# What size LB



## 23chief (Feb 18, 2015)

What size LB should I use at the stub out for a run to a sub panel


----------



## beachguy005 (Feb 18, 2015)

That would depend on the number and size of the conductors you're pulling through it.


----------



## 23chief (Feb 18, 2015)

Sorry I'm gonna run 6-3 uf-b


----------



## beachguy005 (Feb 18, 2015)

I believe it's going to be 5 cubic inches per conductor.  NEC 314.16


----------



## hornetd (Feb 19, 2015)

beachguy005 said:


> I believe it's going to be 5 cubic inches per conductor.  NEC 314.16



Cables have to be calculated as a single conductor for the purpose of conduit and raceway sizing.  

--
Tom


----------



## 23chief (Feb 19, 2015)

I'm confused does anybody know what size LB it should be


----------



## hornetd (Feb 19, 2015)

23chief said:


> Sorry I'm gonna run 6-3 uf-b



The US National Electric Code is very unfriendly to the use of cable inside conduit.  The cables that it accommodates least well are those like type UF that have an oblong or ribbon like cross sectional area.  

The LB must be sized for both the conduits it is to join and the size of the conductors that pass through it.  When a cable passes through an LB it is treated as a single conductor of that same diameter for the purpose of both fill and bending radius.  Worse still the radius of type UF cable is calculated from it's largest diameter as if it were round and not from it's actual cross sectional area which would be much less. So you measure from edge to edge of the flat UF cable jacket and then calculate the cross sectional area of the cable as a circle using that dimension.

When you have done that and selected a conduit size you will have to select an LB based on the required bending radius of a single conductor with the same cross sectional area as the cable as calculated.  

Taking a measurement out of my ear for your UF cable I will guess that it will be 1&1/2 inches across the cable.  That will give you a cross sectional area of the equivalent round cable of 1.77 Square inches.  Since twice that is 3.54 Square inches and the radius of a circle with that area is 2.12 Square inches we need a conduit with a minimum internal diameter of 2.12 or larger.  So it is a gnat's eyelash too large for a two inch conduit and you will have to use a two and one half inch or larger conduit.

Taking the original diameter of the cable you now get to match that to the smallest conductor with the same or larger diameter and select your LB from the minimum bending radius of that wire size at a terminal opposite a cover.  That dimension will be the minimum depth of the LB between the cover and the shoulder of the conduit hub in the back of the LB.  You may well need a Mogul LB to get the required depth.  Alternatively you could install a large enough standard LB and just reduce the hubs of it down to the size of the conduit used.  

Our original radius is 1.5 inches.  Rather than calculate the smallest matching conductor size let me suggest that you either run continuous raceway and use separate conductors or that you buy your type UF cable as individual conductors instead of a single cable assembly.  With either of those approaches the trade size of the conduit needed will drop two full sizes and since the conductors will all be smaller than number four American Wire Gauge the rule on bending radius does not apply.


----------



## bud16415 (Feb 19, 2015)

I don&#8217;t know the code at all but have something quite similar between the house and garage. I believe mine is 2 inch. I know it&#8217;s hard to pull compared to individual wires. I would go as big as I could and then maybe a bit larger based on cost. I wouldn&#8217;t think being too large would hurt anything. How many feet of run and how straight?


----------



## beachguy005 (Feb 19, 2015)

I understand what hornetd is saying but if you go with single conductors, the way I read it,  is three #6s and a ground.  15 to 20 cubic inches of volume.  A 1.25" LB is about 19.5 C.I.


----------



## hornetd (Feb 19, 2015)

beachguy005 said:


> I understand what hornetd is saying but if you go with single conductors, the way I read it,  is three #6s and a ground.  15 to 20 cubic inches of volume.  A 1.25" LB is about 19.5 C.I.



Exactly what I was suggesting.  When you go to individual conductors you drop two full sizes of conduit.  Further if you use Type UF which is done as individual conductors then it will only need conduit protection from the bottom of the Twenty Six inch deep trench to the first place were you can splice to or terminate the Type UF individual conductor cables.  You need to have twenty four inches of earth over wire or cable in a trench with no additional protection.  

If you run the conduit for the hole run then it only needs Eighteen inches of earth over the conduit and you can use any conductor that is listed for wet locations such as THWN.  That would be Thermoplastic, High temperature (up to Seventy Five Degrees Celsius 75°C), Wet Location, with an overall Nylon covering.  Be aware that you must build the conduit as a complete system and then pull the wires into it.  Attempting to build the conduit over the wires to avoid pulling it will expose the conductor insulation to PVC solvent cement which will attack the insulation of the wires.  If you price the conduit and the multi-conductor or single conductor Type UF cable you will probably come out ahead by using a complete conduit run and regular THWN wires.  

--  
Tom


----------



## 23chief (Feb 19, 2015)

It's only 7 feet between garage and house I think I'm gonna use the uf-b


----------

