ring main ?

[frankie310581]

How do you work out volt drop in a ring main as it has now end of circuit

 

[Bacon]

The ends are in the CU if it is fact a (ring main) Ring final circuit as you have stated.

Or have you got 2 radials in the CPD in the CU?

Measurement or calculation if you know the overall distance from first to last or estimate the overall length which would then give you an idea of the overall run of cable used.

 

[SLIPSHOD & SLAPDASH]

A ring main is in the street under the pavement.

 

[PC Electrics]

VD = vd x L x 0.008

where:

VD = calculated volt drop in volts

vd = volt drop of cable in mV/m

L = total end to end length of ring final circuit

This assumes a 32A point load at the midpoint.

the standard circuits in the OSG list the max length of a RFC as 106m, based on a 'standard' distributed load.

---------- Post Auto-Merged at 18:49 ---------- Previous post was made at 18:35 ----------

If you are measuring L, then:

L = (r1+rn) / (2R)

where R = resistance per metre of cable from tables.

 

[Sidewinder]

If you know the loading at the points in the ring and the actual cable runs between then, you can use Kirchoffs laws to determine the current distribution through the ring, then, use this to calculate the volt drop.

However, this would ONLY be valid for the loads and points you use to work this out.

 

[PC Electrics]

Indeed. The 'standard' distributed load used in the OSG has 16A at the mid point with 8A evenly spread on each leg (IIRC) and so can be considered 'worst case' since it is actually impossible to apply a 32A point load on an RFC out of one 2G socket.

 

[revjames]

Blimey Didy, havent had a run in with Mr Kirchoff since BTEC days!

 

[frankie310581]

But voltage flows out on both legs so if your ring main is 60m long when you calculate vd is length 30m

 

[SLIPSHOD & SLAPDASH]

All you need to know about ring mains here

 

[Sidewinder]

But voltage flows out on both legs so if your ring main is 60m long when you calculate vd is length 30m

Only if your load is at one point

 

[PC Electrics]

But voltage flows out on both legs so if your ring main is 60m long when you calculate vd is length 30m

This is taken care of in the formula i gave you. Do you really think I'd have forgotten about such an important point?

---------- Post Auto-Merged at 21:10 ---------- Previous post was made at 20:56 ----------

Only if your load is at one point

It doesn't matter how many load points there are, 1 or 100, current flows around both legs. The current balance can be calculated for any number of loads and their positions around the circuit.

BTW: the total voltage drop is the same wherever the load(s) is/are on the ring, too.

Kirchoff.

 

[Sidewinder]

Will it?

IIRC the volt drop will not be equal at every point in the ring if the loads are not equal, I can't remember now it's late and it's a long time since I have done a ring volt drop / current balance calc!

Don't see much point as you can never be sure of the loads and if you put 20A at every outlet your ring is overloaded probably!

The sum of the current at every node = zero is the current one yes?

Can't remember the voltage one now.

 

[PC Electrics]

TBH it's pretty academic really. I don't suppose many people design 4 or 6mm rings on 40 or 50 amp breakers for industrial do they?? In which case as long as you measure the total length as 106m (without spurs) or less it complies. Slightly more, code 3 it, loads more and it's a 2.