How many CTs needed to monitor US 230V heat pump?

I have this heat pump:

It is fed by a 240v split-phase circuit, with 3 conductors. It appears from the schematic & wiring diagram that one is used only as ground, rather than as neutral. 

R.L. amps: 16.7 compressor, 2.8 fan

L.R. amps: 82

1. Can this be monitored with a single CT, or do I need two? 

2. How would I calculate the appropriate burden based on these specifications, assuming I'm using the std YHDC 100A CT? It lists 24A minimum branch circuit ampacity, 40A max circuit protection rating. The breaker feeding it is 30A. Based on the RL amps listed, it appears that max normal load is 19.5A. I'm using the EmonTX Shield SMT, but may move to the new EmonTX. 

3. Do I need to do anything to take the inrush amps into consideration? 



Robert Wall's picture

Re: How many CTs needed to monitor US 230V heat pump?

According to the schematic, if I've read it correctly, there are indeed only two conductors (the PE doesn't count!), therefore you need only one CT.

If the circuit protection is rated at 40 A, you won't be losing that much by leaving the burden resistors alone and rating your Shield input at 100 A, it would ensure that any (realistic!) inrush is measured accurately, always assuming you're using the 'continuous' sketch or you use the 'discrete' one and you happen to catch it with a sample. But if you wish, you could double or treble the burden value and rate the input at 50 A or 33 A, though in the latter case you might risk clipping the inrush peaks. All the theory is in "Building Blocks".

ddcarter's picture

Re: How many CTs needed to monitor US 230V heat pump?

That all makes perfect sense. 


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