Clarifying The Earth Cabling Requirements

[petercsea]

There was mention that an earth cable for connection to a circuit should be 4sq mm with 2.5 sq mm Phase and Neutral wires. I notice that 2.5 sq mm, 3 core has 1.5 sq mm earth.

Does this mean that only a discrete earth wire has to be 4 sq mm or should three core cable also have 4sq mm earth wire?

Thank you

[Crossy]

At least you've aquired some 3 core

The rule always used to be the ground conductor must be the same size (or bigger) as the active, this is still a good rule and if you're using discrete wires (building wire) in conduit good to stick with.

However with the advent of quick acting MCBs (rather than wired fuses) and ELCBs the requirement for the earth to pull massive currents in order to trip the safety device is somewhat reduced (in a domestic environment). The wiring regs now specify an earth loop resistance rather than an absolute wire size.

Use the 3 core and then measure the resistance between the furthest socket E connection and the ground bar in your consumer unit, if it's less than 1 ohm you're good to go Don't forget to measure the resistance of any trailing lead you use to make this measurement and subrtact it from the actual reading.

1.5mm² copper is about 0.012 ohms per metre so as long as your runs aren't excessive (>40m) it will be just fine

Edited June 18, 2008 by Crossy

[petercsea]

At least you've aquired some 3 core

The rule always used to be the ground conductor must be the same size (or bigger) as the active, this is still a good rule and if you're using discrete wires (building wire) in conduit good to stick with.

However with the advent of quick acting MCBs (rather than wired fuses) and ELCBs the requirement for the earth to pull massive currents in order to trip the safety device is somewhat reduced (in a domestic environment). The wiring regs now specify an earth loop resistance rather than an absolute wire size.

Use the 3 core and then measure the resistance between the furthest socket E connection and the ground bar in your consumer unit, if it's less than 1 ohm you're good to go Don't forget to measure the resistance of any trailing lead you use to make this measurement and subrtact it from the actual reading.

1.5mm² copper is about 0.012 ohms per metre so as long as your runs aren't excessive (>40m) it will be just fine

Yep, found a great store with good brand products, only a little bit of tat, extensive stock lines. Qualified engineer as one of the sales people with fluent English.

Thanks for the opinion and info again Crossy.

[david96]

There was mention that an earth cable for connection to a circuit should be 4sq mm with 2.5 sq mm Phase and Neutral wires. I notice that 2.5 sq mm, 3 core has 1.5 sq mm earth.

Does this mean that only a discrete earth wire has to be 4 sq mm or should three core cable also have 4sq mm earth wire?

Thank you

4 sq mm min size for a main earth conductor, 2.5 sq mm for a PE conductor run as a separate earth for a final sub circuit.

2.5sq mm is about 0.008 ohms per metre as a guide. A type C circuit breaker must clear a fault to earth within 0.4 secs,

an RCD less than 0.3 secs Ref AS3000/2007. Most circuits in a residential installation do not exceed 30metres.

[petercsea]

There was mention that an earth cable for connection to a circuit should be 4sq mm with 2.5 sq mm Phase and Neutral wires. I notice that 2.5 sq mm, 3 core has 1.5 sq mm earth.

Does this mean that only a discrete earth wire has to be 4 sq mm or should three core cable also have 4sq mm earth wire?

Thank you

4 sq mm min size for a main earth conductor, 2.5 sq mm for a PE conductor run as a separate earth for a final sub circuit.

2.5sq mm is about 0.008 ohms per metre as a guide. A type C circuit breaker must clear a fault to earth within 0.4 secs,

an RCD less than 0.3 secs Ref AS3000/2007. Most circuits in a residential installation do not exceed 30metres.

Thanks David. Grateful for the detail and effort.