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New Build 3 Phase or 1Phase

 
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Electricity, Tariffs, Monitoring, Electrics & Providers
Last Post by Transparent 1 hour ago
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Transparent
 Transparent
(@transparent)
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24/10/2026 10:33 pm  

Posted by: @bobflux
↑

if you have a PV inverter and either sun or charged batteries, and the electricity provider is limited to 25kVA

Not so.

1: It's now the case that the Service Fuse will be 80A, not 100A.

Each DNO will have issued their own directive (known as a Standard Technique) which specifies this.
Read this one from National Grid (NGED).

For three phase domestic supplies, the Service Fuse will normally be 60A per phase for a domestic customer.

image

 

2: The "designated voltage" for 1ph is 230v.
DNOs are now starting to change the taps at Primary substations in order to reduce the usual 240v we have grown used to.

This change has still not been fully ratified, but is agreed on by members of the Electrical Networks Association (ENA).

80A x 230v comes to 18.4kVA.

 

3: If your continuous loading is calculated to exceed 60A (not the 80A fuse rating), there are circumstances by which the DNO can require you to accept a 3ph supply.
The reasons are complex, but pay particular attention to the Neutral current and you'll begin to understand the issues.

The whole point of 3ph is that the load on those phases should be even.
That being so, no current flows in the Neutral.

Deliberately designing a supply for a new house which causes large currents in the neutral will create thermal stress at the local substation.
That causes breakdown of insulation, and hence a higher risk of outages.

 

4: If the reason you are applying for a single-phase supply greater than 18.4kW, but not more than 23kVA is due to an EV charger, then that charger may not be fixed to the building.

The charger must be fixed to a separate/dedicated post, with the position dictated by airflow for cooling.

 

Talk with the Network Planner in the DNO Team assigned to your area.

They use a software calculator which takes into account a number of parameters which you won't know about.

 

 


This post was modified 1 hour ago by Transparent

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