electricalforum.nz

Hi All,
With large heat pumps and ducted systems over 10a, just curious as to whether we should be using 'running current' or 'max current' when calculating max demand on an installation.
I've always gone with max current/worst case scenario, but wondering if I should just be going on running current instead.
Interested to hear what others are doing.

Cheers,

Domestic - table C1 group D

Non-domestic - table C2 group C

Thanks Jamie,
No issue with the table, I'm more after what amperage to use from the heat pump specifications. Eg, A heat pump im looking at has both 'rated amps(17.5A)' and 'max amps(30A)' in their specs. Should we be basing the demand calcs on the 17.5 or 30A figure?

Apologise, I thought there would be something a bit more to the question haha

Both seem to indicate "connected load" so I'm not 100% but I'd say probably the rated amps as that's going to be a more usual value where as max will only be for short periods of time I'd assume

I would be leaning towards the max amps, there is no way you can determine the usage of the heat pump, a safe assumption would to use the max amps.

I havnt done heat pumps for a long time but I feel like I remember seeing one in the past that listed running current and current under fault conditions such as a locked compressor.
I wonder if your two current ratings could be something similar but it would be hard to say without more information

Starting current or overload current (locked rotor, over pressure etc) , isn't usually allowed for in installation wiring. Such as the note in 3.6.2 for example.

But also the wiring and protective devices have to work in the real world.
So for example (without getting into the technicalities of soft start or VSDs in AC units) - if a unit has a nominal running current of say 9.5A, you are never going to put it on a 10A circuit. You would always be at least 16A, and more likely a 20A circuit.

But as your question is about maximum demand, you would have to use the highest input power. Some units have input power as "min-rated-max".
I think you would have to use the max, as they can actually run at that power input. But as the units get larger, usually the difference between rated and max gets less. And with the MD factor being 75%, you're only looking at 75% of the difference. Probably less than 5A difference in your max demand calcs at most.