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Amtech Ring Circuit Grouping

Firstly, Amtech recommends the design current of a ring circuit to be 20A as the attached image and it also the deafult, at first glance that appears correct as it assumed that no leg of the ring will carry more than 20A, however this doesn’t seem to make sense as when the design current is set at 32A Amtech calculates the Iz at 25A (20A/0.8) as the below table, so uses the 20A in this case, so I am not sure why 20A Design current is the default as this will be split between legs...

I have also been trying to establish how Amtech calculates the grouping factors of ring circuits (with no luck), I have been changing the design current to see if I could see a trend but there doesn’t seem to be one. I have attached the table below to see if anyone else has any ideas on the calculation, there doesn’t seem to be anything within the manual.

Design Current

Grouping Factor (2 circuits)

0.8

21.1

0.8

21.2

0.8

21.3

0.8

21.5

0.8

21.6

0.8

21.7

0.8

21.8

0.8

22.7

0.8

23.4

0.8

24.2

0.8

089fa81c383511347efc18dbb2a8d03c-huge-annotation-2019-10-17-113612.png

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0 Former Community Member over 4 years ago

Correct in terms of tabulated numbers.

However, the reason we don't burn out 2 or 3 rings installed in dado is that we are probably running them at 10A not 32A - but that's the real world rather than the theory we are discussing.

You can use Ib rather than In - but you have to be pretty bloody sure that the simultaneous overload beyond Ib isn't going to be sustained for any length of time if you want to group the circuits - BS 7671 warns us against small overloads of long duration.

Equally - is the clipped direct rating really 27A and is the enclosed rating really 23A - the numbers themselves are fluid and based on incredibly conservative assessments design to cover all circumstances. As an example, is there really the same rating for a cable in dado if that cable is in plastic dado or metallic dado- there will be a difference (in a previous life, I spent a happy summer with a major british cable manufacturer measuring these things in the day and spending the money on cigarettes, beer and girls in Enfield Lock each evening)

Regards

OMS
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0 Former Community Member over 4 years ago

Correct in terms of tabulated numbers.

However, the reason we don't burn out 2 or 3 rings installed in dado is that we are probably running them at 10A not 32A - but that's the real world rather than the theory we are discussing.

You can use Ib rather than In - but you have to be pretty bloody sure that the simultaneous overload beyond Ib isn't going to be sustained for any length of time if you want to group the circuits - BS 7671 warns us against small overloads of long duration.

Equally - is the clipped direct rating really 27A and is the enclosed rating really 23A - the numbers themselves are fluid and based on incredibly conservative assessments design to cover all circumstances. As an example, is there really the same rating for a cable in dado if that cable is in plastic dado or metallic dado- there will be a difference (in a previous life, I spent a happy summer with a major british cable manufacturer measuring these things in the day and spending the money on cigarettes, beer and girls in Enfield Lock each evening)

Regards

OMS
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Vote Up 0 Vote Down

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Amtech Ring Circuit Grouping

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