Outside Electrics

[Antscran]

Hi all,

Moved into a property in late 2020 and in the process of updating lots of things. There is a large shed at the bottom of the garden which has various lights and sockets inside, this was connected to the house by standard PVC cable which was in some cases just a few inches below the grass, needless to say this is all disconnected currently.

The consumer unit has a separate RCD for the shed which is currently off, the wire ran through a hole in the wall and down the side of the property in what was a lean-to which we had taken down. I would like to renew this cable and also run a new length down the garden.

As far as connection to the consumer unit I will only use a qualified electrician and want to carry out the less skilled work myself.

The question is should I use armoured cable or run (7m approx.) this in suitable trucking along the wall of the house?

For the garden run (25m approx.) I intend to dig a trench 60cm deep laying the cable at the bottom, with a length of barrier tape about 20cm above that, should the garden cable be armoured or in suitable trucking?

There is a possibility that a new shed will be built that could support upto 4kW of solar, so presume the garden length will need to be atleast 4mm^2?

Thanks,
Ant

 

[ericmark]

4 kW = 17.5 amp so current wise 2.5 mm² is enough, however there is more involved with solar power.

To feed in it has to be installed by a registered installer, the solar panel produces DC and a grid tie inverter converts it to AC, and this needs to monitor the voltage and auto shut down if not within 207 to 253 volts, so although lighting is allowed 3% and power 5% volt drop, with a solar panel this may not be suitable.

So 32 meters of cable at 4 mm² at 17.5 amp looking at 5.7 volt drop, which would mean if the voltage reaches 247.3 volts the system will lock out, the cure is likely to send the power to house as DC then convert to AC which likely means extra cores in the cable or two cables.

There is a problem with DC imposed on the AC stopping RCD's working, so likely two independent cables.

Some one needs to sign that they have designed the system, I would get an electrician on board before starting, who has the paperwork to connect solar panels and let him design the system, he will likely jump at you doing some of the ground work, however as electricians were are wary.

When my so tried working as a sole trader he got caught, owner would say I will chase walls etc, and son agreed on a price with him doing some of the donkey work, however when he returned to do his bit, the guy had not done his bit, to juggle the jobs was not going to help due to travel involved, so he ended up doing the chasing of walls etc. Result he vowed never to let the client do prep work, work after job completed yes, but he wanted work promised done on time.

There is also the reverse, he had some jobs which were spanning over years where the owner was doing major works, he would get a phone call ready for next stage, and he would then find a slot, however his wife left him, so the house was sold, and he became of no fixed abode, he lived on a narrow boat, and also went to work cards in, condition of employment was no work on the side, so people with work in progress had to now agree new rate with sons employer, which was more expensive. And in one case seems they did not have his mobile number, so could not tell him when ready.

So theory may be a job can be on going for years, in practice want it done within 6 months, OK a tradesman can die at any time, but I have seen it where the electricians was a scheme member to start the job, but has retired before the end, and had problems getting the paperwork as a result.

A simple supply to shed OK, but solar panels needs a lot more thought.

 

[SUNRAY]

I'll simply say this particular job is more complex than can be advised on a forum like this.

Get you electrician in now and work it with him, the situations Eric mentions are real world.

 

[Antscran]

Hi Ericmark,

Agree I was thinking about the cable drop for solar and options. Ideally you want the inverter as close to the consumer unit as the DC side is usually 6mm^2 and also won't suffer from skin effect.

The option here thinking about it is to run the length with 2.5mm^2 SWA and then have a small length of trunking under a new patio I intend to lay, this can be blanked off and if solar is installed DC cable could be run directly from the shed to the house inverter which over the system life should cover the extra costs in cable and work.

I would only use a local electrician and with WhatsApp could agree before anything starts and update them with work I have completed that way everyone is happy.

Thanks for the answer that has set my mind at ease and can start planning the next few steps.

Cheers,
Ant

 

[Antscran]

I'll simply say this particular job is more complex than can be advised on a forum like this.

Get you electrician in now and work it with him, the situations Eric mentions are real world.

Agree and not after specific advise more guidance, it has helped me look at it is a slightly different way.

Powering the shed properly and safely is my main concern but I would like to future proof the installation if solar is ever installed and running DC back to the house would mean a far better system, exactly how that is done in future is definitely a discussion to be had with an MCS registered electrician.

Laying down some conduit whilst the old patio is up won't be a lot of work just incase, I can hide the ends under the gravel boarder.

 

[SUNRAY]

Agree and not after specific advise more guidance, it has helped me look at it is a slightly different way.

Powering the shed properly and safely is my main concern but I would like to future proof the installation if solar is ever installed and running DC back to the house would mean a far better system, exactly how that is done in future is definitely a discussion to be had with an MCS registered electrician.

Laying down some conduit whilst the old patio is up won't be a lot of work just incase, I can hide the ends under the gravel boarder.

Good result, I'll always advise to run a bigger cable than needed as the cost of the 'next size up' is usually about 50% and i'll guess 25m of 2.5 or 4mm² will be £50 or 75 ish.
I haven't purchased any recently so my guess at price rises may off.

 

[JohnW2]

... the solar panel produces DC and a grid tie inverter converts it to AC, and this needs to monitor the voltage and auto shut down if not within 207 to 253 volts, so although lighting is allowed 3% and power 5% volt drop, with a solar panel this may not be suitable. .... So 32 meters of cable at 4 mm² at 17.5 amp looking at 5.7 volt drop, which would mean if the voltage reaches 247.3 volts the system will lock out, the cure is likely to send the power to house as DC then convert to AC which likely means extra cores in the cable or two cables.

I'm rather confused by that reasoning - could you perhaps clarify/explain a bit?

Kind Regards, John

 

[ericmark]

I'm rather confused by that reasoning - could you perhaps clarify/explain a bit?

Kind Regards, John

The inverter must auto shut down with loss of PEN, and standard method is monitor voltage, also supply should not exceed 253 volt as that is official limit.

Where I use to live this caused me a problem. The voltage was around 245 volt, and my old 65 watt fluorescent tube was happy, even when the old fat tubes were not avaible any more the 58 watt still worked, but then we had a group of homes with solar panels which were forever tripping out on over voltage, so volts dropped to 230 volt, and my fluorescent stopped working.

I had to fit an LED tube.

 

[JohnW2]

The inverter must auto shut down with loss of PEN, and standard method is monitor voltage, also supply should not exceed 253 volt as that is official limit.

Yes, I understand that, but what confused me was ..

.... So 32 meters of cable at 4 mm² at 17.5 amp looking at 5.7 volt drop, which would mean if the voltage reaches 247.3 volts the system will lock out, the cure is likely to send the power to house as DC then convert to AC which likely means extra cores in the cable or two cables.

Since you said that the 'cure' would be to move the inverter into the house, I assumed you were talking about the situation in which the inverter was remote from the house (near the panels). If that's correct, then the inverter would presumably only be able to sense the voltage as it was at that remote location - so if it were 'seeing' 247.3V, I haven't yet managed to grasp why that would 'shut down' the inverter.

What am I missing?

Kind Regards, John

 

[Antscran]

In theory even if the inverter was remote from the house the voltage drop would be relatively small as the load would be relatively small as most inverters draw 30-80W.

If you wanted to export to the grid this might cause an issue as the voltage drop from the inverter to the house would be evident?

 

[plugwash]

Voltage drops between the source of power and the destination of that power.

So if you have a grid-tie inverter pushing power into the instalation, the voltage will be higher at the inverter than it will at the origin of the installation. If it gets too high the inverter will trip. If the house is already towards the top end of the acceptable voltage range then a long cable run to the inverter may be the difference between it working and tripping.

 

[JohnW2]

So if you have a grid-tie inverter pushing power into the instalation, the voltage will be higher at the inverter than it will at the origin of the installation. If it gets too high the inverter will trip. If the house is already towards the top end of the acceptable voltage range then a long cable run to the inverter may be the difference between it working and tripping.

I thought that eric was talking about the (grid) 'supply voltage', rather than the output voltage of the inverter. Are the inverters not designed to not (be able to) generate output voltages above the 'acceptable voltage range'?

Kind Regards, John

 

[DetlefSchmitz]

I thought that eric was talking about the (grid) 'supply voltage', rather than the output voltage of the inverter. Are the inverters not designed to not (be able to) generate output voltages above the 'acceptable voltage range'?

They are required by their spec to turn off for network safety reasons.

 

[Antscran]

They are required by their spec to turn off for network safety reasons.

G59 I think so you don't inadvertantly electric someone carrying out maintenance on the grid by back feeding when the power is off. I think they have some settings you can adjust with minor increments.

 

[JohnW2]

They are required by their spec to turn off for network safety reasons.

Yes, I understand that, but I thought that they sensed (and thought that eric was talking about them sensing) the grid supply voltage, rather than just their own output (which presumably will also be 'capped'). Is that not the case?

As you presumably understand, it was eric's comment about voltage drop affecting what situations would cause the inverter to shut down that I didn't really understand.

Kind Regards, John