EV consumer unit tails

[adsmith]

Hi

My relation has had an EV charger fitted and upon me being nosey checking out the job I noticed something I thought I'd query with you guys.

-Her main cut out fuse is 100amp
-From the meter there is 25mm tails that go through DP isolator to Henley blocks.
-From the Henley blocks are 25mm tails feeding the main house consumer unit aswell as 10mm tails feeding a fusebox EV distribution board (soley for the car charger)with spd protection with a 6mm swa cable feeding the 7.4kw EV car charger.

My question is are 10mm meter tails safe as the supply to the Ev distribution board with the main cut out fuse being 100amp? I would have assumed 16mm tails minimum would have been used?

Your help will be greatly appreciated

 

[littlespark]

It’s ok.

The 10mm tails are protected downstream in the ev did board. I presume a 32 or 40A OCPD?

If there’s a problem with the ev, that OCPD will trip protecting the 6mm downstream and the 10mm upstream.

 

[Bidges]

It's common practice, but it's still done as a minor deviation to save money and make the job simpler.
The 10mm tails will not safely carry 100A in the event of a fault (nor would 16mm tails), but in reality the fix load brigade will site that the 32A EV charger cannot overload the tails, which is true. But imagine if the tails became damaged say due to a faulty/no grommet where they enter a metal enclosure. Someone could well be calling the fire brigade when the tails go up in flames before the main fuse blows.
For this reason I don't subscribe to fixed loads making it ok to fit under sized cables for the OCPD.

 

[James]

It's common practice, but it's still done as a minor deviation to save money and make the job simpler.
The 10mm tails will not safely carry 100A in the event of a fault (nor would 16mm tails), but in reality the fix load brigade will site that the 32A EV charger cannot overload the tails, which is true. But imagine if the tails became damaged say due to a faulty/no grommet where they enter a metal enclosure. Someone could well be calling the fire brigade when the tails go up in flames before the main fuse blows.
For this reason I don't subscribe to fixed loads making it ok to fit under sized cables for the OCPD.

Don’t get confused between overload and fault current

 

[DefyG]

But imagine if the tails became damaged say due to a faulty/no grommet where they enter a metal enclosure.

That would result in a current far higher than 100A but for milli seconds until the main cutout fuse pops! but would be fault current not overload current!

 

[Bidges]

That would result in a current far higher than 100A but for milli seconds until the main cutout fuse pops! but would be fault current not overload current!

All depends on the severity of the fault. A dead short would take the fuse out fast enough for it not to be an issue, but a fault with a low enough resistance could allow the fuse to hold long enough for the tails to burn out. Otherwise what's the point in fitting cables which are rated to their OCPD?

 

[Pretty Mouth]

All depends on the severity of the fault. A dead short would take the fuse out fast enough for it not to be an issue, but a fault with a low enough resistance could allow the fuse to hold long enough for the tails to burn out. Otherwise what's the point in fitting cables which are rated to their OCPD?

Did you mean 'high enough resistance'?

Consider what would happen if a theoretical fault such as you describe happened between Line and CPC. The CPC, in this case the earthing conductor, probably wouldn't be sized to take a continuous 100A would it? A similar fault could also happen on a 32A radial wired in 4mm T+E. Again the 1.5mm CPC wouldn't be rated for the full 32A that the fault could theoretically sustain. There are many other common circuits where the CPC isn't sized to take a continuous current of up to the OCPD rating.

In any of these scenarios, would you be concerned about the CPC burning out due to it being rated lower than the OCPD? Would you design the circuits so that the CPC could take the full continuous current of the OCPD?

 

[mainline]

All depends on the severity of the fault. A dead short would take the fuse out fast enough for it not to be an issue, but a fault with a low enough resistance could allow the fuse to hold long enough for the tails to burn out. Otherwise what's the point in fitting cables which are rated to their OCPD?

The cpc only has to carry current under fault conditions, and then only long enough for the circuit breaker or residual current device to trip. In contrast, the live/neutral has to carry load current.