I'm talking about something like this Type LB Conduit Body
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Only then can the conduit body contain splices.
In your specific case, I don't see the volume listed in the specifications section on the Home Depot page. So you'd have to check the fitting itself, to determine if the manufacturer has marked the volume. You didn't say what size wires you're pulling, so I can't answer that definitively. You also didn't mention the number of conductors, so it's impossible for me to do the fill calculations.
National Electrical Code 2014
ARTICLE 314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures
314.16 Number of Conductors in Outlet, Device, and Junction Boxes, and Conduit Bodies. Boxes and conduit bodies shall be of an approved size to provide free space for all enclosed conductors. In no case shall the volume of the box, as calculated in 314.16(A), be less than the fill calculation as calculated in 314.16(B). The minimum volume for conduit bodies shall be as calculated in 314.16(C).
(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)(2), 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.
(2) With Splices, Taps, or Devices. Only those conduit bodies that are durably and legibly marked by the manufacturer with their volume shall be permitted to contain splices, taps, or devices. The maximum number of conductors shall be calculated in accordance with 314.16(B). Conduit bodies shall be supported in a rigid and secure manner.
(3) Short radius conduit bodies. Conduit bodies such as capped elbows and service-entrance elbows that enclose conductors 6 AWG or smaller, and are only intended to enable the installation of the raceway and the contained conductors, shall not contain splices, taps, or devices and shall be of sufficient size to provide free space for all conductors enclosed in the conduit body.
For each current carrying conductor that terminates within the box, you'll add 1. So if you're pulling one ungrounded (hot) conductor, one grounded (neutral), and one grounding conductor. If you want to splice within the box, the calculation would look like this.
Ungrounded (hot) 1 Ungrounded (hot) 1 Grounded (neutral) 1 Grounded (neutral) 1 -------------------------- 4
If grounding conductors are used, you'll add 1.
Current Carying 4 Grounding 1 -------------------------- 5
Once you have the conductor count, you'll use Table 314.16(B) to determine the required volume.
NOTE: You'll always base the calculation off the largest conductor used. so if you have a #14 grounded (neutral) and a #12 ungrounded (hot), you'll use 2.25 because of the #12. If you're using grounding conductors that are a different size than the current carrying conductors, you'll multiply them separately (#12 hot, #12 neutral, #14 ground = 2 x 2.25 + 1 x 2.00).
If the largest conductor used is 14 AWG, you'll multiply the conductor count by 2.00 according to Table 314.16(B).
`5 * 2.00 = 10.00'
Which means the box must have a volume of at least 10 cubic inches, if you want to splice within it.
If the largest conductor used is 12 AWG, you'll multiply the conductor count by 2.25.
5 * 2.25 = 11.25
So in this case, the box must have a volume of at least 11.25 cubic inches.
If you were pulling 3 ungrounded (hot) conductors, the calculations would look like this.
Ungrounded (hot) 1 Ungrounded (hot) 1 Ungrounded (hot) 1 Ungrounded (hot) 1 Grounded (neutral) 1 Grounded (neutral) 1 Grounding 1 -------------------------- 7
7 * 2.00 = 14 cubic inches
7 * 2.25 = 15.75 cubic inches
12-2 will work, 12-3 is too much.
If you are using a 3/4" Carlon, it's 12 cubic inches and should be marked. 12-2 with ground, spliced, needs 5X 2.25", 11.25 cubic inches, passes. 5 is two neutrals and two hots that terminate in the box, plus one for the ground. 12-3 with ground needs 7X 2.25, 15.75 cubic inches, fails (unless you increase to a 1-1/4" LB with 32 cubic inch volume).