Using open bottom panel with skirt in lieu of conduit risers

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DMuns

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I've seen several installations where the electrical contractor has decided to use panel skirts on the bottom of the wall mount panels in lieu of providing conduit risers into the panel. There have been several instances where the bottom of the panel was removed and the panel mounted directly to the concrete slab. The slab penetrations have been plastic and the ones that I've seen with metal were all bonded together with bond bushings and then bonded to the panel can.

I was wondering if this could be an issue with the Arc Flash rating or possibly a code issue due to the fact there is no bottom to the panel.
 
Presumably by "arc flash rating" you mean incident energy (IE). The style of cabinet, or whether there is a cabinet at all is not considered when calculating IE.

There is no code requirement to have a bottom to any enclosure. It does have to be suitably closed, and a concrete pad is suitable IMO.
 
Yes that is a common installation method, especially with MCCs and switchgear. You raise a good point and its all dependent on the listing and installation instructions.
 
I've seen many variations of this over the years with panels, wireways, and pull boxes. With the exception of open bottomed switchboards I have yet to find a place in the NEC where this is permitted.

Welcome to the Forum. :)
 
tom baker said:
I recall an exception or allowance for this in the NEC. But the challenge is to find it.
Click to expand...

Could it be 300.12, exception #2?
There is a list of some equipment, but the exception does not appear to be limited to that list.
 
jim dungar said:
Could it be 300.12, exception #2?
There is a list of some equipment, but the exception does not appear to be limited to that list.
Click to expand...

I don't see how that is intended to include cutting the metal bottom out of a panel, in fact the requirement for cabinets (which would be the enclosure for a panelboard) is explicitly mentioned in 300.12.
 
infinity said:
I don't see how that is intended to include cutting the metal bottom out of a panel, in fact the requirement for cabinets (which would be the enclosure for a panelboard) is explicitly mentioned in 300.12.
Click to expand...
not for the cabinet, but for the raceway or cable.

300.12 Mechanical Continuity — Raceways and Cables. Raceways,
cable armors, and cable sheaths shall be continuous
between cabinets, boxes, fittings, or other enclosures or outlets.
Click to expand...

Take a look at 312.5 (A).
 
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infinity said:
I don't see how that is intended to include cutting the metal bottom out of a panel, in fact the requirement for cabinets (which would be the enclosure for a panelboard) is explicitly mentioned in 300.12.
Click to expand...
But it would be allowable if the item was manufactured with removable end wall and optional skirting.
312.5 simply requires openings to be 'closed in an approved manner'.
 
I have installed listed, open bottom, traffic signal cabinets that get mounted on concrete pads. The instructions state a 1/2" drain line is to be installed in the bottom of the cabinet, they are listed ad NEMA 3R
 
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