Derating Standards for Bundled Wires

In Professional BoatBuilder No. 195, contributing editor Nigel Calder’s “Amped Up” explores the effects of increasingly high-amperage onboard DC systems that exceed the capacities of standard marine-grade electrical cabling and overcurrent-protection devices. This companion text specifies for builders and repair technicians the differences between ISO and ABYC derating factors for bundled conductors they need to accommodate.​​​​​​​           —Aaron Porter, Editor

Derating Bundled WiresNigel Calder | Professional BoatBuilder Magazine

The author has conducted experiments with ducted airflow around high-output alternators to minimize the effect of high engineroom temperature on adjacent bundled wiring that must still be derated to meet industry standards.

The American Boat and Yacht Council (ABYC) and ISO specify the maximum allowable ampacity for electrical conductors based on bundling. The ABYC has a single derating factor for DC conductors and multiple derating factors for AC, depending on the number of current-carrying conductors in the bundle. The ISO has similar (but not identical) derating factors, the major difference being that it applies all of them to DC circuits as well as to AC.

The ISO begins its derating with bundles of four to six conductors, applying a derating factor of 0.7. Thereafter, the derating factor is 0.6 for bundles of seven to 24 conductors, and 0.5 for more than 25 conductors.

Although I have participated in the ABYC Electrical Project Technical Committee for the better part of 30 years, it was not until I wrote the first draft of my recent article in Professional BoatBuilder and had some email communication with Maciej Rynkiewicz at the ABYC concerning conductor derating factors that I realized the ABYC’s E-11 has an additional bundling step for two-to-three current-carrying conductors. This applies the 0.7 derating factor to both DC and AC conductors. Thereafter, the derating factor of 0.7 is applied to all larger DC bundles, while a derating factor of 0.6 is applied to AC bundles of four to six conductors, 0.5 for bundles of seven to 24 conductors, and 0.4 to bundles exceeding 25 conductors.

ABYC and ISO Derating Standards differ

Note that ABYC and ISO standards are incongruous because of the additional two-to-three bundling step in ABYC’s E-11 standard. The final ISO step has a derating factor of 0.5, while the same step under ABYC rules has a derating factor of 0.4. Note also that E-11 requires the hot, neutral, and grounding conductors in an AC circuit to be run together (E-11.5.3.4), which means there are always two current-carrying conductors. Thus, all triplex AC cabling on a boat must have the 0.7 derating factor applied to it.

If an alternator, instead of relying on the case for grounding, is given a negative conductor (true for most high-output alternators and for all isolated ground alternators) and if this negative conductor is strapped to the positive conductor, as is also commonly done, as currently written, E-11 requires the two conductors to be derated to 70% of the single conductor ampacity. If this derating is applied, many existing installations will be found to violate the ampacity requirements of E-11.

I have been dealing with these installations for 30 years. I know of no history of problems arising from not applying this derating factor, and installing what are, according to E-11, undersized conductors.

On looking back through past standards, I see we made a change between 2007 and 2008 that I have never noticed, and that extends bundling derating down to two current-carrying conductors. This seems to me to be excessively conservative. One of my projects is to question the two–three conductor derating factor to see if we can remove it from E-11 at the next review cycle, with a revision of the derating factors for the subsequent steps to bring E-11 into alignment with ISO 13297.