Getting the mains connection wrong is one of the most expensive mistakes you can make before a project starts. Too small and you're hiring a generator to cover the gap. Too large and you're paying standing charges on capacity you never use.
Start with peak demand, not average demand
Your Distribution Network Operator (DNO), the company responsible for electricity supply in your area, will size your connection based on your stated peak demand in kilovolt-amperes (kVA), which is the measure of total electrical load on a circuit. Most people underestimate this because they think about what's running most of the time, not what could all run at once. Walk through your plant schedule and ask: what's the worst-case moment? Typically it's when the tower crane is hoisting, the hoist is loaded, and welfare units are at full draw in the morning. That overlap is your peak.
Apply a diversity factor
Not everything runs at 100% simultaneously. A standard diversity factor for mixed construction loads is 0.5–0.7. Tower cranes and hoists are the exception. Treat them at full draw during overlap periods.
Standard connection sizes (UK, Three Phase and Neutral — TPN)
| Current | kVA | Typical use |
|---|---|---|
| 63A | 43 kVA | Small welfare only |
| 100A | 69 kVA | Welfare + light loads |
| 125A | 86 kVA | Medium site, no crane |
| 160A | 110 kVA | Medium site with small crane |
| 200A | 138 kVA | Large site, single crane |
| 315A | 218 kVA | Multi-crane, heavy plant |
| 400A | 277 kVA | Maximum standard single connection |
Anything above 400A TPN typically requires a substation — factor in 12–16 weeks lead time with the DNO.
When mains isn't enough
If your peak demand exceeds what the connection can supply, you have two options: upsize the connection (if lead time allows) or bridge the gap with battery storage. Battery storage is increasingly the smarter call: no fuel, no noise, and you only hire it for the weeks you actually need it.
The number most people get wrong
Mains capacity is constant. Site demand isn't. A 400A TPN connection handles your crane and hoist fine in weeks 1–20. But when the second crane arrives in week 21, you're over. That's the week you need battery storage. Not the whole project.