Power Wiring for branch circuts
Power Wiring for branch circuts
(OP)
I have a stituation with one of my electrical engineers. The problem is with the design of his power destribution for branch circuits and the gauge of wire to be used.
The rule-of-thumb that I was taught was that the wire going to the branch fuse needs to be the same size (gauge) as that from the main disconnect. For example if my main feeder wiring is gauged for 30 amps (10 AWG), and I have lets say a 5 amp branch circuit (fused at 5 amps).
My quetion is does the wire that goes form the main feeder to the fuseholder need to be 10 AWG or can it be a smaller gauge (like 16 or 18 AWG).
My engineer insits that it can be smaller.
Can someone clearify this issue
The rule-of-thumb that I was taught was that the wire going to the branch fuse needs to be the same size (gauge) as that from the main disconnect. For example if my main feeder wiring is gauged for 30 amps (10 AWG), and I have lets say a 5 amp branch circuit (fused at 5 amps).
My quetion is does the wire that goes form the main feeder to the fuseholder need to be 10 AWG or can it be a smaller gauge (like 16 or 18 AWG).
My engineer insits that it can be smaller.
Can someone clearify this issue





RE: Power Wiring for branch circuts
RE: Power Wiring for branch circuts
RE: Power Wiring for branch circuts
RE: Power Wiring for branch circuts
RE: Power Wiring for branch circuts
what exactly is your electrical engineer saying that you do not agree with?
RE: Power Wiring for branch circuts
What he is saying, that the wire going to the fuse can be a smaller gauge then that of the main line wire, which is 10 gauge. He telling me that a 16 and 14 gauge wire can be used up to the fuse.
RE: Power Wiring for branch circuts
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Power Wiring for branch circuts
I don't see anything about enclosures. This appears to be covered by NEC 241.21B. Are you citing a different document? The first NEC consideration is the Length of the tap ahead of the fuse.
bigdog,
Why the "situation"? Can't you two pull out the book(s) and discuss what applies?
RE: Power Wiring for branch circuts
UL 508 I beleive, I don't have a copy with me here. It's only recently that the NEC got into the business of what goes on inside an enclosed controller, that used to be the realm of UL. Not everything that's in UL 508 is in the NEC and vice versa.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Power Wiring for branch circuts
CEC;
For a power circuit. The wire needs to be <7m long, >1/3 the main conductor in size and all within the same enclosure.
According to UL508;
18.2.1 Conductors of control circuits that are connected to the load side of the motor branch-circuit short-circuit protective device – common control – shall be protected against overcurrent in accordance with Table 18.1 by protective devices located within the controller. Overcurrent protective devices shall be provided in each ungrounded conductor, located no more than 12 inches (305 mm) from the point where the conductor is connected to the source of power.
I don't know of anything in either UL508 and UL508A about splitting the main power into branch power circuits. This would be something like splitting one main breaker to feed a bunch of small motor starters in the enclosure. However, we have done it by using the power wire required for the smaller circuit and the UL inspector has looked at the
I don't have the NEC here.
RE: Power Wiring for branch circuts
I'm on the road and don't have my codes. There used to be a provision in the CEC for 1/10 size for shorter distances and possibly other restrictions. Can you check that out for us.
Thanks.
RE: Power Wiring for branch circuts
As and example, if you were to look inside a residential clothes dryer. The electrical code requires you to install a 30 amp circuit breaker in your electrical panel and to use a 3 conductor # 10 awg cable to bring the power to the dryer. Once the power is delivered inside the dryer it is subdivided at a terminal block. None the wires attached to this terminal block are protected by individual fuses. These wires provide power to the timer, the drive motor and the heating elements. None of these wires are the same size as the cable supplying the power from the panel. In fact they are 14 awg and 16 awg in size, the only overload or short circuit protection they have is the 30 amp circuit breaker at the distribution panel. Based on the fact that these dryers are UL approved I would have to agree with your electrical engineer's point of view.
RE: Power Wiring for branch circuts
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Power Wiring for branch circuts
I don't know where it is in the code if it's there.
NewfieBoy;
That could be an invalid example. The dryer was likely UL tested as complete assembly which means the manufacturer can do things not allowed by someone following the NEC or UL508.
Jraef;
I know you're right or that it's acceptable practice but I couldn't find anything in UL508 or UL508A that supports this type of installation. As I didn't finish before, we are UL508A listed and split large circuits into smaller subcircuits only feeding the circuit protection with the power wire size it requires and the UL inspector has looked at this a number of times without questioning it.