Current carrying capacity

[cameronj]

Morning

The company that I work for has recently had an electrical inspection carried out. One of the findings consisted of 'ineffective overcurrent protection of cable due to rating of protective device and CSA of cable'

Basically a 100A isolator was feeding a 16mm SWA. The SWA conforms to BS 5467 and according to Table 4E4A a 16mm Ref. Method E can carry 99A.

Would it be prudent to downgrade the isolator to 63/80A or for the sake of 1A would the 100A Isolator suffice?

In addition numbers quoted in ERA report 69-30 Part V allows 107A for a 16mm SWA.

Thank you for your help

Cameron.

 

[Darkwood]

Morning

The company that I work for has recently had an electrical inspection carried out. One of the findings consisted of 'ineffective overcurrent protection of cable due to rating of protective device and CSA of cable'

Basically a 100A isolator was feeding a 16mm SWA. The SWA conforms to BS 5467 and according to Table 4E4A a 16mm Ref. Method E can carry 99A.

Would it be prudent to downgrade the isolator to 63/80A or for the sake of 1A would the 100A Isolator suffice?

In addition numbers quoted in ERA report 69-30 Part V allows 107A for a 16mm SWA.

Thank you for your help

Cameron.

Hi Cameron.

So this said cable, judging from the quoted tables is a 90c SWA installed on a perforated tray or free air throughout its length and is a 3ph supply.

-Firstly I assume the switchgear is standard and only rated at 70c which means you have to use the tables suitable for that temp or derate the figures from 4E4A. (see note 1 on same page also reg' 512.1.5)
-I take it by 100amp isolator you actually mean Switch/fuse as these are very different things.
-Is this cable only 1m long then, you have not even accounted for any factors to be applied to the cable due to the installation 'crucially the length'.

Without more info about the install I cannot give a actual fuse rating but I would be expecting you need to derate to 63amp as a max size fuse - was this a recent install or just an existing cable been inspected?

Given the info supplied I feel the cable is unlikely as suggested to comply, VD will probably not comply as well as permitted Zs although cable length, proximity to origin would have a big influence on the compliance on these points so if a very short run close to origin then this may not be an issue.

 

[cameronj]

Hi Cameron.

So this said cable, judging from the quoted tables is a 90c SWA installed on a perforated tray or free air throughout its length and is a 3ph supply.

-Firstly I assume the switchgear is standard and only rated at 70c which means you have to use the tables suitable for that temp or derate the figures from 4E4A. (see note 1 on same page also reg' 512.1.5)
-I take it by 100amp isolator you actually mean Switch/fuse as these are very different things.
-Is this cable only 1m long then, you have not even accounted for any factors to be applied to the cable due to the installation 'crucially the length'.

Without more info about the install I cannot give a actual fuse rating but I would be expecting you need to derate to 63amp as a max size fuse - was this a recent install or just an existing cable been inspected?

Given the info supplied I feel the cable is unlikely as suggested to comply, VD will probably not comply as well as permitted Zs although cable length, proximity to origin would have a big influence on the compliance on these points so if a very short run close to origin then this may not be an issue.

Thank you for replying. Yes, the cable is 3 phase, rated at 90C and is installed on tray. It is an existing install fed from a Hager 100A h125d IEC 60947-2 device and the length of run is 20m.

I hope that clarifies some points. If you need any further info just let me know.

Cheers

 

[Darkwood]

I suspect the mccb you quoted is rated approx 70c for the terminals, don't confuse with an max ambient value which maybe approx 40c, check any rating with manufacturers guides as these values vary as to what they imply.

So you have the info you need now - derate the SWA for 70c use, use the BS7671 tables for CCC unless you can provide manufacturers guidelines to the cable if they are higher, work out the CCC of the cable with respect to permitted VD and then you have the required rating for the front end device.

I take it this won't fall under the fixed load clause for OCPD omittance, even if it did you would still need to meet Zs values required for the mccb.

I will stop short of doing the sums for you as I believe I have pointed you in the right direction to do these, if your still unsure then please show your calcs and we can pick up any errors or confirm your sums.
Re' darkwood

 

[cameronj]

I suspect the mccb you quoted is rated approx 70c for the terminals, don't confuse with an max ambient value which maybe approx 40c, check any rating with manufacturers guides as these values vary as to what they imply.

So you have the info you need now - derate the SWA for 70c use, use the BS7671 tables for CCC unless you can provide manufacturers guidelines to the cable if they are higher, work out the CCC of the cable with respect to permitted VD and then you have the required rating for the front end device.

I take it this won't fall under the fixed load clause for OCPD omittance, even if it did you would still need to meet Zs values required for the mccb.

I will stop short of doing the sums for you as I believe I have pointed you in the right direction to do these, if your still unsure then please show your calcs and we can pick up any errors or confirm your sums.
Re' darkwood

All good. Thank you for your help