Ampacities (wire) and a Pannel box?

As I understand it free air is not inside anything, like the utility wires on a pole. If wiring is inside any structure or conduit then the conduit ratings apply.

Well I thought that some thing like that was the case, but it appears that the manufacture of the panel assembly ( GE ) used the free air sire sizes to feed one of the sub panels, and then again, when they wired another sub panel off of the first with sub feed lugs.

Thanks

William.....

Once you get inside a "listed assembly" you will find a lot of things that don't meet the NEC rules. These require engineering and testing.

key word here is manufacture. I used to work for a electrical equipment manufacture. As long as it passed testing in the lab and UL bought off on it. It became part of the standard. I could do it but a contractor could not. Cause I was considered the factory. Kept my head spinning for a long time.

Once with parallel conductors for an MCC tie, between 2 sections I was told to go out and "tie" the cables together. When I questioned how I got screamed at. So I called the UL rep for the plant and got the skinny. There was an actual method outlined using 1/2 nylon rope using square knots every

6 inches. So I went out and did exactly what the paper stated. The local inspector when nuts. I showed him the paper and the phone number and told him to make the call. I handed him my cell phone and he made the call. The installation was passed first and last time I have ever installed rope in electrical equipment. Still have 20 feet of the rope in the garage if anyone is interested.

Listed assemblies use different charts then the NEC charts.

Interestingly, "free air" does not appear to be defined in the code, although in a few cases it seems to imply the conductors do not touch each other, having a specific distance between them (such as on a ladder style cable tray).

It appears that "free air" means it is not enclosed inside of anything.

Listed assemblies use different charts then the NEC charts.

Interestingly, "free air" does not appear to be defined in the code, although in a few cases it seems to imply the conductors do not touch each other, having a specific distance between them (such as on a ladder style cable tray).

It appears that "free air" means it is not enclosed inside of anything.

Well there ya go, in the installation manual for the panel board it shows the feed wires from the under ground conduit tied up to a strut with rope! It shows a wrapping and knot "plan" for attachment of the feed wires. I was even thinking about doing it but I really can't see where they are going to go as the box is going to be bolted to the floor with at least 4 bolts @

1/2" X 3"

William...

I suspect the concern is for strain relief of the wire/terminals, and not the physical security of the panel itself. Imagine the pullout stress on the wire/terminal connection if there was much vertical drop (panel mounted on the xth floor) in the wire run, with the conductors run nearly straight into the lugs (bottom feed bus), or to keep the wire from digging into various edges with the conductors up the side & 180 into the lugs (top feed bus).

Louis --

********************************************* Remove the two fish in address to respond

I think you must be right. I don't think that the wire moving in my case will be of any concern. The 350 kcml wires are entering the building in 2,

3" conduits that are made up in a "U" 2, 90 deg L's and about 1' of riser on one side and a 1.5' slip joint on the outside into the power co required box. It penetrates the foundation wall ( and is grouted in ) then up through the 6' concrete floor. The total length of the wire runs is about 16' I think that there will be more of a problem pulling it in the conduit than it moving once it's in place.....

William.....

The NEC allows the calculation of conductor ampacities by a formula given in 310.15(C) (2002 Edition) as an alternate to the values in the tables. If you think its worth having an engineer sign off on the analysis to save a little wire, go for it.

When a manufacturer is cranking out thousands of units, its worth doing the engineering to save a few pennies each.

The reason for tying the feeders is to support them under the mechanical loads from a fault current. This is required in order to meet the manufacturer's SCA rating on the panel. UL has tested and confirmed the manufacturer's instructions.

Ben Miller

So the UL wants the rope with the feed wires tied up, but the NEC doesn't require it?? So should I do it or not??? The wires are going to come out of the conduit at about 6" above the floor Then they will rise up about

16" and make a "U" turn back down and pass through the CT's then another "U" back up and a 90 deg to the main breaker lugs. I would guess about 5' of the dual 350 kcml ( given the bending restrictions on that heavy of wire ) would be contained in the panel. If I was to tie them it would need to be about 12" out of the ground / conduits.

William

The NEC does not address design details inside of equipment. Instead, it requires a listed panel, with the knowledge that the listing agency (ie. UL) will take care of those details. The instructions were submitted to UL, and the panel was tested accordingly. The panel installation must be in accordance with those instructions in order to comply with the UL listing, and therefore, to comply with the NEC.

Ben Miller

Actually, most practical Neher McGrath Calculations (reference 310.15(C)) have already been performed and are in the NEC in Annex B in the form of Tables. There is reference to free air ampacities for cable trays in Article 392. And there are several free air ampacity tables in the NEC including those in Annex B. If the conductors are not bundled and have at least a wire diameter of clearance between them and the space where they are installed has ventilation openings then free air ampacities probably apply. Prudent persons often make decisions like this in the field and not every decision in the field requires a full engineering analysis. I make practical decisions everyday in the field that engineers would take hours to make and after all their calculations would probably miss the answer anyway.

It depends on how the local authority classifies a panelboard. If they consider it to be similar to a covered cable tray, there are derating factors to be applied to conductors free air ampacities. If they are similar to an uncovered cable tray, then less strict or no derating applies. Logically, panelboards appear (to me) to be similar to covered cable trays in that ambient air does not circulate around the cables.

Doing a heat loss calculation for a single installation isn't reasonable. Manufacturers do it to save a few cents across thousands of units. So it comes down to how conservative you and your inspector are and how much money is at stake. I know the inspector will buy the most conservative values.

There is a reference in Note 2 to Table 430.72(B) to Table 310.17 for free air ampacities for motor control conductors installed in an enclosure.

That's why I asked the question because this panel board appeared to have lots of free air in it :-) it's vented and mostly empty space ( 3 section unit that is 7' tall and 6.5' wide by 34"deep)