Distribution Circuit Question

[Davisonp]

Hi,

The background.

A church bought a derelict farm about 4 years ago. They demolished several non listed buildings and put up new wood buildings in their place. A new 100 Amp single phase supply was brought into one of those buildings and a henley block used to supply a local consumer unit, then two switch fuses that supply the two other big buildings.

Non of the new electrical work was certified.

The church need to move one of the buildings supplied by one of the switched fuses and have asked me to disconnect and make safe, then reconnect at it’s new location.

I have had a look and the supply cable to this building is 25mm 3 core SWA, and the building is 100 meters away. The consumer unit had a Maximum Demand of about 80 Amps
I looked at the switch fuse and it is 100 Amp.

Question 1)
My main concern is the current supply cable appears to be undersized. If it is expected that 100 Amps is going to be available for use at the end of the cable, then 50mm SWA should have been used to compensate for voltage drop.

So, Should I be changing the BS88 fuse to a 50 Amp fuse as 50 Amps over 100 meters will meet the voltage drop regulations, or what should I be doing in this situation?

Question 2)
The building is now moving a further 40 meters away from the supply, and they want a supply for building works at the end of the existing 100 meters (32 amp Cform and 16 amp Cform) and I am not sure how to proceed. I was thinking of a henley block feeding a local small cu for the builders supply and another length of SWA to the new building supply. Do I need another switched fuse before the new 40 meter SWA? And, do I calculate the cable size for the extra 40 meters as from the new Henley block, or from the supply 140 meters away?


Any Advice appreciated.

 

[Spoon]

You say in paragraph one that "A new 100 Amp single phase supply was brought into one of those buildings". Is this a DNO supply or is it tapped from the original DNO incoming supply?
Where is the discrimination between the DNO 100A fuse and the 100A switched fuses?
If the max demand that you have measured was 80A then changing the fuse to 50A will not help. Doesn't help with the voltage drop if the supply will include lighting. Got to go down to 32A for lighting.
If extending the SWA can you not just join it in a metal box?

 

[Davisonp]

You say in paragraph one that "A new 100 Amp single phase supply was brought into one of those buildings". Is this a DNO supply or is it tapped from the original DNO incoming supply?
Where is the discrimination between the DNO 100A fuse and the 100A switched fuses?
If the max demand that you have measured was 80A then changing the fuse to 50A will not help. Doesn't help with the voltage drop if the supply will include lighting. Got to go down to 32A for lighting.
If extending the SWA can you not just join it in a metal box?

Thanks for the reply.

It was a new DNO 100A Supply.

There is no discrimination between the DNO fuse and distribution switched fuse, hence the post.

“If the max demand that you have measured was 80A then changing the fuse to 50A will not help.” The cable is in the ground and cannot be changed now, so what needs to happen to help the situation?

The circuit is a distribution circuit so lighting does not come into the voltage drop calculation (only 3% on final circuits).

“If extending the SWA can you not just join it in a metal box?” The original suggesting was a simple underground joint, but the client has asked for a couple of outlets at the end of the original cable, hence the harting block feeding a new mini CU and the new extension.

 

[Wilko]

Hi ,
On the V drop concern, the site design should have spec'd the cable sizes for the blg load to meet Vdrop and this leaves Customer in a tough spot now where all the wanted services and appliances may not be possible. May I suggest a sit-down with Customer to review the situation and clarify the load requirements? Then a new design can be done and you may find the original single phase supply for the '3 buildings' is actually not enough long term, or agree to something more modest. Cheers, David.

 

[Wilko]

Hi - the distribution vs final cct for V drop calcs - putting on my tin hat - I read 525.202 "voltage drop from the origin of the installation (usually the supply terminals) " to mean the DNO supply terminals where the meter is. This would then include the distribution to the outbuildings as its all part of one supply? Cheers, David.

 

[Spoon]

The cable is in the ground and cannot be changed now, so what needs to happen to help the situation?

If you need 80A then you will have to add another cable, in parallel with the original one, if they are not willing to change it.

The circuit is a distribution circuit so lighting does not come into the voltage drop calculation (only 3% on final circuits).

Doh..

The original suggesting was a simple underground joint, but the client has asked for a couple of outlets at the end of the original cable, hence the harting block feeding a new mini CU and the new extension.

I thought you said that the original building is moving 40 meters. What will be where the original cable ends?

 

[Davisonp]

Hi - the distribution vs final cct for V drop calcs - putting on my tin hat - I read 525.202 "voltage drop from the origin of the installation (usually the supply terminals) " to mean the DNO supply terminals where the meter is. This would then include the distribution to the outbuildings as its all part of one supply? Cheers, David.

As usual, the regs do not appear clear. They say “The voltage drop between the origin of an installation and any load point should not be greater than the values in the table below” in this installation, the “Load Point” (light fittings) of the final circuit of the lights has a maximum draw of 1.5 Amps. The load at the CU is potentially 80Amps.

The regs also say “The voltage drop within each final circuit should not exceed the values given in (i)” (3% lighting and 5% Other)

This circuit being discussed ends at the consumer unit so I would argue that the applying voltage drop at the consumer unit is 5% for the 80 Amp potential current draw.

 

[Davisonp]

Hi ,
On the V drop concern, the site design should have spec'd the cable sizes for the blg load to meet Vdrop and this leaves Customer in a tough spot now where all the wanted services and appliances may not be possible. May I suggest a sit-down with Customer to review the situation and clarify the load requirements? Then a new design can be done and you may find the original single phase supply for the '3 buildings' is actually not enough long term, or agree to something more modest. Cheers, David.

They are getting a 3 phase supply. They buried a suitable cable to the building, but arranged this cable laying and the switch over to three phase without talking to me at all, so Southern Electric arrived to connect to the three phase supply, and could only attach to one of the phases as the current installation is a single phase setup.

 

[Leesparkykent]

The maximum volt drop figures stated are recommended values. If the distribution circuit is feeding a DB/CU that supplies lighting then you should really be using the 3% figure. What you also have to factor in is the length/load of the final circuits that are fed by the distribution circuit as they should be included in the calculation. If your that worried about it then carry out a load study by monitoring over a week a so and base your calculations on that. Chances are after diversity is applied the load will be no where near 80A so the 25mm may be adequate. Also adding 40 meters to the length of distribution circuit have you considered EFLI?

 

[Davisonp]

If the distribution circuit is feeding a DB/CU that supplies lighting then you should really be using the 3% figure.

That surprises me. How often do you come across consumer units with no lighting at all? I have in a factory with multiple CU’s for groups of machines, but CU’s without lighting are a rare exception rather than a common occurrence, so I would expect the reg’s to require a maximum volt drop of 3% with an exception for a circuit from the main intake that does not have any lighting on it, in which case 5% volt drop is allowed.

In the reg’s the emphasis is on the 5% with repeated instructions for lighting which appears to be the exception to the 5% rule.

If your that worried about it then carry out a load study by monitoring over a week a so and base your calculations on that. Chances are after diversity is applied the load will be no where near 80A so the 25mm may be adequate.

Are you allowed to use a case study? The Max Demand with diversity for the consumer unit is 85 Amps (2 32A rings and electric heaters on 2 20A radials and lighting). However, the building has two separate offices and a workshop at the end with people coming and going all day. The offices are empty 50% of the time (only manned on 3 working days a week), and the most people I have ever seen in there working at one time is 4 people, each with a laptop. With occasional power tools and kettle boiling, there is very little power used in reality.

That is the thing, I know that the 25mm is fine for the actual use, however, I thought the regs require a cable suitable for the Max Demand (although many houses greatly exceed the 60 Amp main fuse).

Very interested in this.

 

[Leesparkykent]

The tabulated mV/A/m figure can be multiplied buy the design current (Ib) and not necessarily (In) the rated current of the protective device and the length of run. Where Ib is less than the CCC of the cable the volt drop will be less than calculated because the resistance will be lower than the tabulated mV/A/m values due to conductor operating temp being less than what the tables are based on. Ib should be your starting point for most calculations when selecting a cable.