SE Cable Ampacity According to the 2014 NEC

Image 1. SE Cable installed in thermal insulation.

By: JADE Instructor | Feb 25, 2016

Properly sizing an SE cable installed in thermal insulation requires a careful examination of the Code. Section 338.10(B)(4)(a) is all about interior installations of SE cables and states: Where installed in thermal insulation the ampacity shall be in accordance with the 60°C (140°F) conductor temperature rating. The maximum conductor temperature rating shall be permitted to be used for ampacity adjustment and correction purposes, if the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.

SE Cable installed in thermal insulation.
Image 1: Notice the gray SE cable entering the top of the exterior wall, installed in thermal insulation.  Click image to enlarge.

For example, if a 1/0, XHHW SE cable with aluminum conductors is installed in thermal insulation, it can only carry 100 amps, from the 60˚C column of Table 310.15(B)(16), even though XHHW conductors are rated for 90˚C in a dry location in Table 310.15(B)(16).

However, if the 1/0, XHHW SE cable with AL conductors is installed in thermal insulation in an attic where the temperature is 110˚F, then the 90˚C column of Table 310.15(B)(16) can be used to correct for the higher temperature. The correction factor for 110˚C ambient temperature is .87 and the ampacity of a 1/0 XHHW from the 90˚C column of Table 310.15(B)(16) is 135 amps.  So a 1/0 XHHW AL conductor can carry 135 amps x .87 = 117 amps.  This value of 117 amps has to be compared with the rating from the 60˚C column because the cable is installed in thermal insulation.  The ampacity from the 60˚C column is only 100 amps, so that is the maximum permitted load on a 1/0 XHHW AL conductor installed in thermal insulation.

Here are some common questions about installing SE cable in thermal insulation:

Image 2. SE Cable
Image 2: This entire SE cable can be calculated from the 75 degree column of Table 310.15(B)(16).  Click image to enlarge.

Q: What is considered installed “in thermal insulation”? Does the 60°C column of Table 310.15(B)(16) still apply if the cable is just passing through a few inches of thermal insulation but the majority of the cable is not actually “in thermal insulation”?

A: This will always be an AHJ call and is a bit of a gray area. In some jurisdictions, Section 310.15(A)(2) will be considered. Where more than one ampacity applies for a given circuit length, the lowest value shall be used.

Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft.) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

See Image 2.

 

Q: Which temperature column of Table 310.15(B)(16) should be used if the cable is NOT in thermal insulation?

SE cable conductor insulation
Image 3: SE cable ampacity is partially dependent on the conductor insulation marked on the cable jacket. Click image to enlarge.

A: The answer to this is based on two things:

  1. The markings found on the SE cable jacket (see Image 3).    Example: An XHHW SE cable has a temperature rating of 90°C in a dry location and 75°C in a wet location. Use the corresponding column in Table 310.15(B)(16) based on the location.
  2. Temperature ratings for the termination points of the SE cable.    Simply put, if connecting a 90°C SE cable to a 75°C terminal, the final derated value of the SE cable cannot be higher than the value in the 75°C column. See 110.14 and the parent text in 310.15(B).

 

Main breaker
Image 4: Notice the two-pole 30-amp breaker just under the main breaker that feeds an outdoor air conditioner. Since the SE cable to the interior panel does not carry the entire load of the dwelling, it is not considered a “main power feeder”. The 83% allowance in 310.15(B)(7) cannot be used when sizing this feeder. Click image to enlarge.

Q: Which temperature column should be used for a main power feeder to a dwelling if the SE cable is installed in thermal insulation?

A: Informational Note No. 2 after Section 338.10(B)(4) states: For the installation of main power feeder conductors in dwelling units refer to 310.15(B)(7). This part of the Code makes a special allowance for feeders or service conductors that carry the entire load of the dwelling (main power feeders). Main power feeders can be smaller in size than what would normally be required to carry the load based on the principle of load diversity. This is the idea that as more loads are connected to a dwelling unit panel, the less likely it is that all connected loads will be operating at the same time. Utility companies use the same concept when sizing a transformer based on how many homes it supplies.

Main power feeders are permitted to be sized at 83% of the required service size. Example: After load calculations, it has been determined that a dwelling unit requires a 100 amp service. 100 amps X 83% = 83 amps. Select an SE cable from the 60°C column that can carry 83 amps if the SE cable is installed in thermal insulation.

 

For the 2017 NEC, a proposal for 338.10(B)(4) has been submitted to only require the use of the 60°C column for SE cables size 10 AWG and smaller installed in thermal insulation.

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