Split-Phase Installation
This chapter assumes the reader has read the CT Basics section.
What is split-phase?

Worldwide, virtually all of the residential power delivered to residential
homes is 230V-240V single phase. In North America, the single-phase 240V
supply is split so that there are two 120V legs that are typically used for
lighting and light-duty appliances. To learn more see the Split-Phase Wiki.



Split-phase load center





A typical split-phase load center has two Main circuit breakers, one for
each of the two “legs” coming from the service entrance. There is also
a
third neutral wire that is directly connected to the neutral bus - a long
bar with holes and screws to connecte conductors. Another ground bus is
provided which is connected to a reliable local earth connection like a
ground stake. In most entrance panels, but not sub-panels, the neutral and
ground busses are “bonded” (connected together).

The service is typically described in terms of the amperage rating of the
Mains circuit breakers or fuses. Most common are “100A service” and
“200A service”.

See the North America section of this Load-Center Wiki. to see how the
split
-phases correspond to alternating breaker rows.

	
	 
	Maybe neater than normal load center




Monitoring split phase
When monitoring a circuit in a split phase service, it’s helpful to
recognize exactly which of the mains circuits it is utilizing. There are
three possibilities:


120V phase A
These circuits are two conductor (plus ground) and typically use a black
conductor connected to the circuit breaker and a white conductor connected
to the neutral bus. As described in the Wiki, circuit breakers in odd rows
will be on phase A. Odd rows include breakers numbered [1,2] [5,6]
[9,10]..etc.



120V phase B
Same as above, except circuit breakers will be in the even rows with
breakers numbered [3,4] [7,8] [11,12]… etc.



240V
 These circuits are typically larger appliances like Hot-Water, Range,
Pumps, Electric Heat, Dryers, Heat-pumps, sub-panels, and Air-conditioners.
They use two circuit breakers in adjascent rows, so one of the breakers is
on phase A and the other is on phase B.
 As explained in the CT Basics section, 240V loads can be two-wire or three
-wire.
 Two-wire loads typically use two conductor cable with the white and black
conductors connected to the two circuit-breakers. For these circuits, a
single CT can be placed on either one of the two conductors and the
“double” box checked to indicate to IoTaWatt that the voltage is
doubled
to 240V. Orientation of the CT is dependent on the row that the CT is
associated with.

 Three-wire loads typically use three conductor cable with black, red, and
white conductors. The red and black are connected to the two breakers and
the white is connected to neutral. These loads must be measured with two
CTs, one on the red and one on the black, or by passing the two conductors
through one CT in opposite directions so that the resulting orientation of
the CT is correct for the row of the breaker that each conductor is
connected to. An illustration canm be found in the CT Basics section.



Voltage Reference
The voltage between the two mains conductors is nominally 240V. That is
brokenb down into two 120V potentials between each of the Mains and the
neutral, but they unless they are in perfect balance, there will always be
a
difference in voltage between the two phases. That said, the difference in
most situations is practicxally negligable and the sum of the two will
always equal the voltage between the two Mains.

This is an important point, because in order to measure real-power, what
your meter measures and you pay for, it’s necessary to also have a
reference voltage on that circuit.

IoTaWatt supports multiple voltage references, but as a practical matter,
you only need one to measure any of the three circuits described above.

The simplest and most effective reference is a 120V wall transformer
connected to an ordinary 120V plug as close to the load center as possible.
That will provide a reference for the phase it is plugged into, and a
reference that is the exact opposite of the other phase, or to put it
another way, the reverse of the other phase.



Mains CT orientation
The first CTs to be installed should be the Mains.

As mentioned in multiple discussions, this is a job for someone familiar
with
the working with live, partially exposed electrical wiring and familiar
with

all of the risk factors. An electrician is recommended.

Because each of the Mains is on a different phase, and we are using the
same
voltage reference for both phases, we need to reverse one of them so that
the current it measures aligns with the reversed voltage reference. So
which
one? The answer is that because the wall transformer plug is not polarized
as well as other uncontrolled factors, we don’t know yet.

So what we do is install just install the CTs with opposite orientation,
configure them, and look at the status display of the IoTaWatt. If
reversed,
they will show a ↺ symbol. To correct this, you can do one of two things.

Reverse the wall transformer in it’s socket.
Click the “reverse” box in the VT configuration menu.


Now the two Mains inputs should show the Watts for each Main and no reverse
↺.



Load CT orientation
First, it’s important to note that the only consequence of installing a
load CT backward is that it will show a ↺ symbol next to the input in the
status display. This indicates that IoTaWatt has recognized that the
voltage
and current are opposite and is producing the correct measurement by
reversing the output numerically. There is no error attributable to this
correction. If the ↺ symbol doesn’t bother you, you can place the CTs
without regard for phase.

Another approach is to simply install the CTs without regard for phase
orientation and then simply check the “reverse” box for any inputs that
show the ↺ symbol in the status display or physically reverse those CTs
in
the load center.

To install with correct orientation initially, the easiest method is to
install one CT on an active circuit and note if the ↺ symbol appears in
the status display. If so, reverse that CT. Now note which way the
correctly
oriented CT is installed and whether it’s row is even or odd.

If it’s an even row, all of the CTs that you install on even row circuit
-breakers should be installed with the same orientation and the odd row
circuit breakers with the opposite orientation. And visa-versa.