Blast from the Past

This new series will review the most popular questions that have appeared in NECA’s online Code Question of the Day and have generated the most comments from the subscribers. All answers are updated to comply with the 2008 National Electrical Code (NEC). If you are not a subscriber and would like to receive the Code Question of the Day online, go to and follow the links.

> In a recent Code Question of the Day, I was asked about the use of the ampacities shown in the 90°C column of Table 310.16. Judging by the comments my answer generated, there appears to be considerable confusion about when the ampacities shown in that column can be used. So let’s go through it again.

Equipment temperature limitations

Some conductor types have an insulation temperature rating of 90°C. The 90°C column of Table 310.16 lists these conductor types and the ampacity permitted for each conductor size. These ampacities are higher than the ampacities shown in the 60°C and 75°C columns for conductors of an identical size. For example, an 8 AWG TW conductor has an ampacity of 40 amperes. This is because a conductor with TW insulation has a temperature rating of 60°C, and the ampacities shown in the 60°C column must be used. An 8 AWG THWN conductor has an insulation temperature rating of 75°C, so refer to the 75°C column. It permits 50 amperes. An 8 AWG THHN conductor has an insulation temperature rating of 90°C, so refer to the 90°C column, which permits 55 amperes.

As explained above, we have three different ampacities for the same-size conductor based on insulation temperature rating. Why not just use the 90°C rated conductor and take advantage of the higher ampacities permitted for each conductor size? Well, it sounds good and was being done all too often, as there were no specific NEC requirements governing the use of the insulation temperature rating of conductors.

This led to the acceptance in 1993 of new text 110.14(C) titled Temperature Limitation. This new part is used to determine the ampacity of conductors permitted for termination on equipment based on the temperature rating of the equipment. Part 110.14(C) states, “The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor or device.” This requirement is specific to equipment termination ratings and the use of conductors consistent with those ratings. This rule further requires the ampacities of the conductors to terminate on the equipment based on Table 310.16, as appropriately modified by 310.15(B)(6), unless the equipment is marked otherwise. The reference to 310.15(B)(6) pertains to equipment that is 120/240-volt, three-wire, single-phase service or feeder related and means the ampacities shown for conductors in Table 310.15(B)(6) may be used instead of the ampacities shown in Table 310.16.

NEC 110.14(C)(1)(a) states, “Termination provisions of equipment for circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors” shall be used only for the following:

(1) Conductors rated 60°C

(2) Conductors rated 75°C or 90°C are permitted, but the ampacity for these conductors must be based on the 60°C ampacity of the conductor size.

(3) Conductors rated 75°C or 90°C are permitted if the equipment is listed and identified for use with these conductor ratings.

(4) For motors marked with design letters B, C or D, conductors having a 75°C or 90°C rating may be used provided the ampacity does not exceed the 75°C ampacity.

The four conditions above are the only termination methods the Code permits for use to comply with 110.14(C)(1)(a).

NEC 110.14(C)(1)(b) states, where the equipment is rated for circuits over 100 amperes or marked for conductors larger than 1 AWG, the termination provisions must be one of the following:

(1) Conductors rated 75°C must be used.

(2) Conductors with higher temperature ratings may be used provided the ampacity of such conductors does not exceed the 75°C ampacity of the conductor size or up to their ampacity (90°C) if the equipment is listed and identified for use with such conductors.

Going back to 110.14(C), the article states, “conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.”

This permits using the higher ampacities of 90°C conductors for derating purposes. For example, using four 12–3 w/gr Type NM cables installed, without maintaining spacing between the cables, through the same opening that is to be fire or draft stopped, the allowable ampacity must be adjusted according to Table 310.15(B)(2)(a). There would be eight phase conductors, and the Code would require a derating to 70 percent. The 25-ampere ampacity shown in the 60°C column of Table 310.16 must be reduced to 25 × .70 (17.5 amperes), and the overcurrent protection permitted would be reduced to 15 amperes. However, being permitted to use the 90°C column of Table 310.16 for derating purposes, the allowable ampacity of 30 amperes shown in the 90°C column is reduced to 21 amperes, and a 20-ampere overcurrent device still is permitted. Note the conductors used in Type NM cable are rated 90°C.

> Here’s an interesting question relating to the need for an emergency power-off (EPO) switch in an information technology equipment (ITE) room.

IT equipment

Who determines if a data center or a computer lab needs to have an EPO circuit for a room? What criteria are used to make this decision? Is this the responsibility of the authority having jurisdiction (AHJ)?

I have a very large room that was used as a data center in the past. The raised floor was left in place, but carpeting was placed over the raised floor in some places for offices and a lab. A smaller computer room was installed in one section, and partitions were placed under the floor (under the walls) to keep the HVAC air within the computer room. This room is a dedicated data center and has an EPO circuit and smoke detectors.

One of the labs is used as a setup area for the computer room equipment. It has permanent equipment installed in many cabinets. It also has space in these cabinets where equipment is set up and then removed to go into the computer room. It has a UPS panel and an HVAC unit in the room. Partitions were placed under the floor (under the walls) in this area also for the HVAC air. It also is fed by circuits under the floor. The HVAC unit is not used in this area at this time.

Does this qualify as a computer room and need to be Code-compliant as a computer room?

NEC 645.10 requires the installation of an EPO in an ITE room. To qualify as an ITE room, the special requirements of 645.4 must be followed. According to NEC 90.4, the AHJ has the responsibility for making interpretations of this Code and for approval of equipment.

It is difficult to completely picture your installation, but it appears that 645.4(5) pertaining to separation from other occupancies by fire-resistant walls, ceilings and floors is not being followed, and the room does not qualify as an ITE room. Therefore, the equipment and wiring installed under the raised floor must follow 300.22(C). For further information, check out the fine print note to 645.4(5).

> The following was a follow-up question concerning the installation of EPO disconnecting means in an ITE room.

ITE rooms

I have a question about the change made in 645.10 for the 2008 NEC. The NEC now allows ITE rooms to be divided into separate zones that may separately disconnect the power to ITE equipment in the event of an emergency. What sorts of separation requirements are necessary to define the individual zones?

There has been a concerted effort over the last couple of Code cycles to eliminate the disconnecting means required by 645.10. This has not been accomplished, but the panel has accepted an alternate method. The ITE room may now be divided into separate zones with each protected by a disconnect for that zone. In this way, all ITE equipment in an entire ITE room would not be disconnected by an unintended shutdown.

However, the last sentence reads, “Where multiple zones are created, each zone shall have an approved means to confine fire or products of combustion within the zone.” I know of only one way to accomplish this, and that is by separation by fire-rated walls. This separation will have to extend down to include the underfloor area.

> While we are on the subject of ITE rooms, here’s a question relating to underfloor wiring having fire-resistant qualities.

Underfloor wiring

It looks to me that NEC Article 645 has made some changes intended to differentiate between cords and cables. If I’m reading this correctly, 645.5(D)(6) now covers cables but not cords, thereby eliminating this requirement for supply cords of ITE equipment.

Yes, the panel voted unanimously to permit the installation of flexible cords supplying listed ITE equipment under raised floors (645.5(D)(3) in accordance with 645.5(B), which means they can be 15 feet in length and must be protected from physical damage where run on the surface of the floor. There is no requirement that supply cords for ITE equipment be listed as Type DP, having adequate fire-resistant characteristics suitable for use under raised floors of an ITE room. The panel apparently did not deem it necessary to add any fire-resistant characteristics to the requirements for cords used under raised floors.

Submitting a question to the Code Question of the Day:

If you have a question relating to the National Electrical Code, or if you wish to comment on a question, you may send it to codequestion@necanet.orgEC

TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at 352.527.7035.

About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

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