OSG Final Circuits voltage drop table question

[cornburn]

Just been flicking through and table 7.1 to enable final circuits to be installed without calculating voltage drop has one of the criteria as

ii)The final circuit is connected to a dis board or CU at the origin of the installation

Just seeing that I understand this and the reasoning behind it correctly really. If you had the supply in to a large house with the meter and main board in the cellar, and 2 submains fed from this main board in the east and west wings of the house (think Wayne manor), this means that the table in the OSG can't be used for any final circuits from either of the submains.

Have I got this correct and is the reason that we also need to take the voltage drop of the distribution circuit into account?, so obtaining an accurate voltage drop from the origin of supply to the extremity of the final circuit we're installing?

If I am right with this, would it be acceptable to measure the voltage drop of the distribution cable as I can't see how this could be calculated without knowing an accurate length, size and condition.

Likewise in commercial installations where a final circuit can be installed to a submain that is in essence daisy chained through several other submains and control boxes (for example DB5A2), how would you go about calculating the overall voltage drop on the circuit you're installing?

Thanks

 

[cornburn]

No one?

 

[Guest123]

Sorry, missed this one.....

VD is calculated from the origin of an installation i.e main intake.

The mV/A/m values in BS 7671 and OSG apply to their particular cable wether it's connected directly to the origin or via a sub-main.

If you wer calculating a VD of a lighting circuit fed off a sub-main for example, you would calculate for the lighting circuit and the sub-main as two seperate circuits, with the total of the two circuits then being used to compare to the maximum permissable VD or 3% for lighting circuits or 5% for a power circuit, 6.9V and 11.5V respectively.

You can only measure the VD of a cable when it is under full load....which can be difficult to do, making sure that all loads are on at the same time particularly on a sub-main.

 

[cornburn]

Thanks for that Lenny, that's what I was thinking. What would you do in a situation with many distribution circuits from the supply to origin of the new circuit? ie circuit from single pahse board, fed from 3 phase board, which is fed from another 3 phase board, which is fed from an isolator which has an SWA buried outside going to another building that has the meter etc in it?

I see this sort of setup very often in commercial premises and it would take many hours to trace the run back all the way to the origin of supply and pull everything apart to check cable sizes and calculate. I can't see anyone bothering but what would you do in such a situation (a) when your assessor asks how you would go about it and (b) in reality when he's not there watching over you!

 

[Colonel Hathi]

I see what you are saying Lenny, but I just spoke to 2 people yesterday from the NICEIC - who told me that you just have to comply with 5% VD on the distribution circuit.
You then have to comply with 3% just on the final circuit for the lights.
I know it sounds strange - but they were both clear on this point.
It really means that you could have a total VD of 8% on a lighting circuit fed off a distribution circuit.
Your way of seeing it makes more sense to me though.
Can you clarify?
Cheers

 

[Chr!s]

I see what you are saying Lenny, but I just spoke to 2 people yesterday from the NICEIC - who told me that you just have to comply with 5% VD on the distribution circuit.
You then have to comply with 3% just on the final circuit for the lights.

I know it sounds strange - but they were both clear on this point.
It really means that you could have a total VD of 8% on a lighting circuit fed off a distribution circuit.
Your way of seeing it makes more sense to me though.
Can you clarify?
Cheers

Unless private supply you should be working to 3% or 5%. If lighting is being utilized then the volt drop is deemed to comply if the difference in voltage at the origin and at the load is 3% or less.Based on the nominal voltage.

I would disagree with the niceic there, give them a call and ask to clarify.

 

[Colonel Hathi]

I suppose the question is whether "the origin" is at the original CU or an additional CU at the end of the distribution circuit?

 

[Guest123]

I disagree with the NIC also. As do ECA, Select, City + Guilds.

 

[i=p/u]

quote....I suppose the question is whether "the origin" is at the original CU or an additional CU at the end of the distribution circuit?

theres your awnser
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