Sizing LBs

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Jim1959

Senior Member
Location
Longmont, CO
Having a hard time finding how to size LBs for larger conductors.
Per 314.16(C) Conduit Bodies.
(1) General. Conduit bodies enclosing 6 AWG conductors or
smaller, other than short-radius conduit bodies as described in
314.16(C)(3), shall have a cross-sectional area not less than twice
the cross-sectional area of the largest conduit or tubing to which
they can be attached. The maximum number of conductors permitted
shall be the maximum number permitted by Table 1 of
Chapter 9 for the conduit or tubing to which it is attached.

How about larger conductors?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
For larger than #6 conductors you need the LB to be a minimum of 6X the trade size of the conduit just like pull box on an angle pull. It can be smaller if the conduit body lists the maximum conductor size and you do not exceed it.
 

Carultch

Senior Member
Location
Massachusetts
Having a hard time finding how to size LBs for larger conductors.
Per 314.16(C) Conduit Bodies.
(1) General. Conduit bodies enclosing 6 AWG conductors or
smaller, other than short-radius conduit bodies as described in
314.16(C)(3), shall have a cross-sectional area not less than twice
the cross-sectional area of the largest conduit or tubing to which
they can be attached. The maximum number of conductors permitted
shall be the maximum number permitted by Table 1 of
Chapter 9 for the conduit or tubing to which it is attached.

How about larger conductors?


Standard LB's and conduit bodies in general, are consistently sized too small to use with the maximum wire that fits in the nominal conduit size that connects to them. So it is very common that you need to upsize the conduit body and connect via reducing bushings, if the conduits are loaded to capacity.

Conduit body datasheets will indicate a capacity of wires that are allowable in the fitting. Suppose you pick a size that indicates 3x 4/0. This refers to use of XHHW-2 conductors unless otherwise noted. THWN-2 conductors are generally close enough in terms of overall diameter. So you are allowed to pull up to 3 qty 4/0 wires in this conduit body, or any combination of wires that add up to the same cross-sectional area of 3 qty 4/0 wires. Suppose you have larger conductors than 4/0, even if the conductors add up to less area than 3x 4/0, such as 2x 250 kcmil + 1#4 egc. You'd need to either upsize the conduit body or select a Mogul conduit body, to be compatible with pulling your desired group of wires.

Unlike standard conduit bodies, Mogul conduit bodies are built with solving this particular problem in mind. They are built to meet 314.28 rules on pull box sizing, and generally capable of fitting any wire that can fit in the conduit connected to it, per the 40% area rule of conduit sizing.
 
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