90 degree rated MCBs

I'm still on the quest to fix the design challenges on my theatre project and we are nearly there.

Does anyone know of a manufacturer with a 125A TPN DIN-rail MCB that has terminals rated for 90 degree operation?  The ABB breakers we're using elsewhere do have a 105 degree rating but they don't have 125A available as they re-designed it and are awaiting a new approval.  Most manufacturers don't seem to publish the terminal temperature rating but you can sometimes get it from their tech support if you can get them to respond!

The situation I have will generally have a 90-degree rated cable in trunking with grouping factors applied, and this will terminate on DIN-rail terminals (105 degree rated) at the panels/socket boxes at each end.  These terminals will then connect to the appropriate breakers/connectors via lengths of 105 degree tri-rated cable, over a reasonable length (0.5-1m-ish) inside the breaker panel where there is more fresh air generally.  The designer is however insisting that to be able to use the 90-degree basis for the cables in the trunking then all components right through to the MCB terminals need to be 90 degree rated.

Hence if anyone knows of a 125A TPN MCB with a 90 degree terminal rating then it would be of great assistance as this is the last piece of the puzzle.


  • Not wishing to say your Designer is wrong, but, basic common sense says the terminals will never get to 90 degrees. The cable will only get to 90 degrees if it is at its full load, with all the other cables in the trunking at full load too. And that temperature will only be in that section of trunking. The terminals should be in free air, so a lot more cooling available, hence unlikely to get above 70 degrees, and, in real life, very unlikely to get above 50 degrees.

    This is from personal experience when testing industrial units. We've to investigated hot cables on a wall. It was at 50 degrees C, though, of course, anyone touching will think its pretty hot, but it was running well within its limits.The panel board the cable ran from had a maximum terminal temperature of 38 degrees C.

    Do some working out, how long will the load be at its maximum, how long will the other cables in the trunking be at their maximum load, and,as in the world of domestic cookers, apply diversity. If they will be at their maximum for hours, then he is justified in asking for 90 degree terminals, but I doubt it, so use a bit of engineering common sense.

    As for 90 degree breakers, I thought a good bet would be the MCCB type used in panel  boards, but a quick look online shows Eaton, Schneider and RS are 70 degrees. Possibly time to go back to metal clad fuse boxes?

  • Thanks Alan - I agree with you but we're in a bit of a 'them's the rules' scenario with BS7671 saying you cant use a 90 degree basis for calculation of cable sizes unless the terminals are also rated at 90 degrees., and the designer is reluctant to apply what we think is sensible judgement, and it will be his name on the certificate and not ours!

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