Earth cable size on three phase

[Pond digger]

Should the earth cable be bigger than the phase conductors, or is it ok to use the same?

 

[milkloss]

When I looked into this it got complicated as it depends on your earthing system. A tt (earthrod) for a 100amp 3ph supply needs to have 16mm2 earth for up to 35mm2 incoming tail size if not mechanically protected, I think it can be much smaller if protected but both answers should be calculated depending on earth rod resistance etc etc.

I’m no leccy so I advise professional opinion but we used a 16mm earth.

 

[Pond digger]

When I looked into this it got complicated as it depends on your earthing system. A tt (earthrod) for a 100amp 3ph supply needs to have 16mm2 earth for up to 35mm2 incoming tail size if not mechanically protected, I think it can be much smaller if protected but both answers should be calculated depending on earth rod resistance etc etc.

I’m no leccy so I advise professional opinion but we used a 16mm earth.

Sorry, I should’ve been more specific: I’m leaving the complicated stuff to an electrician. I was meaning wires in 3phase machine isolators, sockets etc. Most stuff that I have will be fed by 2.5mm2 wire: I was just wondering if the earth wire is ok at the same size, or should you perhaps use 4mm2 for the earth.

Interesting what your saying about main earthing though. I have three phase already, and am taking power from a 63amp fused (32amp) isolator switch via armoured cable to a new distribution board about 15 m from where the main incoming feed is situated. The armoured cable is 4 core 10mm2. Ive been reading a bit about the earthing capacity of the steel armour and the conductive deficiency of steel in comparison to copper: but as said, I’ll be leaving that one for the electrician.

 

[milkloss]

Sorry, I should’ve been more specific: I’m leaving the complicated stuff to an electrician. I was meaning wires in 3phase machine isolators, sockets etc. Most stuff that I have will be fed by 2.5mm2 wire: I was just wondering if the earth wire is ok at the same size, or should you perhaps use 4mm2 for the earth.

Interesting what your saying about main earthing though. I have three phase already, and am taking power from a 63amp fused (32amp) isolator switch via armoured cable to a new distribution board about 15 m from where the main incoming feed is situated. The armoured cable is 4 core 10mm2. Ive been reading a bit about the earthing capacity of the steel armour and the conductive deficiency of steel in comparison to copper: but as said, I’ll be leaving that one for the electrician.

It’ll be fine the same size I expect. It might be that it doesn’t have to be as big as the lives but to save argument I’d go equivalent.

Anything I run now doesn’t use the armour as earthing, much easier to buy 5 core than worry about getting the glands spot on etc. It’s still terminated and earthed as best can be done but I’ve seen enough shaky gland connections to metal isolator boxes to give me the willies!

 

[Pond digger]

It’ll be fine the same size I expect. It might be that it doesn’t have to be as big as the lives but to save argument I’d go equivalent.

Anything I run now doesn’t use the armour as earthing, much easier to buy 5 core than worry about getting the glands spot on etc. It’s still terminated and earthed as best can be done but I’ve seen enough shaky gland connections to metal isolator boxes to give me the willies!

Thanks

I actually quite enjoy fitting glands. I wish I knew more about the technical side of this electrickery stuff, I mean mcb class b, c, and d . Can’t beat a bit of fuse wire I say. But seriously, most of the job just entails screwing stuff to walls and making terminal connection, which I find really satisfying.

 

[cows sh#t me to tears]

If you have a 3ph extention lead , unscrew the plug and have a look at the 4 wires. Pretty sure they will all be same size.

 

[Netherfield]

You will find in Twin and earth cable that the earth is of a smaller diameter, and that technically the neutral does not need to be a big as the live, a three phase board can use the three live wires and just the one neutral for return. And many years ago the electricity companies used to use what was known as a three and a half core cable, where the neutral was only half size of the lives.

 

[jamj]

It is frowned on now to use armour as the earth.
Not sure if regs permit it or not, but I would use 5 core as mentioned above.
As for size, if you are fitting surge protection then larger earth size is needed. Current takes easiest path and that will be down wire/path with least resistance.

 

[Pond digger]

It is frowned on now to use armour as the earth.
Not sure if regs permit it or not, but I would use 5 core as mentioned above.
As for size, if you are fitting surge protection then larger earth size is needed. Current takes easiest path and that will be down wire/path with least resistance.

Bit late to be suggesting 5 core I’m afraid. They still sell 4 core armoured so can’t imagine it’s not permitted to use the armour for earthing. I’ll see what the electrician says when he comes to check things out, but the issue has never been raised previously.

 

[TheTallGuy]

Use of armour is still permitted where the calculations show it gives an adequate CSA for steel.

 

[milkloss]

Use of armour is still permitted where the calculations show it gives an adequate CSA for steel.

This would be up to a conductor size of 240mm2 for 4core swa!

 

[TheTallGuy]

https://www.diynot.com/wiki/Electrics:SWA-earthing

 

[milkloss]

https://www.diynot.com/wiki/Electrics:SWA-earthing

Wasn't far out for an amateur!