Just as an example, if this was in the UK and I was looking at the design, my first step would be to look up 90C rated single core cables (Table 4E1A in our wiring regs) and in column 4 we have the rated CCC (ampicity) for 2 conductors enclosed in conduit on a wall (table for ambient of 30C). The choices that look applicable are:
- 2.5mm (4% under #13 AWG = 2.62mm) = 31A
- 4mm (4% under #11 AWG = 4.17mm) = 42A
- 6mm (10% under #9 AWG = 6.63mm) = 54A
Then on the assumption the ambient temperature could be 70C we would refer to Table 4B1 for de-rating and under the "90C thermosetting" column we get a factor of 0.58 for 70C ambient, leading to a new set of CCC values:
- 2.5mm = 31A (at 30C) * 0.58 => 18A at 70C
- 4mm = 42A (at 30C) * 0.58 => 24A at 70C
- 6mm = 54A (at 30C) * 0.58 => 31A at 70C
The spec you have gives minimum ampicity at 19A so we would have to chose 4mm or 6mm for this circuit.
Next we would check voltage drop and from the next page Table 4E1B column 3 has the VD for 2 single phase cables in trunking or conduit as:
- 2.5mm = 19 mv/A/m (but not acceptable on 70C ampicity)
- 4mm = 12 mV/A/m
- 6mm = 7.9 mV/A/m
You give the distance as 40' = under 13m so at the 19A rating for the A/C unit:
- 4mm = 12 mV/A/m * 19A * 13m = 2.9V = 1.2%
- 6mm = 7.9 mV/A/m * 19A * 13m = 1.9V = 0.8%
So both meet VD of 5% for final circuit and so 4mm (approx #11 cable) would probably be our choice.
The other thing we do is compute the earth loop fault impedance Zs which is based on the supply impedance (Ze at source of installation, Zdb at sub-panel) and the resistance of the line conductor (R1) and that of the CPC (circuit protective conductor = ground wire) commonly given the notation R2. We require sufficient fault current in the event of a L-E (hot-ground) fault that the breaker disconnects in under 0.4s, typically this means you have to hit the magnetic trip point.
Here the USA is more complicated as your 240V is actually 120V-0V-120V two-phase and we don't know your supply impedance or that of the EMT conduit you would be using. Assuming the conduit is mechanical protection only and not the CPC (i.e. even though the EMT is grounded you run another #11 ground wire through it for the load), then R1+R2 is going to be the same as the above voltage drop figures:
- 4mm = 12 mV/A/m * 13m = 0.16 ohm
- 6mm = 7.9 mV/A/m * 13m = 0.1 ohm
We don't know the Zs at the DB feeding this, but
assuming it is 200A rated and 5% drop on the 240V then the prospective short circuit current = 4kA so impedance around 0.06 ohms. Again,
assuming (always a dodgy thing to do...) that the CPC and line are same ratings, then PFC=PSCC so we get our end of circuit Zs values as:
- 4mm case Zs = Zdb + R1+R2 = 0.06 + 0.16 = 0.22 ohm
- 6mm case Zs = Zdb + R1+R2 = 0.06 + 0.1= 0.16 ohm
Worst-case fault to ground would be, say during -10% on your nominal 120V to ground (108V) and fault currents are then:
- 4mm = 108V / 0.22 ohm = 490A
- 6mm = 108V / 0.16 ohm = 675A
I don't know your breaker characteristics, but in the UK/EU the smaller systems (panels typically to 250A in, 63A max out) we have three MCB curves to chose from that alter the ratio of magnetic trip to thermal rating. Here it would be a 32A breaker as specified by the manufacturer and so our choices are:
- B-curve = 3-5 * In = 5*32A max = 160A
- C-curve = 5-10 * In = 10*32A max = 320A
- D-curve = 10-20 * In = 20*32A max = 640A
So on the above assumptions we would need to use 6mm cable if a D-curve breaker was in use, but typically for something like A/C it would be C-curve in which case both 4mm and 6mm would lead to fast enough disconnection in the event of an earth fault.
TL;DR Under UK regs #11 wire with 90C rating would be acceptable (if assumptions true). Disclaimer: the above is
not the USA code!
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