Dual Feeder

[Essex1]

Came across a sub-main circuit the other day that was an MCCB panel board being fed from another MCCB panel board by two separate SWA cables. Both were connected the same and were the same size. 

Anyone come across this?  If so, what is the reason behind it?  Is it acceptable?

 

[Manator]

What size were the cables? I would presume they are short runs. It depends on the cables individual capacity to withstand any fault currents for the required 2 hour period.

 

[Essex1]

What size were the cables? I would presume they are short runs. It depends on the cables individual capacity to withstand any fault currents for the required 2 hour period.

2 X 50mm 4 core SWA. About 100 meters. 

Can you explain more ? Not sure I'm with you @Essex1 ?

Would not be the first time.  :innocent

so I have 1 MCCB. From this MCCB there are two SWA cables with each core doubled up and terminated into another MCCB board about 100 meters away. 

It it is as if the installer thinks they are getting the equivalent of a 100mm CSA by doubling up two 50mm cores. 

 

[Manator]

Its a small supply then at least.

100m swa cable sizes

55.43 kW 100amp = 25mm

83.14 kW 150amp =35mm

110.85 kW 200amp=70mm

138.56 kW 250amp=70mm

166.28 kW 300amp = 95mm

193.99 kW 350amp = 120mm

221.7 kW 400amp = 150mm

 

[poni]

yes this is quite common, twin feeder's, a bit like a ring circuit, it keep's the cost down, two 50mm2's carry more than one 100mm2(by the way I know that they don't make 100mm2 ), I have not seen one that small before,

 

[Davethsparky]

It it is as if the installer thinks they are getting the equivalent of a 100mm CSA by doubling up two 50mm cores. 

Actually they will be getting the equivalent of a bigger cable than that. A 100mm cable will only be approx 1.5x the ccc of 50mm, whereas 2x50mm in parallel will have approx 2x ccc of 50mm.

It it is a very common method of running high current circuits, most big submains are run in this manner along with large machine supplies and transformer tails. 

 

[Manator]

@poni I see what you are saying but the individual cores will only carry the amount of current for that cable. Doubled up will not allow each 50mm cable to stand just a tad over 300amp. Which is somewhat nearer to what 100mm cable would (if it existed). If we take the PFC for the section board as over say 3000amp you would have trouble containing the heat induced on the cables before the MCCB tripped out.

 

[NozSpark]

But isn't there something in this reg about each cables protective conductor having to be able to carry the all the fault current??

 

[Essex1]

Right. Now I understand why. 

So the question now becomes. Why not?  Why do we not do this more often with larger supplies?  

 

[poni]

we do

 

[Manator]

But isn't there something in this reg about each cables protective conductor having to be able to carry the all the fault current??

You are right, but the issue here is that doubling up at over 300amp with cable not designed to withstand that is very bad indeed.

Each 50mm 4 core will provide 122mm cpc through the armour.

 

[Essex1]

we do

The first time I have seen it. Or even heard it mentioned. 

 

[poni]

You are right, but the issue here is that doubling up at over 300amp with cable not designed to withstand that is very bad indeed.

Each 50mm 4 core will provide 122mm cpc through the armour.

you must have seen this type of supply before, normally on supply's over 400A , the Local Supply Authority  do this all the time, I know they play their own rules just saying   

 

[Essex1]

you must have seen this type of supply before, normally on supply's over 400A , the Local Supply Authority  do this all the time, I know they play their own rules just saying   

Ok. To be honest I am talking about installations that fall under BS7671. 

 

[Davethsparky]

Right. Now I understand why. 

So the question now becomes. Why not?  Why do we not do this more often with larger supplies?  

The short answer is we do do it more often with larger supplies. Most cubicle type boards I come across have 2/3/4 feeder cables supplying them. 

But it takes a bit more than the simple addition of the ccc to design the circuit, you have to ensure that overload and fault protection is provided sufficiently for all of the cables along with various other considerations. Bs7671 touches on the subject but doesn't go in to too much detail. 

 

[Manator]

I have seen this before poni and you are right. Only trouble is that with 50mm cable over 100m thats a big ask of a small cable if indeed it is supplying 300amp or so.

I²T ?

 

[NozSpark]

This is in the regs @Essex ,,,,, it's the only place that I would have heard of it

 

[Essex1]

I will have a look next week. 

The he current installation is an empty board so no load at all. We have been asked to take it out and install a new 'conventional supply' but wanted to understand the reasons behind the choice of this type of wiring. 

So so it a cost saving excersise?

 

[poni]

as long as this is met

t = k2 S2/ I2

t is the duration in seconds

S is the cross-sectional area of conductor in mm2 

I is the effective fault current, in amperes, expressed for a.c. as the rms value, due account being taken of the 

current limiting effect of the circuit impedances 

k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor
 material, and the appropriate initial and final temperatures.

Essex1 you never said how big the Mccb was

 

[Essex1]

as long as this is met

t = k2 S2/ I2

t is the duration in seconds

S is the cross-sectional area of conductor in mm2 

I is the effective fault current, in amperes, expressed for a.c. as the rms value, due account being taken of the 

current limiting effect of the circuit impedances 

k is a factor taking account of the resistivity, temperature coefficient and heat capacity of the conductor
 material, and the appropriate initial and final temperatures.

Essex1 you never said how big the Mccb was

To be honest I cannot recall. 

The client want 100A 3 phase for the new supply.