New 1.5-ton AC circuit on a 22 m run with a weak supply and high earth resistance
A shop owner wants a new 1.5-ton fixed-speed split air conditioner on the rear wall of his shop. Nameplate: 1,450 W input, running current 6.8 A, locked-rotor current 32 A. The distribution board is at the front of the shop, 22 m of cable route away, mostly in PVC conduit above a false ceiling alongside existing light and socket wiring. The board has a 40 A main switch, a 30 mA RCCB, a 6 A MCB for lights and a 16 A MCB for sockets, none spare. Measured supply at the board at 7 p.m. is 214 V. The shop's earth rod gives 9 ohm on an earth tester. The owner wants to save money by running the AC from the existing 16 A socket circuit with 1.5 mm2 wire, or preferably from the 6 A lighting circuit which 'has a spare way'. Summer load will run the AC about eight hours a day.
1. Calculate the voltage drop at full load and at starting for 1.5 mm2 and 2.5 mm2 copper, and select the cable with justification.
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Given: one-way length 22 m, so loop length 44 m; resistivity of copper 0.0175 ohm mm2/m; Vd = 2 x I x L x rho / A.
1.5 mm2: R = 0.0175 x 44 / 1.5 = 0.513 ohm.
- Full load: 6.8 x 0.513 = 3.5 V, about 1.6 percent - acceptable.
- Starting: 32 x 0.513 = 16.4 V, so terminal voltage falls to 214 - 16.4 = about 198 V for the start interval. That is below the minus 10 percent limit of 207 V and will strain the compressor and its contactor, and may cause repeated start attempts.
2.5 mm2: R = 0.0175 x 44 / 2.5 = 0.308 ohm.
- Full load: 6.8 x 0.308 = 2.1 V, about 1.0 percent, well inside the 3 percent branch limit.
- Starting: 32 x 0.308 = 9.9 V, giving about 204 V momentarily, recovering to 212 V.
Selection: 2.5 mm2 copper. It meets the steady-state requirement with margin and limits the starting dip. 1.5 mm2 is not selected because of the start dip and because it also constrains the protective device to 10 A. Note the supply is already 7 percent low, so leave the option of 4 mm2 (drop about 1.3 V full load, 6.2 V at start) if the summer voltage falls further, and derate for the hot false ceiling and grouping with existing cables.
2. Select the MCB rating and type, state the isolation arrangement and the earth conductor, and explain why the existing 6 A lighting circuit and the 16 A socket circuit are both unacceptable.
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MCB: 2.5 mm2 in conduit carries roughly 18-19 A, so the protective device must not exceed that - a 20 A MCB is marginally too large. Use a 16 A Type C MCB dedicated to the AC. The Type C magnetic threshold is 5-10 times rating, i.e. 80-160 A, well above the 32 A locked-rotor current, so there is no nuisance trip on start, while the thermal element holds the 6.8 A running current indefinitely. A Type B 16 A (48-80 A magnetic) would also ride through the start, but Type C is the better match for a motor load with a long run.
Isolation: a 20 A double-pole switch or isolator next to the indoor unit so the unit can be serviced without going to the board; a 2.5 mm2 protective conductor run with the phase and neutral to the unit.
Why not the existing circuits
- 6 A lighting circuit: the MCB trips just above 6 A, so a 6.8 A running load will trip it in normal use; the lighting cable is typically 1.0-1.5 mm2 and feeds other points. Completely unsuitable.
- 16 A socket circuit: it is a shared circuit with an unknown future load, a socket outlet is not a substitute for a dedicated fixed-appliance supply, and there is no local isolation. Adding 6.8 A for eight hours a day to that circuit risks overloading the cable and nuisance tripping for other equipment.
Keep the 30 mA RCCB protection at the board and check its rating after adding the new way.
3. What do you do about the 9 ohm earth reading and the 214 V supply? Give the owner a prioritised recommendation.
Read the worked answer
Earth, 9 ohm. For a small shop a dedicated electrode should normally read well under 5 ohm; 9 ohm is high, usually from a single dry rod, a corroded joint or a loose earth bar connection. The immediate shock risk is still limited because a 30 mA RCCB should operate, and touch voltage at 30 mA through 9 ohm is only 0.27 V - but a poor earth lets low-level faults persist, stresses the AC's electronics, and leaves the installation dependent on one device. Actions, in order: clean and tighten the earth bar and the rod clamp and re-measure; if still high, drive a second rod about 2 m away (two similar rods in parallel give roughly half the resistance, around 4.5 ohm) or use chemical earthing; confirm continuity from the AC body to the bar.
Supply, 214 V. Already 7 percent below nominal and close to the minus 10 percent limit. At low voltage a fixed-speed AC draws more current, runs hotter and its compressor and PCB age faster; starting will be sluggish. Advise the owner to report the low voltage to the utility, to fit a voltage stabiliser rated for the AC's current if the problem persists, and not to add further load to the same board. The 2.5 mm2 cable keeps your own contribution to the drop to about 1 percent, so the remaining loss is upstream and outside the shop's wiring.