This information is used to get a
basic understanding of three phase wiring for control panels. Many technicians work around three phase
everyday, yet they do not understand how to design a control panel. First
we start with the current law. The diagram below is just theory. It shows
A4 (Incoming current) = A1 (Load current 1) + A2 (Load current 2) + A3 (Load
current 3). The arrows indicate direction and flow of charge. The
black dots indicate a connection point.
A4 = A1 + A2 + A3
Next we have the single line diagram. This notation is the
next step after the current law. Below a 3 phase system is shown as a single
line. We just add a main disconnect, circuit breakers, motor starters,
motors and a single phase transformer. A single line diagram is often used
by the person who is hiring the electrical work to be done. It allows you
to change the size of the loads quickly for bidding purposes.
Here you can see that 5 HP, 10 HP, and 5 KVA are
the key elements that indicate the current load. These key elements
dictate the size and cost of your control panel.
Finally a three phase wiring diagram. Now this information is used
by the guys wiring the control panel. It illustrates the connections in
more detail. Note that the black dots above have been replaced with a
Power Distribution Block below.
A very common question when designing a control panel is "Does my
control panel need a Main Disconnect?" Main disconnects are expensive.
Answer: If the source of the power is in the same room and it
has a labeled disconnect near the floor already, then you
don't need a Main Disconnect on your control panel. You only need a Power Distribution Block.
Next question: How do I size the Main Disconnect? Answer:
Add up the loads plus 25% of the largest load. This gives you the FLA
(Full Load Amps) for the control panel. For the above model you have
