Archived Forum

As no-one with local knowledge is going to chip in, here's a UK view:

[Note: "8-wire" means nothing to me, I assume you are supplied by a star-connected distribution transformer with 3 centre-tapped windings, the centre taps being the common neutral, and earth, the 3 windings being 120° apart, so "208 V" refers to the voltage between phases 120° apart, i.e 120 V phase-neutral].

Broadly speaking, you need 3 emonTx's to monitor three phases. However, someone was looking at adding two extra voltage inputs (off-board) to the EmonTx to give that capability - you'll need to search the forums, but then as there are only 3 c.t. inputs, you have another problem - see below. If you come across comments that say the three phase voltages are identical, then you need a healthy dose of scepticism because that is true only for a perfectly balanced system that you rarely see in practice. However, one of our US correspondents has reported that the voltage balance between the two legs of a phase and neutral is good (I think though it was a domestic supply so it may not hold true for you), so it may be unnecessary to monitor both halves of voltage, though of course if any load is connected across 120 V, then you do need to measure two currents per phase.

If you feed multiple conductors through a c.t., the result is the vector sum of the currents.

I can't offer a comment about the number of emonTx's that can be serviced by single emonBase. The Tx's transmit asynchronously so it's possible that readings may be missed if there's a collision. I have a vague recollection that someone has used Xbee instead to get round that problem - again a search of the forums might reveal more information.

[Added] There's comment here about the number of emonTx's per emonBase: http://openenergymonitor.org/emon/node/651

This post mentions using an off-board 16 channel multiplexer:

openenergymonitor.org/emon/node/619

Alas if the project bore fruit, it wasn't documented in the forums.  emonTx begs for three voltage inputs so that three phase power can be accurately measured.  Unfortunately I'm completely unqualified to implement that =(  

> If you feed multiple conductors through a c.t., the result is the vector sum of the currents.

Can I take what you've said to mean that I can clamp as many conductors in the same phase as my CT can handle, needing only to use one voltage measurement reading for that phase, assuming that ultimately all the conductors being measured are from the same panel (and therefore same the same feeder)?

I've since found evidence that one can `hang' a few emonTx's off an emonBase.

You're a consistent, huge help to the community Robert!  Thanks again, -mt 

"Can I take what you've said to mean that I can clamp as many conductors in the same phase as my CT can handle,"

I was being cautious because I don't know American practice, nor do I know what sort of accuracy you need, nor how closely (in terms of individual loads) you need to know the usage. From what you are hinting at, if you just need the total consumption, can you not put the c.t's on the incoming cables to the panel? You can of course use different c.t's to our standard 100 A one.

Perhaps you could give me a bit more background information - I see who you work for, do you have in-house electrical engineers who can flesh out the details for me - like the actual loads (Amps) and the degree of current unbalance, the actual neutral voltage, diversity, where it would be possible to fit c.t's, etc?

You could also try a private message to jstoaks (click his name and "send this user a message") to see how he got on with his multiplexer. (He owes it to the community for the help he got to report back!).

> ...can you not put the c.t.'s on the incoming calbes to the panel?

Our 208 VAC 3-phase service panels have three conductors coming into them, each of which is capable of carrying 225 Amps.  The cables are too fat to accomodate the 100A clamps sold on this site.  Even if they did, I'm assuming I'd have to change the resistors that bridge each of the CT circuits to allow measurement up to 225 A.  If I could source clamps of larger diameter that fit the bill, then it would be great to be able to have one emonTx per service panel.  This would make pitching the emon system easier here at work.

> ...nor do I know what sort of accuracy you need

I believe even +/- 10% accuracy would be fine.  Currently we have no idea what our power consumption is, so this level of accuracy would be acceptable.  

> ... like the actual loads (Amps) and the degree of current unbalance, the actual neutral voltage, diversity, where it would be possible to fit c.t's, etc?

I'll forward this query along to the facilities electrician with whom I've been working.  I'm not sure what you mean by `diversity'.

I'll ping jstoaks.  

Thanks R!  -mt

If your electrician can take the panels out of service, he should be able to disconnect and fit ordinary ring-type c.t's. You could ask him to talk to his suppliers and find one suitable for the current with a 1 A secondary and VA rating enough to give the required voltage - a little more than 1 VA. You would then hang a 1.2 Ohm burden resistor across it which with the existing 18 Ohm burden resistor in the emonTx in parallel would develop 1.125 V. That is fractionally too much - you want 1.1 V rms - but if needs be you could easily reduce that with a resistor change on the emonTx or a bit more resistance in parallel.

This c.t http://www.owen-brothers.com/clip-together-split-core-current-transforme... is not ideal - it has a 2.5 A secondary which means a lower value and higher power burden, but worth considering.

I think if your power factor is reasonably constant and the phases are balanced, you will probably be able to get an estimate within 10% for the power, when reading only one voltage. If those assumptions aren't valid, then if you are happy to (or you can enlist somebody who can) build up a few components on stripboard and connect them to the emonTx, then with 3 voltage adapters you can measure all 3 phases and be much more accurate.

Diversity is the allowance you can make when you know that the whole installed load will not all be switched on and in use at the same time - so it allows you to have smaller cables than is strictly necessary. You might know it by another name.