I am new to this forum and would like some help with wire sizing for a Delta Wye transformer. My situation is as follows:
Incoming to the transformer is a three wire (three phase) 480V supply. The secondary is 120v, but is broken down into three separate 83amp circuits. The total secondary amp draw is 198.
Do I size the four wires going to the control panel for the entire 198 amps or can I size each line at the 83 amps (plus 125% and whatever fuse size there is for each leg). There is only one nuetral (X0) and three hot wires (X1,X2,X3). I am thinking I need to run these wires as 3/0, but would like another opinion to see if they could be smaller.
The transformer is 30KVA, The 198 amps is what my control circuit is actually pulling. So my plan is to break my circuit down into three smaller circuits. One (X0 - X1)will have two sets of heat tracing each fused at 30 amps and a convience outlet fused at 20amps. The second circuit (X0 - X2)will have two sets of heat tracing at 30 amps each and a PLC curcuit with instrumentation at 4 amps, and the third (X0 - X3) will be two heat trace line fused at 30 each. So I will pull one neutral and three hots and have amp draws of 80,64, and 60.(or at least fused at those numbers)
If this makes sense and I understand your previous comments I should be able to run all four wires using #4 and a ground at #6. Do you agree with that.
Time to ask a question. Is this an engineering excersize, or is this something that will be fabricated and inspected?
You have 3.6KW of heat trace in each circuit. (120 volt, 30 amp) Or, are you protecting the circuit at 30 amp, and the actual load is less?
Further, have you planned for ....."427.22 Ground-Fault Protection of Equipment. Groundfault protection of equipment shall be provided for electric heat tracing and heating panels. This requirement shall not apply in industrial establishments where there is alarm indication of ground faults and the following conditions apply: (1) Conditions of maintenance and supervision ensure that only qualified persons service the installed systems. (2) Continued circuit operation is necessary for safe operation of equipment or processes.
Does the install qualify as an industrial establishment?
How do you propose to provide the OCP you mention?
I see a 100 amp secondary main OCP (breaker) minimum #3 THHN Cu, and branch cbs as required. The 4 amp PLC protected by a 15 amp cb, or in-line, or fuse block if you so choose.