Volt drop calcs

[flyingsparks]

Need to price for running a sub main 300m from a generating plant back to house and workshops. 3 phase will be brought back to the buildings near the house where the mains supply for the house comes in.. Here's the problem..
At the moment the house is on single phase. Bringing the 3 phase cable from the generating plant to the house and fitting new 3 phase DB and making the house a 3 ph installation is certainly the best way to go to keep things balanced. However there is a chance the customer may want to bring the 3 phase up and then only use one phase for the house keeping the existing house DB, the other 2 phases being used around the other buildings.

calculating the volt drop on the cable should I be using 230V figures if the house is split off to be single phase only? And hence 3% for lighting of 6.9V rather than 12V.

It's bit of a complicated set up and difficult to explain but hopefully that kinda makes sense!

Edit. Thinking about it the house circuits are 230V anyway even if 3 phase is at the house so therefore everything should be worked out on 230V, right?

 

[JohnW2]

Need to price for running a sub main 300m from a generating plant back to house and workshops. 3 phase will be brought back to the buildings near the house where the mains supply for the house comes in.. Here's the problem.. calculating the volt drop on the cable should I be using .....

I must admit that, although it's way beyond my experience, I might wonder whether LV was really ideal for distribution over a distance of 300m!

Whatever, do you actually have to consider it as a 'submain'? Voltage drops are calculated relative to 'the origin of the installation', which is normally where the supply enters the premises, not all the way back to the 'source' of the power. I would have thought that VD in your 300m cable was only relevant in terms of ensuring that an acceptable voltage (maybe the same as the DNOs nominal +10% -6%) was delivered to 'the origin of the installation'.

Or am I thinking totally wrong?

Kind Regards, John

 

[flyingsparks]

The generating plant is an anaerobic digester on the customers farm which usually exports to the grid. Now they want to use the power instead of exporting it.

 

[JohnW2]

The generating plant is an anaerobic digester on the customers farm which usually exports to the grid. Now they want to use the power instead of exporting it.

Fair enough - but I still wonder where 'the origin of the installation' starts as far as (per regs) VD calculations start - as I said, it usually starts where the power enters the premises, regardless of how far away it was generated (or transformed).

A DNO-supplied supply would be allowed to vary from 216.2V to 253V at the point it entered the premises, regardless of what VDs may have occurred upstream of that point, and VDs (per BS7671) would be calculated relative to that point of supply. If the voltage at the origin is only 216.2V, you are still allowed drops within the installation of 11.5V (or 6.9V for lighting) relative to that supplied voltage.

Kind Regards, John

 

[Lectrician]

The regs give different percentages for a private sub station. This would essentially be that.

A three phase + N submain should always be calculated at 230V.

What is the actual loading you are calculating too?

 

[flyingsparks]

OK Thanks, will check the percentages in my BGB. Probably looking at 50A per phase to be taken over.

 

[JohnW2]

The regs give different percentages for a private sub station. This would essentially be that.

They do indeed, and the allowed VDs are slightly greater (but have to be restricted to the usual 5%/3% within the final circuits).

However, my point/question remains. Those guidance maximum VDs in Table 4Ab relate to VDs relative to 'the origin of the installation'. You both seem to be saying that, in the case of a private supply, the 'origin' has to be considered as the source of the power (rather than where the supply enters the supplied premises), whereas this is obviously not the case with a DNO supply.

A three phase + N submain should always be calculated at 230V.

That certainly makes sense, at least as far as single-phase loads are concerned.

Kind Regards, John

 

[plugwash]

Can you clarify what exactly you are planning to do?

Is the plan to keep the system grid tied but merge what was previously two installations into one? or is the plan to turn the system into an off-grid setup?

 

[Lectrician]

The regs give different percentages for a private sub station. This would essentially be that.

They do indeed, and the allowed VDs are slightly greater (but have to be restricted to the usual 5%/3% within the final circuits).

However, my point/question remains. Those guidance maximum VDs in Table 4Ab relate to VDs relative to 'the origin of the installation'. You both seem to be saying that, in the case of a private supply, the 'origin' has to be considered as the source of the power (rather than where the supply enters the supplied premises), whereas this is obviously not the case with a DNO supply.

A three phase + N submain should always be calculated at 230V.

That certainly makes sense, at least as far as single-phase loads are concerned.

Kind Regards, John

With the larger %, It essentially allows you to drop a large percentage in the supply cable before the final circuit % applies.

Origin of an installation: The position at which electrical energy is delivered to an electrical installation.

This could be seen in many way, but surely the cable from the generating plant is part of an installation.

 

[JohnW2]

Origin of an installation: The position at which electrical energy is delivered to an electrical installation. ... This could be seen in many way, but surely the cable from the generating plant is part of an installation.

Indeed, but I'm far from convinced about that 'surely' - which is why I asked the question!

A DNOs cable from its 'generating plant' (in practice, the local transformer) to the premises in question certainly doesn't count as part of the 'installation', so is the situation necessarily totally different when the 'generating plant' is a private, one?

What if, say, it was a large industrial size with a dedicated DNO substation on the periphery of the site? You presumably then would not count that substation as 'the origin of the installation' for the purpose of measuring VDs within the supplied buildings, would you?

Kind Regards, John

 

[flyingsparks]

Some interesting debate here but I'm yet to go home and look in my regs book and recalc things.
Plugwash- as far as I'm aware they will get rid of the grid connection at the digester. I'm going there again tommorow so will find out more info then.

Personally I think they may be better off using the supply to heat water in big thermal stores and use this to heat and supply DHW to the house.

 

[ericmark]

It is an interesting case.
I don;t think it matter who supplies the power 209 - 253 volt is the limit and if the voltage drops below 209 or exceeds 253 on any phase then something needs doing to correct it.

 

[JohnW2]

It is an interesting case. I don;t think it matter who supplies the power 209 - 253 volt is the limit and if the voltage drops below 209 or exceeds 253 on any phase then something needs doing to correct it.

Where does 209V come from? 230V minus 6% is 216.2V.

Kind Regards, John

 

[ericmark]

minus 3% + minus 6%

 

[JohnW2]

minus 3% + minus 6%

Ah, right - you're talking about the permissible minimum at the end of the final circuits, not the 'origin'.

That is, IMO, the sensible approach. To attempt to impose a 3% maximum permitted VD when one "knows" (I know, but most of us are pretty sure!) that the voltage at the origin of one's installation is never below, say, 240V does seem a little 'harsh' - even though it can be argued that there's no alternative if one wants to be totally sure under all conceivable circumstances.

Kind Regards, John