Industrial socket cable calculation

[Lc1]

Hi,
I have a question regarding the cable size to a 415v 125A socket. For the design current is it normal to use the protective device rating of 125A as the design current for the calc?

Thank you

 

[Electro-tech]

What is the socket actually going to supply, & length of run.

 

[Lc1]

Thanks for the reply. Socket will supply a temporary building and and length of run from the power source to the socket will be about 15m max. Thanks

 

[Electro-tech]

Thanks for the reply. Socket will supply a temporary building and and length of run from the power source to the socket will be about 15m max. Thanks

Maximum demand for cable then? Seems a bit of a strange setup. What about Isolation, other than a MCCB. Need to know the whole installation design?

 

[pc1966]

If you don't know it is a known fixed load for certain (and for temporary building with open use that is not fixed) then you would have to protect the socket by OCPD of no more than its 125A rating. However, you might have to drop that depending on the disconnection behaviour (end of run Zs, etc) if only OCPD and no RCD aspect.

 

[buzzlightyear]

not knowing the loading ,how come the op asking the question to fit one and not enough information.

 

[Rockingit]

Use of a 125A socket does not necessarily dictate that the circuit is running at that. For example, a 32A draw over any distance will result in a cabe size that no longer terminates into a 32A plug and socket. It could also, classically, simply be what's in the back of the van.

But to answer the OP - the design current is whatever it needs to be: You might have a 125A capacity circuit with a 4A toaster plugged in to it - in which case using a 10A MCB would be perfectly acceptable (odd.... but acceptable!). The question that you actually need to answer is that if you use a 125A MCCB for your CPD you will end up with a very low figure indeed as a max allowable Zs, even at 5secs - and after 15+ metres you'll be really - really - lucky to achieve that.

As others have already said though is that you absolutely cannot go any higher than the max ratings (after de-rates as well) for capacity of the cable or max design current for EVERY component in that circuit.

 

[Lc1]

Thanks for the replies. The client is unsure of the cabin load at this stage. The client has advised a preliminary figure of a 3ph 415v 125A supply.( it has been assumed the cabin load will be 80% of 125A) Also we do not know where the cabin will land yet, it has been advised that a socket will best suit this as the client can cable the cabin to the socket in the vicinity. The distance between the socket and the power supply is 15max. The distance between the cabin and the socket is not known but is believed to be relatively close to the socket.

Thanks again.

 

[Electro-tech]

So Mr Client is going to site his cabin & plug it into your socket in the vicinity. Hopefully, your socket is in an External; weatherproof Cabinet. I think this whole scheme is madness to allow a third party access to your equipment to connect a 3 Phase supply of that rating. I would be asking for more details of Mr Client's cabin Installation as to whether it is wired correctly & safe to connect to your socket.
Who is going to be liable?

 

[Rockingit]

OK - so what we're actually talking about here is you being asked to stick a 125/3 commando outlet on a wall somewhere, protected by a 100A MCCB / CPD. Nowt wrong with that. Zs to the outlet can be 5s on that, if you've the time/budget available you could even stick a 100mA time-delay RCD in there as well for good measure.

The important thing, here, is that what gets plugged into it after you've left is not part of your remit or responsibility. You're not signing off on 'a cabin', you're signing off on a pretty simple socklet outlet.

 

[davesparks]

It's a 125A socket, design it for 125A.
You don't know what it's going to be used for in the future, it could be anything, which is kind of the whole point of sockets (or at least one of the points)

 

[davesparks]

Hopefully, your socket is in an External; weatherproof Cabinet.

Why?

I think this whole scheme is madness to allow a third party access to your equipment to connect a 3 Phase supply of that rating.

Why? Myself and a few other members do this on a regular basis, and with much bigger sockets than 125A.

I would be asking for more details of Mr Client's cabin Installation as to whether it is wired correctly & safe to connect to your socket.
Who is going to be liable?

Again why? Assuming the OP is being employed to install a socket circuit then their responsibility is to install a socket.
Do you always ask for full details for what is going to be plugged in when someone asks you to install a socket?

As far as liability the OP is liable for the socket installation, the builders of the temporary building are liable for that installation and the person who plugs it in/authorises the plugging in is liable for it being energised.

 

[Electro-tech]

Why?



Why? Myself and a few other members do this on a regular basis, and with much bigger sockets than 125A.



Again why? Assuming the OP is being employed to install a socket circuit then their responsibility is to install a socket.
Do you always ask for full details for what is going to be plugged in when someone asks you to install a socket?

As far as liability the OP is liable for the socket installation, the builders of the temporary building are liable for that installation and the person who plugs it in/authorises the plugging in is liable for it being energised.

I would not be happy installing a 125A socket outside without adequate protection & local isolation. I would also ask for details of equipment & maximum demand of the cabin installation, a question often asked by your DNO before connecting a supply. I would then feel that I have designed & installed a adequate installation to satisfy both parties needs.
This is only my personal opinion & way of working & I am sure that the OP will come to there own conclusions to follow. So far all replies are based on assumptions with lack of design information.

 

[davesparks]

I would not be happy installing a 125A socket outside without adequate protection & local isolation.

Which a normal IP rated switched socket will satisfy, no need for additional enclosures. And if you fit it inside an enclosure how are you going to get the cable in there to plug it in?

I would also ask for details of equipment & maximum demand of the cabin installation, a question often asked by your DNO before connecting a supply. I would then feel that I have designed & installed a adequate installation to satisfy both parties needs.
This is only my personal opinion & way of working & I am sure that the OP will come to there own conclusions to follow. So far all replies are based on assumptions with lack of design information.

Personally if I'm asked to install a socket for temporary connections I'll do just that, install a socket for temporary connections.

 

[Lc1]

Thank you for the replies. It has been very helpful