S
Smugley
Afternoon all.
Re-doing my calculations for a hot tub supply after client upgraded the specs.
Details are: Hot tub rated at 16A. Supplier advises to use Type C breaker to handle inrush when pump starts. 26m from CU to Hot tub via rotary isolator switch.
I was proposing to use a 20A Type C RCBO at the board, and take 4mm 3-core SWA from the CU all the way to the isolator switch. Run will be under 1st floor floorboards, through external wall, clipped direct to brick wall for 1m, then the rest underground. I chose to take the SWA all the way to the CU in order to keep the Zs down as low as possible. The calculated R1+R2 for 26m is 0.23 Ohms. (Not taking the armour into account). This would be fine at the moment as I measured the Ze as 0.15 Ohms, giving a total Zs of 0.38 Ohms. Plenty under the maximum allowable 0.93 Ohms for a 20A Type C RCBO.
Those who haven't fallen asleep yet will be asking Ah, but what type of supply is it?
Unfortunately it's a TN-S supply. So although unlikely, the Ze could rise up to 0.8 Ohms and the supplier would still be within the allowable limit. This would bring the design Zs up to 1.18 Ohms and in reality perhaps a bit higher. Should I be calculating the Zs based on the supplier's maximum allowable Ze? If so I'm going to have to install a huge cable to run a relatively small load. Is this overkill and do you think my choice of 4mm cable is acceptable?
Cheers
Re-doing my calculations for a hot tub supply after client upgraded the specs.
Details are: Hot tub rated at 16A. Supplier advises to use Type C breaker to handle inrush when pump starts. 26m from CU to Hot tub via rotary isolator switch.
I was proposing to use a 20A Type C RCBO at the board, and take 4mm 3-core SWA from the CU all the way to the isolator switch. Run will be under 1st floor floorboards, through external wall, clipped direct to brick wall for 1m, then the rest underground. I chose to take the SWA all the way to the CU in order to keep the Zs down as low as possible. The calculated R1+R2 for 26m is 0.23 Ohms. (Not taking the armour into account). This would be fine at the moment as I measured the Ze as 0.15 Ohms, giving a total Zs of 0.38 Ohms. Plenty under the maximum allowable 0.93 Ohms for a 20A Type C RCBO.
Those who haven't fallen asleep yet will be asking Ah, but what type of supply is it?
Unfortunately it's a TN-S supply. So although unlikely, the Ze could rise up to 0.8 Ohms and the supplier would still be within the allowable limit. This would bring the design Zs up to 1.18 Ohms and in reality perhaps a bit higher. Should I be calculating the Zs based on the supplier's maximum allowable Ze? If so I'm going to have to install a huge cable to run a relatively small load. Is this overkill and do you think my choice of 4mm cable is acceptable?
Cheers
