Panel wiring
Panel wiring
(OP)
Hi, I work for a company who has proposed for me to make alteration to a 24v dc control circuit. I have looked at the proposed alteration and can see from the drawings that the current circuit has a 50A breaker feeding several other smaller breakers between 4A and 10A. These feeds are routed through ice cube type relays but are done in 1.5mm and 2.5mm cable. To me this is under-rated as the both the relay contacts and the length of cable between the 50A breaker and the 4A breaker are being protected by the 50A breaker so should therefore be of suitable csa to take 50A. Am I correct in this or being over fussy. Does the fact that it is 24vdc or wired inside a control panel allow for underrating of cables? Is this common practise. I don't want to question the circuit designers without due cause.





RE: Panel wiring
In my opinion, the wire is suitable with the breaker setting-copper wire even PVC insulated, 1.5 mm^2 copper, in 30 oC ambient air, in conduit withstand 14.5 A and 2.5 mm^2 19.5 A [IEC 60564-5-52].For 50 A –in the same conditions-you need 16 mm^2.
RE: Panel wiring
RE: Panel wiring
However, as far as violating code, you might be OK. Assuming the small conductors you are worried about are not more than 3 m long, they don't leave the panel, and they are rated correctly for their entire downstream load, "protection shall be permitted to be omitted." See rule 14-100 (b) of CSA 22.1. Someone please correct me if I am interpreting this rule incorrectly. I don't know whether this applies to overcurrent protection that is oversized or only if protection is absent altogether.
I don't recommend relying on this rule, though. It's always better to have a design that is clear to anyone who looks at it.
RE: Panel wiring
"Will work for (the memory of) salami"
RE: Panel wiring
In the official Feeder Tap Rule for the NEC, which governs field installations, there is another requirement that the conductors not be rated less than 1/10 the ampacity of the main circuit protection device.
"Will work for (the memory of) salami"