will get you somewhere quite close to the 3-phase column. (If you're running 2-conductors in parallel then maybe think of it as a group of 3 single-phase cables, rather than two 3-phase cables).

Unless all the conductors are widely spaced.... (which I suspect isn't usual).

- Andy.
 07 January 2013 04:09 PM
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It will depend on the disposition of the single cores as a group and relative to each other.

If you keep each "pair" of the phase "tight together" then you can use the last column - but you then need at least 4 x the cable OD between each group which may give you reactance increase.

Most usual in this case (using H07BN4-F) would be to group the whole lot ( probably two layers of 4 conductors) and derate the "2- single core cables, touching" by a grouping factor of about 0.9.

Effectively, a trefoil group is approx the same CCC as 2 single cores touching - hence the relatively high grouping factor.

Derate again for ambient temperature of 50C, of course - about 0.8 - so an overall derating of 0.9 x 0.8 = 0.72 to the 2 x single core touching value.



Let the wind blow you, across a big floor.
 07 January 2013 06:42 PM
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are you saying 2 times 240mm2 singles per phase? thats a nice size generator to be feeding a DB or its one hell of a run, Are you going to run half size or full size neutral?

I hope you have a nice big aliminium gland plate, I think you need a 40mm stuffing gland and 8 of those glands need a big area
 16 January 2013 10:29 AM
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Many thanks for your help.

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