Values In Table 7.1 Of Osg

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Sam Donaghy

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Where do the values in Table 7.1 of the OSG come from?

If I look at a 6A Type B Circuit Breaker with 1.5/1.0 mm2 cable, Table 7.1 (p57) gives a max circuit length of 90 metres.

But using Table F6 gives a Volt Drop of 29 mV/A/m for 1.5mm2 cable so 90 metres at 6A would give a volt drop of (29x6x90/1000) = 15.66V. This is way above 3% which is 6.9V. In fact about 27 metres would give a volt drop of 3.88V.

Where do the values in Table 7.1 come from?

What am I missing?

Sam

 
Welcome to the forum Sam...

First you have identified one of the key issues about wiring regs....

THEY ARE NOT A SET OF BLACK AND WHITE RULES THAT MUST BE BLINDLY FOLLOWED WITHOUT UNDERSTANDING.

1/ THE TYPE OF INSTALLTION..

2/ HOW IS THE PARTICULAR REG APPLIED IN THE REAL WORLD.

So first off...

Your table on page 57 says its..

Lighting Circuit 3% volt drop load Distributed 

Did you notice...  LOAD DISTRIBUTED??

You tried calculating on the full load at the far end of the circuit..  (as you may have with a shower or cooker or immersion heater)

But lighting circuits typically has lights distributed along its length....

So say your 6Amps is equally divided along the 90m circuit with 1amp every 15m..

first 15m is carrying 6A

next 15m is carrying 5A

next 15m is carrying 4A

next 15m is carrying 3A

next 15m is carrying 2A

Last 15m is carrying 1A

Work out the volt drop for each section of cable....

I think you will find that the full volt drop will be something like 9.13v

which is still too high for the 3% value of 6.9v..

But as it is a lighting circuit we can also consider diversity as every lamp is probably not on  at the same time...

Table A2 page 111 suggests that 66% is a reasonable figure for diversity on a lighting circuit...

This diversity will adjust the total load and its volt drop.....

66% of 9.13v is 6. something volts!!!

So the bits you are missing are:-

Distributed load

and

Diversity due to the type of circuit.

Guinness

 
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The OSG tends to give conservative numbers as a 'Catch-all' set of recomendations. Personally i think this is crape dumming down of out industry and should be stopped. What should be taught is the proper applicaction of formulas and the regulations themselves, then people might just work out how technical a job this really is, and we might get some credit (and wages) that reflects this.

 
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