This is why...
- "Part of the garage is about to become the WORKSHOP"
Workshop implies potential h/d tools...
- "2kW+ circular saw"
... such as this one - 2000W continuous. Startup 30A?
- "Lighting"
... so about 100W.
- "Deep Freeze"
... startup is 30-40A. Continuous 210W.
- "In winter electric heater"
... 2000W potentially continually.
Ok...
1) Identify Circuit Design Current (Ib)
Continuous load is 4300W. Startup load for freezer & circular at same time 60-70A. That might get interesting if a low In rated Type-B breaker, eg, 20A.
4300W = 18.7A = Ib, Circuit Design Current.
2) Identify OC rating (In) which must > Circuit Design Current (Ib)
Ib = 18.7A, closes OC circuit protective device is 20Amps.
3) Select cable based on Circuit Design Current (Ib)
Tabulated Current Carrying Capacity (It) of 2.5mm SWA is for 90oC due to XLPE insulation, but assume 70oC limit use BS6004.
It at 70oC for 2.5mm SWA is 23Amps, It of 23Amps > In of 20Amps > Ib of 18.7Amps - so all ok.
Cable is 2.5mm SWA.
4) Verify Cable Corrected-Current-Carrying-Capacity (Iz) for Installation Factors > In.
That is, derate cable It figure for installation factors Ambient, Insulation, Grouping & Rewireable fuse (Ca, Ci, Cg, Cr).
Ambient - I assume 30oC. Insulation - I assume cable is not surrounded in insulation. Grouping - I assume cable is not grouped. Rewireable fuses - I assume not used.
So no adjustment required, Iz = It in this instance.
5) Verify Cable Selection & Length satisfies Voltage Drop Limitations.
17th requires lighting to 3%, power to 5% IIRC. That 3% of 230V = 6.9V, 5% of 230V = 11.5V.
Cable route.
- Length A - 10m of FTE 2.5mm from house-CU to SWA on house wall
- Length B - 25m of SWA 2.5mm into garage-CU
- Length C - 10m of FTE 2.5mm into sockets or lights
Voltage drop for Length A @ 20Amps...
- 10m x 20Amps x 18mV/A/m = 3600mV = 3.6V Voltage drop for LengthB @ 20Amps...
- 25m x 20Amps x 19mV/A/m = 9500mV = 9.5V Voltage drop for LengthC @ 20Amps...
- 10m x 20Amps x 18mV/A/m = 3600mV = 3.6V
i) Your total voltage drop on 25m of 2.5mm SWA alone is 9.5V.
17th regs limit lighting to 6.9V drop, power to 11.5V.
ii) Voltage drop on startup is going to be significantly worse. However a circular saw does NOT start in a stalled state.
6) Calculate EFLI, Fault Current, Shock Voltage & Disconnect Time
EFLI Zs = Supply-Resistance Ze + (Phase R1 & CPC R2 resistance).
Length A R1+R2 & Length C R1+R2...
- R1 (Phase) = 10m * 7.41mohm at 20oC or 8.89mohm at 70oC
---- for length of 10m = 88.90mohm
- R2 (CPC) = 10m * 12.10mohm at 20oC or 14.52mohm at 70oC
---- for length of 10m = 145.20mohm
- R1+R2 = 88.90+145.20 = 234.10mohm
Length B R1+R2
- R1 (Phase) = 25m * 7.41mohm at 20oC or 8.89mohm at 70oC
---- for length of 25m = 222.35mohm
- R2 (CPC) = 25m * 7.41mohm at 20oC or 8.89mohm at 70oC
---- for length of 25m = 222.35mohm
- R1+R2 = 222.35+222.35 = 444.70mohm
Length A-B-C R1+R2 = 234.1+444.7+234.1 = 912.9mohm - 0.91ohm at 70oC.
70oC is used because in a fault condition high currents flow, heating, which increases resistance.
i) Assuming your supply is TN-C-S, Ze < 0.35ohm.
- EFLI Zs = 0.35 + 0.91 = 1.26ohm
ii) Assuming your supply is TN-S, Ze < 0.80ohm.
- EFLI Zs = 0.80 + 0.91 = 1.71ohm
Fault current under i) at 230V = 182A. Fault current under ii) at 230V = 134A.
A 20A Type-B MCB trips at 5*In or 100A. Under both i) & ii) your fault current is >100A so ok.
Fault disconnect time will be 0.1sec at 5*In.
Prospective shock voltage for Length A-B-C R2 is...
- Pros Shock Voltage = Fault Current * A-B-C CPC R2
- Pros Shock Voltage = 182 * (145.20+222.35+145.20/1000)
- Pros Shock Voltage = 182 * (512.75/1000) = 93.32V
For a shock voltage of 93.32V you need a fast disconnect time. However we know a Type-B MCB trips at 5*In or 100A in 0.1sec. Ok.
Actual shock voltage for is thus 100 * (512.75/1000) = 51.28V. The wrong side of 50V for a touch of an appliance casing.
So most people would prefer a larger CPC. That could be achieved by combining SWA armour (steel) with a core (I'm assuming 3 core SWA BTW).
7) Verifying CPC CSA for Disconnect Time
We want to verify via the adiabatic calculation that the CPC will not overheat.
Assuming copper CPC throughout, k = 115 at 70oC (IEE 16th Table 54C).
- CPC min-size = SQRT (Fault-Current^2 * disconnect-time) / 115
- CPC min-size = SQRT (182^2 * 0.4) / 115 = 1mm CPC.
The FTE 2.5mm has a 1.5mm CPC > CPC min-size so ok.
Basic problem with 2.5mm SWA at 20A is voltage drop.
- For 2.5mm SWA *alone* Vdrop @ 20Amps = 9.5V.
- For 4.0mm SWA *alone* Vdrop @ 20Amps = 5.5V.
- 17th requires lighting to 3%, power to 5% IIRC.
- That 3% of 230V = 6.9V, 5% of 230V = 11.5V.
Even 4.0mm SWA does not leave much room.
- Any House-end cable run before the 25m of SWA
- Any Garage-end cable run (within the shed)
Next problem may be your fuse of 20A may not be enough.
- May not be enough for the circuit - 4300W is 18.7A.
- May not be enough if saw & freezer startup together.
Remember a Type-B 20A MCB will trip