4mm 90 Degree Thermosetting cable on a 32 amp MCB

[Flanders]

Hello Can I just confirm that if 90 degree cable is used in switch gear that is only ratted at 70 degrees then you use the 70 degree tables to find the maximum current carrying capacity in this case Table 4D1A

I ask this as I carried out an EICR and found 4mm 6491B cable run in trunking supplying 3 phase 32amp socket outlets protected by 32amp Type C Hager MCB

Table 4E1A (90 Degree singles) 37amps

Table 4D1A (70 Degreee single) 28 amps

I did Speak to Hager to try and find the operating temperature of their equipment and they informed me that the MCB are designed to operate correctly at a temperature of 30 degrees if the temperature is any higher then the mcb will operate at a lower ampage ,just confused me more , So tried NIC Tech who just referred me to the tables so no joy there either.

My thinking is the MCB are overatted for the cable
At the moment some supply fork lift chargers with a max current of 20 amp other sockets are not in use, so could be used for anything .
I am looking at given them a Code 2 ( over ratted MCB for cable size , Just wanted to confirm I am using the correct table and the person who designed it didn't.

 

[Leesparkykent]

Yeah if the switchgear is rated to 70 degrees you will have to use the CCC table for 70 degrees not 90 degrees.

 

[Flanders]

Thanks Lee

 

[Pete999]

Hello Can I just confirm that if 90 degree cable is used in switch gear that is only ratted at 70 degrees then you use the 70 degree tables to find the maximum current carrying capacity in this case Table 4D1A

I ask this as I carried out an EICR and found 4mm 6491B cable run in trunking supplying 3 phase 32amp socket outlets protected by 32amp Type C Hager MCB

Table 4E1A (90 Degree singles) 37amps

Table 4D1A (70 Degreee single) 28 amps

I did Speak to Hager to try and find the operating temperature of their equipment and they informed me that the MCB are designed to operate correctly at a temperature of 30 degrees if the temperature is any higher then the mcb will operate at a lower ampage ,just confused me more , So tried NIC Tech who just referred me to the tables so no joy there either.

My thinking is the MCB are overatted for the cable
At the moment some supply fork lift chargers with a max current of 20 amp other sockets are not in use, so could be used for anything .
I am looking at given them a Code 2 ( over ratted MCB for cable size , Just wanted to confirm I am using the correct table and the person who designed it didn't.

NIC being helpful again then?

 

[westward10]

This is a grey area because where does it say that if the conductors are operating at their full ccc, which is a bad idea anyway that the conductors will be at 90°C. Just because the insulation is rated to this temperature does not mean to say that at full ccc it will be at this temperature. Lets face it what current needs to be flowing for the copper to be at 90°C. The improved temperature rating of the insulation allows for increased ccc, that is the whole point of it that it doesn't break down under increased temperature.

 

[Midwest]

Ain't it said in note (1) of each Table for 90'C?

 

[westward10]

Ain't it said in note (1) of each Table for 90'C?

It does indeed but I gave never bought into that. The OP has a 32A device, even running at 32A are those conductors likely to achieve 90°C so why bother derating them I find it pointless.

 

[Flanders]

It does indeed but I gave never bought into that. The OP has a 32A device, even running at 32A are those conductors likely to achieve 90°C so why bother derating them I find it pointless.

This was my thinking if the circuit was running at 32amp i bet the 4mm cable would only be warm not any were near 70 decrees let alone 90 degrees , and this is why i came on here to see what you guys think , i went for code 2 due to the over ratted mcb if you go by to 70 degree table , but then im thinking is it realy protential dangerous, just to add this installation is 5 years old but looks like it was installed yesrterday , and i feel bad giving it a code 2 , it more to cover my arse if somone plugs a 32amp load into one of the sockets and the terminals over heat which i dont think will hapen but the regs says it will

 

[westward10]

I wouldn't Code it at all. What will happen if someone plugs in a 32A load, does everything heat up to 90°C, never going to happen for it to reach that temperature the mcb will trip long before.

 

[Flanders]

NIC being helpful again then?

Yes i dont why i bother calling them ,I never seam to get an answer , this place is much better,

 

[Flanders]

I wouldn't Code it at all. What will happen if someone plugs in a 32A load, does everything heat up to 90°C, never going to happen for it to reach that temperature the mcb will trip long before.

Yes very true as hager did say they the higher the temprature the lower the current required to trip the mcb , things like this make me want the throw the regs book in the bin and use common sense, thinking now to go for a code 3 , afterall it apears to be a reg fail . Just to add The reason that i feel the installtion was not designed correctly as the sub main to the db is a 16mm 4 core pvc swa about 20meter long fed from a 125 amp mccb , I gave tnat a code 2

 

[westward10]

I still wouldn't Code it I don't see an issue this 90°C business is overplayed.

 

[Wilko]

That cable is a waste (in my view) as it is not in a 90C environment. So it's not a good design to use it with that mcb. Whether a design that doesn't conform to regs is a no comment, C3 or C2 I will pass on - as I'm scrambling for my tin hat and heading for the trench...

To Westie's point in #5, afaik the ccc ratings are determined by temperature rise, ie the 90C cable is installed method X with a 30C ambient temp, current Y is passed through it and conductor temp rises to 90C, but no further. Current Y is then ccc. Unless I've misunderstood .

 

[westward10]

BS7211 wiring has the advantage of being LSZH which is why it is popular. The temperature rating of a conductor is the maximum it can withstand before the insulation breaks down, because whether BS6004 or BS7211 the conductor size remains constant and it is only the temperature withstand of the insulation which dictates ccc. I still maintain there is no need to consider 90°C switchgear, who designs a general circuit where conductors operate at such a temperature.

 

[Flanders]

I think at 6491B is used a lot because thats what the wholesalers stock on the shelf , well every time I have asked for 6491x , they say they only have B .

Wilko I started by putting Info on the report.Then after doing some reading up in the regs , I went to code 2 now I have come here and applied common sense , I going to give it a code 3 ,

 

[Darkwood]

I see issues all the time with cable burn outs, mainly because I work with alot of heating equipment like elements etc, what I think you may have forgotten here is how heat dissipates from a cable, pushed beyond its thermal limits you get what is known as a runaway effect as temp' will continue to rise well beyond the thermal breakdown point of the insulation.
IMHO I would heed caution about becoming complacent with these values just because you feel you never see any issues, you have to remember exceeding the rated value for long enough will damage the dielectric strength and properties of an insulator, if the temp' does not exceed it for long or too high then it is very likely you will see no visual damage but that's not to say that it hasn't started to breakdown, it can become prone to moisture ingress which inturn would cause the obvious issues expected with that.
Another point would be to also note that a very short momentary current spike like in a fault will not damage the cable as long as trip times are met, exceeding these trip times is when these currents will be expected to damage the insulation hence we have to show a circuit meets them, also inrush currents associated with inductive loads are generally fine too but if one has a high inertia load with a long start-up time then you often need to increase the cable size well beyond the actual running current max value associated with such a load or not only would the cable suffer damage but it acts as a choke on the motor start-up and can lead to damage of the motor as well as dragging the mains voltage down.

Like I expressed earlier, I deal with element connections where high temp' terminations are required and sometimes its down to trial and error as finding out the exposed temperatures of these terminations is somewhat a guess work if you have no prior reference points to actually physically measure from.

 

[Midwest]

I just thought if it's written as a regulation in BS7671 (512.1.5) it has to be complied with, otherwise we could all start picking and choosing which regs to comply with and those not. Its pretty clear, unlike some of the others

 

[Midwest]

Yes i dont why i bother calling them ,I never seam to get an answer , this place is much better,

I find emailing them gets you a more detailed measured response

 

[Flanders]

Thanks Midwest , I Have never thought of emailing them I will try asking them the question when i'm back in the office