Verification of voltage drop

[SgtTrojan]

Reg 612.14 - BS7671:2008 States:
Where required to verify compliance with Section 525 the voltage drop may be evaluated by:
i) measuring the circuit impedance
ii) using calculations using cable length v load current for different cable sizes.

Now..

Issue 1 - The regulation does not mention "by measurement". Why not?

Issue 2 - Regarding option i), what do you assume this is referring to?
Does it mean use the sum of R1 and Rn and multiply that by the design current? (for parallel v.d)
Does it mean use R1 and multiply it by the design current (for series v.d)
Does it mean that as long as Zs is verified we assume the voltage drop is acceptable? (which doesn't seem likely having just done a random calc using a design current and the Max Zs for a device). This clearly includes the Ze though.
Does it mean something else?

Issue 3 - If i) does require a calculation why does it not state it?

 

[echoes]

Does it not mean that in order to evaluate voltage drop, one can:
(1) Measure the impedance of the conductors directly and use that to calculate the voltage drop at given loads; or
(2) Calculate the impedance of the cable according to its length and CSA and use that to further calculate the voltage drop at given loads.

Issue 1 - The regulation does not mention "by measurement". Why not?

Because one does not have to perform a measurement to calculate voltage drop when the length and CSA are known.

Issue 2 - Regarding option i), what do you assume this is referring to?
Does it mean use the sum of R1 and Rn and multiply that by the design current? (for parallel v.d)
Does it mean use R1 and multiply it by the design current (for series v.d)

It means that the impedance of the conductors determines the voltage drop for a given current at the far end of the circuit. e.g. Vd = I (R1 + Rn). Since the same current flows through R1 and Rn in this single phase example, both these impedances must be taken into account.

Does it mean that as long as Zs is verified we assume the voltage drop is acceptable?

No, although R1 is a common factor in determining Zs and Vd. Some cable lengths will be limited by Zs, and others by voltage drop.

Issue 3 - If i) does require a calculation why does it not state it?

It implies a calculation based upon the measured impedance.

 

[ricicle]

You can measure the L to N impedance with a loop impedance tester.
If you measure it at the origin and at the extremity of a circuit the difference is the circuit impedance. Using Ohm's law you can can work out the VD for a known load current.
Conversely you can work out the impedance of a circuit by measuring VD for a known load.

 

[plugwash]

Issue 1 - The regulation does not mention "by measurement". Why not?

Direct measurement is a problematic way to determine volt drop because to get it by direct measurement you would have to ensure the circuit is loaded to it's maximum configuration

Issue 2 - Regarding option i), what do you assume this is referring to?
Does it mean use the sum of R1 and Rn and multiply that by the design current?

What we are trying to determine is the total volt drop that the end device sees. In a simple case of one load at the end of a single phase circuit that is the calculation you would be doing.

Issue 3 - If i) does require a calculation why does it not stateit?

Applying equations blindly is a receipe for mistakes. In particular the calcuations involved for volt drop depend hugely on the topology of the circuit and where the load is located (all load at the far end being obviously the worst case). For three phase circuits there is the further complication that for a balanced load In is 0 but if all the load is on a single phase it will be as high as the current in that phase. Real life situations are likely to be somewhere in between.