Ring, Radial and Earth

[DpreshunGromit]

Hey guys!

Hope some of you can help me out quickly!

So I've got incoming PV and battery supply of 2000W at roughly 8.7A. This is through an RCBO and to a RCCB 30mA garage style consumer unit, just with a 16a and 6a MCB. The 16A serves 5 sets of double sockets with about 15M of 2.5mm.

1st Question: I've got a door in the way of the ring so I'd have to backtrack, will I be fine just doing a radial circuit instead? It's not much distance so feeding from both ends doesn't seem like a huge concern to me safety wise.

I'm using an earthing rod outside for this.

2nd Question: As I've not got much that can spike can I get away with running standard 10mm 1-core PVC conduit outside rather than 16mm, so long as I bury it deep?

Thanks guys!

 

[SparkyChick]

These question really depend on factors you've not included, such as expected loading and length of cable runs.

If you're burying the cable directly in the ground you should use SWA.

And... as you're a trainee, do you have access to suitable test gear and the knowledge to use it? If you're training with someone, maybe they could oversee the work and you can get some knowledge at the same time. Plus there is the small factor of the building control notification you're going to need for running in new circuits.

 

[Pete999]

Hey guys!

Hope some of you can help me out quickly!

So I've got incoming PV and battery supply of 2000W at roughly 8.7A. This is through an RCBO and to a RCCB 30mA garage style consumer unit, just with a 16a and 6a MCB. The 16A serves 5 sets of double sockets with about 15M of 2.5mm.

1st Question: I've got a door in the way of the ring so I'd have to backtrack, will I be fine just doing a radial circuit instead? It's not much distance so feeding from both ends doesn't seem like a huge concern to me safety wise.

I'm using an earthing rod outside for this.

2nd Question: As I've not got much that can spike can I get away with running standard 10mm 1-core PVC conduit outside rather than 16mm, so long as I bury it deep?

Thanks guys!

Hi Mate and welcome, I'm confused regarding your 2nd question.
"Ive not got much that I can spike"? and You talk of 10mm 1 core PVC Conduit rather that 16mm and burying deep, might be me but I don't understand. can you explain a bit more please?

 

[DpreshunGromit]

Sorry, it's for a friends garden building/shed run off an inverter. The earthing cable won't actually be buried, it will run down the side of the building for a metre or two and be fed to a rod. At the moment it's only going to be running LED lights but she wants to be able to plug in a TV and maybe a cooker in the future.

 

[davesparks]

Sorry, it's for a friends garden building/shed run off an inverter. The earthing cable won't actually be buried, it will run down the side of the building for a metre or two and be fed to a rod. At the moment it's only going to be running LED lights but she wants to be able to plug in a TV and maybe a cooker in the future.

What is the rod actually doing? Will it connect to neutral at any point?

Where does the ring circuit fit in to all this? Your OP mentions a 16A and 6A circuit but then you talk about a ring?

 

[DpreshunGromit]

The rod will connect to the earth bar of the CU and also ground the inverter.

The 6A is lighting LED circuit.
16A is for the sockets. It's safer to connect both ends isn't it?

 

[SparkyChick]

I think some of the confusion is arising from a lack of clarity in your description of what you're trying to do, so maybe start at the beginning... you mention a solar inverter, so is this the only source of supply for the consumer unit?

A diagram may help make things clearer.

You would only connect both ends if you want to create a ring final circuit, there is nothing stopping you doing so from a 16A circuit breaker, but we would typically use a radial circuit of a 16A breaker because 2.5mm T+E can quite readily handle it. The ring is only required if you're planning on putting it on a 32A breaker.

 

[davesparks]

The rod will connect to the earth bar of the CU and also ground the inverter.

The 6A is lighting LED circuit.
16A is for the sockets. It's safer to connect both ends isn't it?

Is the neutral of the inverter connected to the earth rod at any point?

No its not 'safer to connect both ends'

 

[DpreshunGromit]

Ring halves power on each end doesn't it, preventing potential overloads? Thanks for clarifying SparkyChick, that makes sense. I'll run radial as it's not much distance and only 16A.

I've thrown in a quick diagram to try and make things a bit clearer, hope it makes some sense of what we are trying to do.

 

[davesparks]

Ring halves power on each end doesn't it, preventing potential overloads?

View attachment 56374

No, a correctly designed and installed ring final circuit will evenly distribute the current in the ring so that a cable rated at 20A can be safely used on a 32A OCPD.
A ring does not prevent overloads, it can still be overloaded as easily as any other socket circuit by connecting too many loads and switching them all on at once.

 

[DpreshunGromit]

No, a correctly designed and installed ring final circuit will evenly distribute the current in the ring so that a cable rated at 20A can be safely used on a 32A OCPD.
A ring does not prevent overloads, it can still be overloaded as easily as any other socket circuit by connecting too many loads and switching them all on at once.

But the amount of current needed to reach a distant point in a radial circuit would be more than in a ring circuit. This even distribution of current does make it safer, although I understand there's far more factors in play. Maybe prevention was the wrong word to have used, sorry!

 

[davesparks]

But the amount of current needed to reach a distant point in a radial circuit would be more than in a ring circuit. This even distribution of current does make it safer, although I understand there's far more factors in play. Maybe prevention was the wrong word to have used, sorry!

The amount of current that is needed to reach a distant point for the same load is exactly the same. The laws of physics don't change just because you make a circle out of the conductors.

 

[DpreshunGromit]

The amount of current that is needed to reach a distant point for the same load is exactly the same. The laws of physics don't change just because you make a circle out of the conductors.

OK, but some power is lost through heat. Greater distances, greater loss, greater heat. Add a high load and then there is an increased risk. It is simply something I found while researching my question and doing my own studies. I apologise if I am not wording it well, I'm not as advanced as many on here. If you want to help with my question like SparkyChick then that's great and I appreciate the help but I feel this is going off piste somewhat.

 

[davesparks]

If you want to help with my question like SparkyChick then that's great and I appreciate the help but I feel this is going off piste somewhat.

I've asked twice if the neutral of the inverter is connected to the earth rod, and asked what the purpose of the earth rod is, but you you've not yet answered the questions.

I think rather than worrying about the incredibly small amount of power lost through conductor heating in a final circuit you should focus on more important parts of the design.

Why is there an RCBO in the DC cables from the solar panels?

 

[DpreshunGromit]

Sorry, I think that should be a DC circuit breaker. I made the diagram in a bit of a rush to show what was going on.

I can't see on the inverter instructions anything about connecting the neutral to earth rod. Without the rod there would be no grounding surely?
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Is it advised to use a common ground between AC and DC systems in this way? There is a (I think steel) metal frame/platform raising the room which must be taken into account.