It can keep you warm—or create a fire hazard
“BUNDLE (bundled, bundling):To occupy the same bed without undressing, said of a man and woman, especially during courtship.” —Webster’s Dictionary. This practice was brought to the colonies from England, Wales, Scotland, Holland and Germany, and was widely followed in Pennsylvania, New England and some southern states until the early 20th century.
For one thing, it was said to conserve firewood when a suitor visited his intended overnight. There is no doubt that two in a bed will sleep warmer than one alone.
What does this have to do with the National Electrical Code? In the 1962 NEC, there was mention for the first time of stacking or bundling of cables in Note 8 to Tables 310-12 through 310-15. Cables were assumed to be bundled unless spaced apart at least to the extent required by Article 318, Continuous Rigid Cable Supports (now called Cable Trays). Article 318 (Art. 392 in the 2002 NEC) established ampacities based on both vertical and horizontal separation of the cables.
Bundling with your sweetheart to keep warm on a cold New England winter night sounds cozy, but the bundling of cables is an undesirable condition because the heat generated by current passing through inner cables in the bunch is prevented from escaping by the outer cables, which are also contributing heat to the bundle. The rule applies only to bundles exceeding 24 inches in length, the assumption being that conductors beyond the 24 inches would act as heat sinks to conduct and dissipate the heat from the inner conductors. The ampacity of each conductor is reduced by the adjustment factors in the Table in Note 8 for more than three conductors in a raceway.
In the 1975 NEC the reference to Article 318, Cable Tray, was removed, leaving no criteria for what constituted bundling. This reference to Article 318 was shown as retained in Preprint No.1, (Walter Stone’s editorial rewrite), and in Preprint No. 2, but it disappeared from the Code, despite the fact that there is no record of a proposal to delete it. Old Note 8 is now 310.15(B)(2)(a) in the 2002 NEC.
Many electricians in the effort to produce a neat and workmanlike job in a panelboard will bundle all of the branch circuit wires together, tie-wrap them and feed off individual wires to the circuit breakers without realizing that the bundling increases the heat on some of the conductors. Furthermore, the conductors connected to the circuit breaker terminals serve the purpose of conducting heat way from the terminals and the circuit breakers, and bundling interferes with this heat transfer. Bundling in a panelboard may or may not exceed the 24-inch limit beyond which the adjustment factors apply, but whether it does or not, it may look neat, but electrically it is the wrong thing to do.
There is no question but that a bundle of conductors or cables held together with tie-wraps is indeed bundled, but there are many conditions, especially with the installation of nonmetallic sheathed cable in wood frame construction where the application of the adjustment factors is questionable. If two or three NM cables pass horizontally through a bored hole in a 16-inches-on-center wood stud, is that to be considered bundling? Most inspectors do not think so, but on the other hand there are jurisdictions where NM cable run through wood framing must have a bored hole for each cable. At the other extreme is the inspector who enforces the rule on commercial and industrial properties, but not on residential occupancies on the basis that Article 220 load calculations are overly conservative and dwelling unit demands are so low that it is not necessary to enforce the bundling restrictions.
On another subject: Table 220.18 in the 1989 NEC had the condition in 13 places where the addition of one 5kW dryer connected load resulted in a decrease in the calculated load! Thanks to Frederic P. Hartwell’s proposal, the 2002 Table corrects this anomaly. The new Table shows an increase in the calculated load for each increase in connected load, as it should. Some of the Table loads are slightly increased over those in the ’98 Table, but not to any significant amount. I found the 12-22 and 24-42 lines in the 2002 Table more difficult to follow than in the proposal #2-281 in the 2001 ROP, where the 12-22 line read “[47 – (number of dryers over 11)]%” and the 24-42 line read “[35 – 0.5 (number of dryers over 23)]%”
Here is an of example: In the 1998 Table, 13 dryers @ 5kW x .45 = 29.3kW,
While 14 dryers @ 5kW x .40 = 28kW, a decrease in the calculated load where the connected load is increased. The 2002 Table has the correct formula and answer, 30.8kW. EC
SCHWAN is an electrical Code consultant in Hayward, Calif. He can be reached at firstname.lastname@example.org.