Question About Volt Drop

[KayBull]

As part of my studies, I need some help with a question I have regarding volt drop. I have a copy of Electrician's Guide 17th Edition BS 7671: 2011. I understand everything covered in the volt drop section, but it doesn't appear to mention how volt drop is calculated in a circuit in consideration to spurs. I'm hoping it's not a silly question. I've only had the book a few days and still learning. So I thought it would be easier to put up a diagram.

Untitled

• KayBull
• 1 Oct 2012
• 1

In the radial circuit shown, the cable running from the consumer unit runs directly to socket 4 whereas all the other sockets are spurs. If i was to calculate volt drop for the load at socket 4, would I have to calculate the length of cables used for the spurs also or don't i need to??

 

[ban-all-sheds]

In the radial circuit shown, the cable running from the consumer unit runs directly to socket 4 whereas all the other sockets are spurs.

Are they?

What if #2 had been the first one installed, and then #4 and then #3 and finally #1?

What would make 1, 2 & 3 spurs?

At what stage would #2 change from "directly connected" to "spur"?

If i was to calculate volt drop for the load at socket 4, would I have to calculate the length of cables used for the spurs also or don't i need to??

Does any of the current for the load at socket 4 flow along any of the other cables?

 

[rjm2k]

I thought that technically speaking, Radials have branches rather than spurs?

 

[ericmark]

It is an interesting question and I would not think there is a single answer.

Clearly all sockets can not draw more than the supply MCB/Fuse so taking longest run and seeing if the volt drop will be above the limit when the full MCB/Fuse current is draw may show that you don't need to look further.

On the diagram you show it would have to be a radial circuit so likely the largest MCB would be 20A. With a ring it is considered we draw 20A in centre and the remaining 12A is even drawn so we work on a 26A draw at the centre it does not matter what sizes (singles or doubles) or where they are. I have not seen a recognised similar formula for radials.

Because in most cases a radial is supplied from a 20A MCB and one could plug two items taking the max current into sockets 2 and 4 although there may be somewhere a formula to calculate at less than the 20A from the MCB I would not think you could really use a lower than MCB figure with a radial circuit.

I have not got my University books to hand. I vaguely remember the theroy and it was not easy to work out. This would be level 4 work for people doing a degree or HND/HNC it would not I think be used at level 3. My son has my books but I am sure I could find out some thing. Thévenin's theorem is what you need to look up.

 

[JohnW2]

In the radial circuit shown, the cable running from the consumer unit runs directly to socket 4 whereas all the other sockets are spurs. If i was to calculate volt drop for the load at socket 4, would I have to calculate the length of cables used for the spurs also or don't i need to??

As BAS has implied, we don't talk about 'spurs' of radial circuits, not the least becasue it is sometimes impossible to define which is a spur [consider a supply going to socket 1 and from that socket separately to skts 2 and 3 - is the main circuit 1&2, with 3 as a 'spur', or is the main circuit 1&3, with 2 as a 'spur']. We therefore normally just call them 'branches'.

As BAS has also implied, in order to determine the voltage drop for any socket, you have to consider what route the current takes between CU and that socket, and then determine voltage drop just for that route (total length of cable between CU and the socket in question). From the point of view of circuit design and regulations, you need to know the greatest voltage drop at any of the sockets, so you would have to work out, or discover, which involved the greatest length of cable back to the CU. If the circuit involves cables of different sizes (CSAs), it obviously gets a little more complicated (the longest cable run is then not necessarily the one with the graetest voltage drop), but the principle remains the same.

Kind Regards, John

 

[stillp]

Name began with T but could not spell it well enough to get internet to say anything about it. Sorry.

Are you thinking of Thevenin Eric?

 

[ericmark]

Just found it yes Thévenin's theorem although I would not really expect electricians to work that out!

 

[ericmark]

One could also use Norton's method you get same result. Again don't think that's level 3 I would think it's higher education (degree work).

As to the name spur Appendix 15 BS7671:2008 yes still called spurs with radials.

I have said before I don't like the names used. To me unfused was a spur but fused was a radial but that's not what the regulations say.

 

[JohnW2]

Having seen Eric's reply, I would point out that the responses from BAS and myself assumed that you wanted to know the voltage drop at socket 4 when there were no loads on any of the other sockets. As Eric says, if that is not the case, the whole situation becomes much more complicated, and there is not necessarily a single answer. Although none of the current going to socket 4 goes along any of the branches, currents from all the other sockets (1, 2 & 3) go along parts of the route between socket 4 and CU, introducing additional voltage drops in the path between CU and socket 4. Determining the voltage drop at socket 4 in that situation therefore requires knowledge of, or assumptions about, the currents being drawn at each of the sockets in the circuit, and also detailed knowledge of the various cable lengths concerned. Even determining the 'greatest possible' voltage drop at socket 4 would not be a trivial exercise, and might need some trial and error, consdiering different patterns of loading of the other sockets (with, as Eric says, the total limited to the rating of the fuse/MCB protecting the circuit).

Kind Regards, John

 

[ericmark]

The maths is so involved I cheat and use electronics workbench or multisim and let it do it for me.

Thank you JohnW2 really liked that pat on the back. What I would be interested in is what level asks that question?

It is like the often asked question how to work of watts. Those will a low level or DC biased will say volts times amps. But as we go further we realise it's volts times amps times power factor correction.

Same with negative square roots we are first taught it can't be done then we are told it can't be done with real numbers one can imagine the confusion!!

So really we do need to know the level of the guys studies.

 

[JohnW2]

The maths is so involved I cheat and use electronics workbench or multisim and let it do it for me. Thank you JohnW2 really liked that pat on the back. What I would be interested in is what level asks that question?

The OP seemed to suggest that we are talking of a fairly low 'level' - which I guess is why BAS and myself jumped to the conclusion that we were being asked a very simple question about the VD when only one socket was loaded.

So really we do need to know the level of the guys studies.

We do - my assumption, which obviously may be wrong, is above. I would imagine that the 'complicated answer'is definitely degree/HND stuff, and wouldn't be expected of electricians in general.

... which leads to an obvious practical question, which I'm surprised has not occurred to me before - when completing I&T forms, what is an electrician meant to put down as the VD for a radial circuit?

Kind Regards, John

 

[stillp]

The maths is so involved ...

If you know the current in each socket-outlet, and you know the length of each 'leg' of the circuit, it's not difficult. Just write the currents on the diagram, add them up for each leg, and use Ohm's Law.

 

[JohnW2]

The maths is so involved ...

If you know the current in each socket-outlet, and you know the length of each 'leg' of the circuit, it's not difficult. Just write the currents on the diagram, add them up for each leg, and use Ohm's Law.

Indeed so - but that's unlikely to be the question, except as an educational/exam exercise. More practically, one would want to work out the maximum possible voltage drop at each socket (hence the one with the greatest possible drop), taking into account all possible distributions of loads on all the sockets (within maximum imposed by OPD/'circuit design current') - that is likely to require Eric's 'involved' maths!

Kind Regards, John

 

[ban-all-sheds]

Same with negative square roots we are first taught it can't be done then we are told it can't be done with real numbers one can imagine the confusion!!

Don't imagine confusion.

Imagine some numbers instead. One will do, actually.

 

[stillp]

Indeed so - but that's unlikely to be the question, except as an educational/exam exercise.

But the first few words of the OP state that "As part of my studies, I need some help with a question " which looks like an educational exercise to me!