Odd ball mains supply

[Keef]

Found a strange one at a golf club:

Supply to 2 x no 63 A sockets feeding 2 x temp portakabin supplies.

Fed in 3 core 10mm from a 3 pole isolator Brown = Line 1; Black = Line 2; Blue = N; Sheath = CPC

Goes to a box with the 2 x 63A sockets on with the neutral being split and the sheath isolated and an earth rod so the portakabins are TT about 3M away from sockets & earth rod combo. Before the isolator is 2 RCD's and 2 x 63A MCB's (1 per circuit).

Its a single phase incomer of 100A, there is no overload issue here and isolation for the cable seems to be in order.

Loading in cabins varies but portakabin 1 (office) has got 3 x 2kW heaters in and a water heater, portakabin 2 is lighting, heater and workshop type kit (drills, bench saws) equipment.

Its odd due to the neutral being split and what current is going to go down it as it is all single phase.

Info on how to calculate this and opinions please.

 

[snowhead]

Imagine a 3 phase incomer to a property.
3 phases and Neutral all the same size conductors

2 phases fully loaded, 1 phase nothing.

Is the Neutral overloaded?

 

[Keef]

Imagine a 3 phase incomer to a property.
3 phases and Neutral all the same size conductors

2 phases fully loaded, 1 phase nothing.

Is the Neutral overloaded?

Yea that was my thoughts, but it just got me thinking (too much probably). As its single phase the chances are the neutral can get overloaded, but i need to calculate to be sure. I can never get into the office building (rented by someone else) to turn everything on and check so may need to rely on calculating it.

 

[davesparks]

Imagine a 3 phase incomer to a property.
3 phases and Neutral all the same size conductors

2 phases fully loaded, 1 phase nothing.

Is the Neutral overloaded?

No, but that is entirely irrelevant as the question is about a single phase supply.

The neutral is carrying the combined neutral current from both 63A supplies.

 

[davesparks]

Yea that was my thoughts, but it just got me thinking (too much probably). As its single phase the chances are the neutral can get overloaded, but i need to calculate to be sure. I can never get into the office building (rented by someone else) to turn everything on and check so may need to rely on calculating it.

Surely the answer is obvious?

The neutral currents will combine, giving you a potential 126A down that 10mm neutral, the 100A cutout fuse will happily let 126A flow for quite a while.

This is why the regulations don't allow for two circuits to share a neutral.

 

[Meggerman]

Can you install a TP 32A MCB upstream of the two sockets.

 

[davesparks]

Can you install a TP 63A MCB upstream of the two sockets.

He could, but what would be the point?

 

[snowhead]

No, but that is entirely irrelevant as the question is about a single phase supply.

The neutral is carrying the combined neutral current from both 63A supplies.

Missed the single phase incomer, only saw the " Brown = Line 1; Black = Line 2"

 

[Knobhead]

Neutral currents on a poly-phase system aren’t cumulative

In = √((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 

[davesparks]

May I respectfully draw your attention to posts 1,3,4 and 8 Mr Tony sir, the installation appears to be of the single phase variety

Now why doesn't this forum have that nice 'poking with a stick' smiley? You'll just have to imagine it for now.

 

[Engineer54]

Now why doesn't this forum have that nice 'poking with a stick' smiley? You'll just have to imagine it for now.

You mean this one, .......:30: lol!!

 

[davesparks]

Yeah that's the one!

 

[LeeH]

Neutral currents on a poly-phase system aren’t cumulative

In = √((Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic)))

 

[davesparks]

Seriously, it's the basics of three phase man! What kind of electrician are you?

 

[mechelec]

If I have interpreted your post correctly you have got two circuits ie fed from two separate OCPD's in one cable but sharing (borrowing) the same neutral. I would have thought the OCPD's would need to be two pole or linked so one trips both trip and giving one correct point of isolation.

 

[Knobhead]

May I respectfully draw your attention to posts 1,3,4 and 8 Mr Tony sir, the installation appears to be of the single phase variety

Now why doesn't this forum have that nice 'poking with a stick' smiley? You'll just have to imagine it for now.

Looks like I’m not the only one to miss that. Like others I read L1 L2.

So the neutral currents are cumulative in this instance.

 

[telectrix]

cumulative (def.) ..... added together..... melted cable...... fire.... oops.