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Joined: Dec 2012
Posts: 3
New Member

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.

Thanks for any advice I can get.


Joined: Jul 2004
Posts: 9,688
Likes: 10
If you have individual O/C protection on each secondary conductor I see no reason why the conductors need to be any larger than the O/C will protect.
I am also not sure where you get 198a.

Greg Fretwell
Joined: Apr 2002
Posts: 7,297
Likes: 5
Welcome to ECN!

A little more info will help...
KVA of transformer
Secondary is 4 wire, 3 hots, 1 neutral, so is it a 120/208 volt 3 phase, 4 wire?
As Greg said, 83 amps per leg, how do you calc 198 amps.

Sort of sounds like a 75 KVA, 480/120-208 3 phase which is commonly fused at 100 amp primary and 200 amp secondary; without 'pushing' the transformer.

Joined: Dec 2012
Posts: 3
New Member
Thank you for your responses!

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.

Joined: Apr 2002
Posts: 7,297
Likes: 5
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.

Joined: Dec 2012
Posts: 3
New Member

This is a real project that we are building on our shop floor. The heat tracing circuits are actully between 1.5kW and 2.2kW. All being protected by 30 amp circuit breakers.

Our equipment will be monitored at the installation site and is necessary for safe orperation of the equipment. This will be located outside in northern Canada.

I looked at the numbers and we are planning to run the three hot wires and the neutral as #2.

Thanks for your responses, do you agree with the fact the common neutral is the same size as each hot wire?


Joined: Dec 2001
Posts: 2,495
I assume that the heaters are simple resistive loads, so I'd say yes, there is no need to increase the neutral size. Using a full-sized neutral seems sensible.

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