Panel wiring

[richerdick32]

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.

 

[7anoter4]

If I well understood, the main breaker of 50 A it is a common breaker on the inlet cable supplying the busbar and the all the other[ downstream] outgoing breakers [of 4 or 10 A] are connected through the ice cube relays with 1.5 mm^2 [for 4 A, I suppose] and 2.5 mm^2 after 10 A[may be].
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.

 

[LionelHutz]

The relay contacts are before the smaller breakers? If yes, then I'd say that is backwards. The smaller breakers should be first before any relay contacts. Having a smaller wire tap off a main circuit to feed a breaker or fuse suitably rated for the wire is OK.

 

[irmalcol]

I agree with LionelHutz. It would be better to have the relays downstream of the smaller breakers.

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.

 

[jraef]

The rule, at least in UL 508A, is the same as irmalcol reported from a CSA standpoint )although I'm not sure about the length, I though it was 6ft). It is commonly referred to as the "feeder tap rule" although officially, that is different. The concept behind it in a control panel is that between that 50A breaker and the smaller breakers, the only place a current overload can take place is below the smaller breakers. Between the large and small breakers, the only potential problem is a short circuit or ground fault, and if the conductors are of limited length and never leave the enclosure, and the other wiring rules are properly applied in that enclosure, that likelihood is extremely low. I don't know if a similar rule applies outside of North America, but I imagine it does because most international suppliers offer "splitter blocks" that are used expressly for this purpose.

"Will work for (the memory of) salami"

 

[jraef]

PS, it is 10ft, so 3m.

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"