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!

  • I'd be a bit wary of the differences between "nominal" and "actual" here. The MCB standard puts limits on the temperature rise at MCB terminals - but I have a suspicion that it's more trying to deal with dissipating heat generated by the MCB itself. In effect the cable acts as a heat sink for the MCB. As has been noted, the actual temperature of the wire connected to the MCB due to current carried through that wire is likely to be well below it's nominal conductor temperature as it approaches the MCB - so that leaves a margin where it can dissipate heat from the MCB. So while nominally the cable is simply at 70 degrees, the actuality is messier - 70 degrees in the wall, maybe naturally 35 degrees for  a single core in freer air as it approaches the MCB, but increased to 40 degrees by the heat from the MCB, with the MCB terminal itself somewhat higher again (actual numbers just made up to illustrate of course). Altering things so that the natural conductor temperature just before the MCB is just below 70 degrees might be missing some of the more subtle points of the process.

       - Andy.

  • Any chance of increasing the CSA of the tri-rated lengths so that they don't operate at 90 degrees?

  • Probably no immediate help in dealing with someone who is entirely standards driven, but I have found the peak temperature indicating stickers to be a useful thing to have in the armoury in cases of 'how hot is my terminal block' . We tend to fit them inside kit at manufacture so that we can deal with damage / fault claims - if the customer clearly did not or did not keep the fan intake clear or switch it on  while covered over etc.

    I have also used them to justify a position of 'no action required' on some things that ran cooler than everyone thought they did. Left on critical parts and inspected every year or so, if the 'high tide mark' creeps up then you know that the design is being pushed, before it fails in service. If they do not,, it is fine.

    Note that the '40' degree markers will turn black in a trouser pocket on a  hot day, and a car in the summer sun can ruin the whole strip up to about 60 degrees, so storing the stickers carefully is part of the process. I once posted a pack out to a dry sandy country, only to find that they had all been overheated in transit.