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In the main panel, there are 3 wires for 3 phase power going from 3 pole 40A breaker to a sub panel. the breaker is (CKIPSAL 4CB340/6 C40) The wires are in 2 cables.

Each of these wires has 7 strands 19 SWG each. Online search reveals that 19 SWG handles 4.13amp. which tells me that the wire is expected to handle 29amp (7 x 4.13)

If the breaker should protect the wire, then why it is 40amp when the wire is 29amp? i.e. I expected to see the breaker amp less than the wire amp rating.

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  • \$\begingroup\$ Because somebody had a 40A breaker in the back of the van, and didn't want to drive down the wholesaler to get a 32A one? \$\endgroup\$ – Simon B Mar 6 '18 at 23:31
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Based on your data, your 7 wires of #19SWG are 0.40" (1.016mm) in diameter each.

That makes 1600 circular mil (0.81 sq.mm.) each. x7 = 11200 circular mil (5.672 sq.mm.) the bunch.

By contrast, 10 AWG is 10400 circular mill (5.26 sq.mm.) 10 AWG is legal for 30A@60C, 35A@75C, and 40A@90C in the National Electrical Code, which is more conservative than European codes.

This wire is slightly bigger than 10 AWG, so it should be fine at 40A if the wire, terminations and wire routings are safe for 90C, maybe even 75-ish C.


A bigger problem is the wire in two cables. That is not permitted in the US electrical codes, and for good reason - it will turn anything between the cables into the core of a transformer and induce non-trivial amounts of energy, making heat and vibration which can work-harden, fracture or abrade cables. It may be permitted inside a chassis if UL/CSA/TUV etc. approves it, which they may not.

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To determine the ampacity of the wire, you must use data for the specific wire size and insulation type. You can't count the strands and use whatever information you find about the strand size. You must also consider the maximum ambient temperature and number of conductors in the conduit etc.

A 40 amp breaker may not be appropriate, but you need to do a proper determination.

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