Is a 35mm cable big enough for a 3-phase domestic installation with...

[Spinball]

...2 x 22kW EV chargers, 10kW of solar panels + 15 kW batteries and a heat pump for a 4 bed house and a 1 bed detached garage office?
Got 3 phase being put in and they're planning on putting in a 35mm cable. Just checking with you experts whether this will be fat enough.
Many thanks

 

[pc1966]

It also depends on the length (and hence voltage drop limit).

Typically 35mm is fine for 100A = 69kVA total which is usually the current limit for domestic supplies, but you need to check the length and (occasionally) that the cable is not passing through any areas that are hot or heavy thermal insulation.

 

[Spinball]

It also depends on the length (and hence voltage drop limit).

Typically 35mm is fine for 100A = 69kVA total which is usually the current limit for domestic supplies, but you need to check the length and (occasionally) that the cable is not passing through any areas that are hot or heavy thermal insulation.

Is that 100A per phase?
Was told that can’t have more than 80A per phase on a normal meter box on the front of the house anyway.
It’s travelling about 15 metres under the drive and lawn up to the new meter box. Not near anything hot.

 

[Rockingit]

This will be designed for you by your electrician who will (should!) understand about cable calculations, de-rating and so on. This isn't DIY territory.

 

[Spinball]

This will be designed for you by your electrician who will (should!) understand about cable calculations, de-rating and so on. This isn't DIY territory.

No, but while I’m talking to National Grid, I want to make sure they put in a big enough cable. I started by ordering three-phase and paying 4 1/2 grand to put it in. But then I realised that being looped, I could get delooped for free and when they put in a new connection, if the customer asks for three-phase they will do it. So I got a refund and had to wait six months to get it for free.
Now I have a date and I want to make sure everything is perfect.

 

[pc1966]

No, but while I’m talking to National Grid, I want to make sure they put in a big enough cable. I started by ordering three-phase and paying 4 1/2 grand to put it in. But then I realised that being looped, I could get delooped for free and when they put in a new connection, if the customer asks for three-phase they will do it. So I got a refund and had to wait six months to get it for free.
Now I have a date and I want to make sure everything is perfect.

You won't get 2 * 22kW chargers on single phase!

So you need to adjust the plans or reconsider the supply.

 

[davesparks]

You're going about this the wrong way, you don't just pick a cable size and decide whether it 'fat enough'

You need to start by establishing what size supply you need and then design the installation to suit the intended load.

 

[DPG]

You won't get 2 * 22kW chargers on single phase!

So you need to adjust the plans or reconsider the supply.

I think he is getting 3 phase in.

 

[Rockingit]

I think the OP has a fundamental mis-understanding here of what's required. The DNO simply asks the question of what size and type of supply is required (usually in kW) and they then install whatever they think is suitable. They work to an entirely different method of calculating cable sizes, types and so on. So if for arguments sake you're asking for an 80kW 3ph supply then you'll end up with whatever they consider to be the appropriate method to deliver that to you.

 

[Spinball]

Thanks for the replies. It’s quite difficult (for me) to accurately calculate what’s needed when I don’t know the exact specification of the solar panels and battery plus heat pump of unknown size at some point in the next 5 years.
The only thing I know I’m getting is one 22kW Zappi right away and then potentially another at some point in the next three years.
But in theory if they install a 35mm cable, I should be good whatever I need to plug in within reason, right? I mean do a lot of people have 6kW solar arrays, batteries and heat pumps on a normal single phase supply?

 

[davesparks]

But in theory if they install a 35mm cable, I should be good whatever I need to plug in within reason, right?

You said earlier in the thread that they are installing an 80Amp 3 phase supply. So whether they install 35mm or 300mm cable your supply will still be 80Amps.

 

[Lucien Nunes]

they're planning on putting in a 35mm cable

Who is the 'they' in this case? The DNO or your electrical contractor?
If it's the DNO, what cable they use is out of your control. They will size their equipment according to their regulations and specs, to meet your agreed requirements which should have been calculated by the electrician before the application was made. The methods they use to size cables are different to the ones electricians use within an installation, as they have different performance specifications to meet under the ESQCR, for example.

If it is your electrician installing the said 35mm², then they should be able to calculate the cable to meet the requirements of BS7671 for whatever you have asked them to install either now or in the future. The worst case will be all demand or all generation - i.e. the maximum power that can flow in either direction. From what you have stated, the demand will significantly outweigh the generation, but is likely to be within 69kVA.

As mentioned above there are many factors built into the cable calculations, not just the load. Without all the information from a site survey, it's difficult to give a definitive answer to whether a particular cable size is suitable. 35mm² is in the right ballpark.

 

[Spinball]

Who is the 'they' in this case? The DNO or your electrical contractor?
If it's the DNO, what cable they use is out of your control. They will size their equipment according to their regulations and specs, to meet your agreed requirements which should have been calculated by the electrician before the application was made. The methods they use to size cables are different to the ones electricians use within an installation, as they have different performance specifications to meet under the ESQCR, for example.

If it is your electrician installing the said 35mm², then they should be able to calculate the cable to meet the requirements of BS7671 for whatever you have asked them to install either now or in the future. The worst case will be all demand or all generation - i.e. the maximum power that can flow in either direction. From what you have stated, the demand will significantly outweigh the generation, but is likely to be within 69kVA.

As mentioned above there are many factors built into the cable calculations, not just the load. Without all the information from a site survey, it's difficult to give a definitive answer to whether a particular cable size is suitable. 35mm² is in the right ballpark.

The 'they' is the DNO, National Grid who are doing the install. The cable in the street coming to my property is 95mm according to the DNO engineer. That's about as much as I know.
Thanks for the advice.

 

[IAmSparkytus!]

It's not the size of the cable you yourself need to worry about. You need to tell them how many amps/kva you require and then let them worry about what size cable they need to install to be able to safely provide you with that.

If you have a 3phase 80amp supply and they install a 35mm cable, you'll be able to use 3phase 80amps. If they install a 70mm cable, you'll still only be able to use 3phase 80amps.

 

[pc1966]

What the DNO can give you depends on many factors, not just the cable to your property. For example, the local substation might be close to max capacity based on all other property's loads in which case they might not offer new connections, or if they do it might not be 100A but instead fused down to 80A or 63A protect their system from overload.