Any design must ensure the maximum voltage drop allowed between source and point of use is not exceeded when at full load. In the 16th edition this as 4% of the nominal supply voltage (about 9.2V at 230V AC). The 17th edition (in force from 1 July 2008) limits this to:

• 3% for lighting
• 5% drop for other loads
• (but 6% and 8% respectively are permitted for private supplies)

Conductor CSA (mm²)	PVC (max 70° C) Voltage drop mV/A/m	XLPE (max 90° C) Voltage drop mV/A/m
1.0	44	46
1.5	29	31
2.5	18	19
4.0	11	12
6	7.3	7.9
10	4.4	4.7
16	2.8	2.9

 
These figures are presented for SWA cable here. The values quoted will usually also be correct for other copper cable types. Figures will differ slightly for cables with higher operating temp limits, such as MICC. The length figure above is per length of cable, not per each conductor.

These figures assume that the cable is operating at or near its maximum operating temperature. In designs where the maximum current load is significantly less that the capacity of the cable then these figures will be somewhat pessimistic. For these circumstance, it would be more appropriate to calculate voltage drops based on table 9A of the official I.E.E. “On Site Guide”.

Calculation Examples (PVC SWA):

1) 20m of 4mm², maximum load of 30A would drop 20 x 0.011 x 30 = 6.6V
2) 40m of 6mm², maximum load of 45A would drop 40 x 0.0073 x 45 = 13.14V
3) 10m of 1.5mm², maximum load of 16A would drop 10 x 0.029 x 16 = 4.64V

(1) and (3) are adequately specified with respect to voltage drop. However (2) is out of spec and a larger cable will need to be selected, even though the current handling capacity of the 6mm² cable has not been exceeded. Upgrading to 10mm², gives a result of 40 x 0.0044 x 45 = 7.92V which is acceptable. It initially appears that this still only leaves just over 1V of remaining drop available for any following wiring, however since we will not be operating the larger cable anywhere near its maximum temperature, the situation is actually less tight than the calculation suggests.

Conductor Colours and Harmonisation
Historically the UK used its own set of colour codes for fixed and flexible wiring. In the early 1970s the colours used for flexible wiring were harmonised across Europe, however the colours for fixed wiring remained unchanged until recently. In March 2004 the fixed wiring colours were also harmonised. During a transition period that began on 31st of March 2004 and ended on the 31st March 2006, use of either colour scheme was acceptable. Now only the harmonised colours may be used. The result is that it is now possible to encounter installations that use both colour schemes, and hence great care must be taken to ensure conductors are correctly identified when carrying out any work.

Conductor Colour Coding (single phase T&E)Conductor	Old UK Colour	Harmonised Colour
Live (Phase)	Red	Brown
Neutral	Black	Blue
Earth or CPC	Green / Yellow Stripe	Green / Yellow Stripe

 

Conductor Colour Coding (three phase / Triple and Earth / SWA)
Conductor	Old UK Colour	Harmonised Colour	2 Core (T&E or SWA)	3 Core (3&E or SWA)	4 Core (SWA)	5 Core (SWA)
Live 1	Red	Brown	X	X	X	X
Live 2	Yellow	Black		X	X	X
Live 3	Blue	Grey		X	X	X
Neutral	Black	Blue	X		X	X
Earth (CPC)	Green / Yellow Stripe	Green / Yellow Stripe	X[1]	[1][2]	[2]	X

 
Notes

1. Earth wire in cable will be a bare conductor and will need green/yellow sleeving to be applied at point of termination.
2. Earth wires are present in T&E or 3&E cables. Other cable types such as SWA or MICC will still need to be earthed, however this will be carried using the cable armour / screen.

There is an IEE leaflet explaining these changes. That is available here

Installations that are wired using cables to both colour schemes should carry a warning sticker (see example) on or near the consumer unit that statesand looks something like this:

CAUTION This installation has wiring colours to two versions of BS7671. Great care should be taken before undertaking extension, alteration or repair that all conductors are correctly identified.