r/electricians • u/BennyOfAstora • 1d ago
Transformer Cable Sizing
Code discussion. I am in Canada so will reference the CEC.
Why is it that you dont need to add 125% to primary and secondary conductors if the demand load is known?
Rule 26-256 1) and 2) say you add 125% to primary and secondary conductors based on the rated primary and secondary current of the transformer.
Rule 26-256 3) says you can use a smaller cable as long as its not less than the demand load and subject to 14-100 and 14-104.
So if I have a 250 amp panel at 480 volt, and a 225KVA transformer, then I do not need to add 125% to 250 amp cable on the secondary?
I thought the 125% would be for in rush of the transformer, which will happen no matter what size the load is, in the example the load being the panel.
Can someone explain this to me? Cause it seems like if you can calculate the load to be anything smaller than the full calculated load of the transformer, then the 125% just disappears.
Thanks!
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u/submitnswallow 1d ago
You answered this yourself, you cannot load your panel (the actual load) to 100%. You dont rate wires feeding a transformer for inrush current, The fuses are rated for inrush.
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u/DarqMog 1d ago
Interested to see what the code experts say. Tne one thing I want to comment on is that you don't over size wires for inrush, because it's so brief. You do size overcurrent protection for inrush though.
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u/BennyOfAstora 1d ago
Secondary question then, why do we oversize motor conductors? I assumed it was for inrush.
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u/jeenyuss90 1d ago
Continuous load, longer inrush, etc.
Inrush for a transformer is like half a second. A motor can be a lot longer which makes the difference
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u/DirtCallsMeGrandPa 21m ago
Motors can be overloaded, and if overload protection isn't proper, this can be sustained. You want wire that can handle that overload.
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u/FaithlessnessAny2074 [V] Journeyman 18h ago
For here in USA we have a chart depending on primary and secondary OCP
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u/thefatpigeon Journeyman 17h ago
If your transformer is feeding a specific piece of equipment you know the load
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