Fibrous tubing is frequenly used with knob and tube wiring, and the regulations governing its use are given in ject to moisture. The cost of wiring in flexible fibrous tubing is approximately about the same as the cost of knob and tube wiring.

Rule 24, Section S, of the National Electric Code. This tubing, as stated in this Rule, may be used where it is impossible and impracticable to employ knobs and tubes, provided the difference in potential between the wires is not over 300 volts, and if the wires are not sub-

Fig. 17. Flexible Tubing. Flexduct Type.

Fig. 17. Flexible Tubing. "Flexduct" Type.

Courtesy of National Metal Moulding Co., Pittsburg, Pa.

Duplex conductors, or two wires together are not allowed in fibrous tubing.

Fibrous tubing is required at all outlets where conduit or armored cable is not used (as in knob and tube wiring); and, as required by the Rules, it must extend back from the last porcelain support to one inch beyond the outlet. Fig. 7 shows one make of fibrous tubing.

Table VIII gives the maximum sizes of conductors double-braided) which may be installed in fibrous conduit.

Table VIII. Sizes Of Conductors In Fibrous Conduit

Outside Diameter

Inside Diameter

One Wire in Tube

17/32 inch

1/4 inch

No. 12

21/32 "

3/8 "

" 8

25/32 "

1/2 "

" 6

15/16 "

5/8 "

" 1

1 3/32 "

3/4 "

" 2/0

1 5/16 "

1 "

250,000 C. M.

1 9/16 "

1 1/4 "

400,000 C. M.

1 13/16 "

1 1/2 "

750,000 C. M.

2 1/8 "

1 3/4 "

1,000,000 C. M.

2 3/8 "

2 "

1,500,000 C. M.

2 5/8 "

2 1/4 "

2,000,000 C. M.