Volt drop calculations - a couple of questions

[soms]

Hi, I currently am working on some cable sizing calculations for an exercise in which we apply our knowledge and use of the OSG. We have a ficticious single-phase, light commercial installation for which we have designed the circuits to supply a range of requirements and loads. We have selected the accessories, cable types, containment systems etc.

These have resulted a cooker circuit, water heater circuit, two ring final circuits and two lighting circuits.

I have been applying the process for sizing the cables as follows:
1. determine the design current Ib
2. select the over-current protective device In
3. apply correction factors to determine tabulated current carrying capacity It
4. check volt drop
5. finally I will check ADS/shock protection, when I get that far!

All cables are flat PVC/PVC/Cu thermoplastic.

So I am on step 4 and am a bit stuck. I understand the calculation for volt drop and where to get the appropriate values for the calculation but I am not certain on when and how it should be applied to some circuits.

For example, for ring final circuits serving BS1363-2 socket outlets, if you meet all the specifications in Appendix H, are these circuits exempt from this separate calculation?

And perhaps more importantly, for lighting circuits, I am getting especially concerned, where the volt drop must be a maximum of 3%. Please could anyone offer some advice on how the circuit length should be calculated?

Each of my two lighting circuits serve a few rooms each. Allowing for 100W per lamp position the design currents are 4.3 and 5.9A.
Should the calculation include the total cable length of the feed, switch wires and final supply to the lamp?
In which case, would this be undertaken for the lamp with the longest amount of cable on the circuit?
For example amongst my rooms I have two way and intermediate switched lamps with switches quite far apart so this would soon add up.

Also for the Amps, is the design current used or In, as this would also have quite a different depending on the values?

Thanks for any advice.

 

[Richard Burns]

For final circuits that comply with the tables in part 7 of the on site guide then the voltage drop is taken into consideration and circuits that do not comply are indicated by the maximum length of circuit available, without a qualification reference, as volt drop is often the primary limit of a final circuit.(if there is a zs or something then this is the limiting factor).

For a very long lighting circuit it is possible, but not normally applied in training, to break down the current load across the length of the circuit so if you had five light fitting close to the origin and one light fitting a long distance away then the design current in the long distance would only be one light fittings load.

Use the design current Ib not the In.

 

[davesparks]

Accounting for the switch drop in a VD calculation is an interesting question, and in my opinion the answer will depend on the exact method you use to calculate the VD.

If you are calculating VD based on the full Ib being applied at the furthest light point then you will get a higher calculated VD than you get in reality due to the loads being spaced out along the circuit. The difference between your calculated value and the actual value will be well above the actual VD in the switch drop (except in exceptional circumstances) so your circuit will in reality comply. But this is a common sense approach rather than a strict accuracy approach.

If you are calculating VD by breaking the circuit down and using the actual load in each leg of the circuit etc then you will need to include the switch drop. But whilst this method will give a more accurate assessment of VD it is well and truly over the top for a regular lighting circuit. You would usually only use this approach for long runs of lights such as lamp posts or other external lighting where a worthwhile money saving can be made by dropping cable size along the run.


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[soms]

Thank you for your replies, the approach of breaking down the circuit makes complete sense but did not occur to me. I will look into it and apply the Ib.

The actual load will be very much lower as I specified a mixture of LED and 36W HF batten lamps. To even use 100W lamps all the luminaires would need to be changed, which whilst this could happen I imagine would now be very unlikely in practice.

 

[Richard Burns]

I would do the calculation twice once for the "approved" 100W per fitting and once for the actual load where the lamp rating is limited.
This will show you are aware of the recommendations but can also apply common sense approaches in practice.