Power Wiring for branch circuts 3

[bigdog]

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

 

[davidbeach]

How about a whole lot more information? What are you wiring? You are using a lot of terms associated with building wiring, but have all the sizes wildly out of whack. On the other hand some of it sounds like maybe in a control panel, but the terminology seems wrong. What rules are you under? Is this NEC or are you in a UL-508 control panel, or something else? Not nearly enough information to begin to answer the question.

 

[bigdog]

Industrial panel and motor control circuits. Im using NFPA 79 and NEC.

 

[bigdog]

Your right davidbeach is it a control panel.

 

[stevenal]

NEC 241.21B

 

[NEWFIEBOY]

hi bigdog

what exactly is your electrical engineer saying that you do not agree with?

 

[bigdog]

Newfieboy

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.

 

[jraef]

He's right. As long as it is in the same enclosure and the circuit has a smaller fuse, the wire can be sized for the fuse, even upstream of it. If on the other hand the wire exited the enclosure in a conduit going to a fuse in another panel, then that wire would need to be sized to the breaker feeding it. The fuse in the remote panel is where the circuit size changes.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[stevenal]

jraef,

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?

 

[jraef]

stevenal,
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

 

[LionelHutz]

I only know of 2 references to what you are asking.

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.

 

[waross]

Hi LionelHutz;
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.

 

[NEWFIEBOY]

Bigdog

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.

 

[jraef]

Power splitting within a control panel is a common practice, as evidenced by products dedicated to that purpose such as these ILSCO power distribution blocks. You can connect 350kCMIL wire on one side, and 12 #14AWG wires on the other side, and as long as you are in the same panel you do not necessarilly need individual fusing for each of those 12 wires as they feed other devices. The circuits do need short circuit and overload protection in them at some point, such as MCP breakers and overload relays in motor starters, but from the distribution block to the MCPs, technically those wires are only protected by the main breaker from which the 350kCMIL cable came from.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[LionelHutz]

Waross;
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.