Main protective bonding conductors

[cliffed]

We have tables for the correct sizing of earth conductors & MPBC.
We can also size cpc’s by calculation,can we actually calculate for MPBC.

 

[telectrix]

with PME it's got to be 10mm. otherwise caclulate. with TN-S 6.0mm usually suffices. with TT, a piece of wet string.

 

[loz2754]

Actually main protective bonding conductors cannot be calculated.
544.1.1 except for PME, a main protective bonding conductor shall be not less than half that of the earthing conductor and not less than 6mm.
The same would apply to TT.

 

[cliffed]

Actually main protective bonding conductors cannot be calculated.
544.1.1 except for PME, a main protective bonding conductor shall be not less than half that of the earthing conductor and not less than 6mm.
The same would apply to TT.

Yea thought that,have a look

 

[cliffed]

Sorry that’s not the correct photo, on the Niceic website it says you can calculate the size.
Will try & find it.

 

[Pretty Mouth]

Unless I have misunderstood, 544.1.1 says half of the required (rather than actual) CSA of the earthing conductor. So, for non-PME:

((√I²t)/k)/2

subject to 6mm² minimum

 

[loz2754]

Unless I have misunderstood, 544.1.1 says half of the required (rather than actual) CSA of the earthing conductor. So, for non-PME:

((√I²t)/k)/2

subject to 6mm² minimum

It does say that. I was just being lazy in not typing out the whole regulation. ?

 

[cliffed]

This relates to sizing of MPBC

 

[loz2754]

This relates to sizing of MPBCView attachment 64002

These tables are for sizing conductors other than copper eg steel.

 

[pc1966]

You also need the max I2t for the main OCPD (probably from 5s disconnect BS88 fuse, but not always as MCCB let-though could be higher at max PFC in some cases).

Usually it is simpler just to go with the Table 54.7 values to get the main earth size, and allow for any PME limits from Table 54.8 but for most cases it will be easier just to stock 10mm!

 

[Wilko]

Yes 10mm does me for bondage ...

 

[pc1966]

You also need the max I2t for the main OCPD (probably from 5s disconnect BS88 fuse, but not always as MCCB let-though could be higher at max PFC in some cases).

A quick check for typical main OCPD shows:

• 100A BS88-5 fuse is about 580A at 5 seconds so 1.7MA for I2t leading to 11.3mm for 70C thermoplastic k=115, or 7.4mm for k=176 (separate protective conductor 90C thermosetting)
• Hager 100A MCCB is worst-case just before magnetic trip with I2t around 13MA (from catalogue page 127 graph) leading 31.3mm for k=115, or 20.5mm for k=176

Hence the MCCB needs about 2.7 times the CPC size.

Both MCCB calculated values are above the Table 54.7 sizes for main earth (typically 100A would be 25-35mm line so 16mm CPC there). Halving for 544.1.1 protective bond sizes gives:

• 100A BS88 fuse 5.7mm or 3.7mm => 6mm minimum
• 100A MCCB 15.7mm or 10.3mm => 16mm minimum

 

[cliffed]

These tables are for sizing conductors other than copper eg steel.

Agree & thanks

 

[telectrix]

sooo, basically, it's half the size of the MEC (not necessarily in the right order, oops, or calculate the min.required for MEC and go fer half of that).