11.1kW oven on existing 6mm² cooker circuit - sanity check before I go ahead?
Hi all, hoping some electricians/sparks here can sanity check my numbers before I get this oven installed. I've done the maths myself based on our EICR but want a second opinion before committing.
Setup:
Property: flat, UK, standard 230V single-phase supply
New appliance: 11.1kW electric oven, hardwired (no socket on the cooker connection unit — just isolator/hardwire)
Existing cooker circuit per our EICR (tested May 2026):
Cable: 6mm² live, 2.5mm² cpc
Protective device: 32A Type B RCBO
Measured (R1+R2): 0.17Ω
Measured Zs: 1.37Ω
Ze at origin: 0.23Ω
My working:
Full load current: 11,100W ÷ 230V = 48.26A
Applied cooker diversity (BS 7671): 10A + 30% of remainder = 10 + (0.3 × 38.26) = 21.48A design current (no +5A since there's no socket on the unit)
Breaker check: 21.48A design current vs 32A breaker — fine, ~10.5A headroom
Cable check: 32A breaker well within 6mm² T&E capacity — fine
Zs check: 1.37Ω measured vs 1.44Ω max permitted for 32A Type B — passes but only 0.07Ω margin, which feels tight to me
Cable length: not recorded on the EICR, so I back-calculated from the 0.17Ω (R1+R2) reading using standard copper resistance (3.08 mΩ/m for 6mm², 7.41 mΩ/m for 2.5mm²) → works out to roughly 16.2m
Voltage drop at that length: 7.3mV/A/m × 21.48A × 16.2m = 2.54V = 1.1% of 230V — well under the 3% limit
So on paper this all passes, but a few things I'd love opinions on:
Is the 0.07Ω Zs margin (1.37Ω vs 1.44Ω limit) actually as tight as it feels, or is that pretty normal/acceptable in practice?
Is back-calculating cable length from R1+R2 a reasonable approach, or is there something I'm missing that would throw this off (joints, connectors, etc.)?
Anything else on a job like this that wouldn't show up in the paperwork but you'd always check on-site before signing off? (e.g., condition of the isolator switch, terminal tightness, etc.)
Not trying to skip getting a qualified electrician to actually do the install and certify it — just want to go in with a solid understanding of what's involved and not get blindsided. Appreciate any thoughts