Feeder With Multiple Segments

[jar546]

In a feeder installation comprising multiple segments, how is the allowable ampacity of the intervening segment determined, and how do the temperature ratings of the termination devices and pressure connectors influence this?

Pick one or explain your answer please.

A) The intervening segment ampacity is determined by the lowest temperature rating of any connected termination, conductor, or device, with the continuous load multiplied by 125 percent.

B) The intervening segment ampacity is determined only by the ampacity parameters that apply over its length and the ampacity at the listed and identified temperature rating of the pressure connector.

C) The intervening segment ampacity is determined by the highest temperature rating of any connected termination, conductor, or device, without considering continuous load.

D) The two segments at the terminations and the intervening segment all have their ampacities determined identically, with the continuous load multiplied by 125 percent.

 

[wwhitney]

My attempt at a comprehensive, if complicated, answer, for the case of a single source of supply:

Start with a one-line diagram. Divide the system of feeders up into different segments, where each segment has unspliced conductors of a single ampacity. So each end of each segment will either be at a piece of equipment, at a splicing method, or at a point where the conditions of use change ((ambient temperature and/or number of current carrying conductors in a single cable or conduit (CCCs)).

Each such feeder segment can be labeled with all the info needed to calculate its ampacity: the insulation temperature (usually 90C), wire material (copper or aluminum), wire size, number of parallel sets, ambient temperature, and number of CCCs (by which I mean in a single cable or conduit). Also, Article 220 of the NEC gives us a continuous and noncontinuous load for each feeder segment.

Then to calculate the ampacity for a feeder segment, we start with the insulation temperature, wire material and wire size, and get a base ampacity from the appropriate table in Article 310 (usually 310.16). Then we multiply that base ampacity by the number of sets, the adjustment factor for number of CCCs, and the correction factor for ambient temperature.

Each end of a feeder segment incident at an equipment or splicing method also gets a termination ampacity. This is based on the lesser of the insulation temperature and the temperature rating of the equipment (usually 75C) or splice (usually at least 90C), respectively. Then the termination ampacity is the Table 310.16 value for that temperature rating multiplied by the number of sets, without any adjustment or correction for number of CCCs or ambient temperature.

The NEC requires the following checks on these ampacity numbers:

(1) The ampacity of each segment needs to be at least equal to the sum of the continuous and non-continuous loads on that segment. (Ampacity of a conductor is a continuous rating, so no need for a 125% factor on the continuous load for this check).

(2) Each segment needs to be protected by an OCPD in accordance with Article 240 (OCPD size may also be restricted by other articles). E.g. for the case of OCPD of 800A or less, the ampacity has to be greater than the next smaller standard size OCPD per 240.4(B).

(3) The termination ampacity of any feeder segment at a piece of equipment not rated for continuous operation at 100% of its rating (e.g. most OCPD) needs to be at least 125% of the continuous load plus 100% of the non-continuous load. Other feeder segment termination ampacities need only be equal to the sum of the continuous and non-continuous loads.

Cheers, Wayne

 

[jar546]

My attempt at a comprehensive, if complicated, answer, for the case of a single source of supply:

Start with a one-line diagram. Divide the system of feeders up into different segments, where each segment has unspliced conductors of a single ampacity. So each end of each segment will either be at a piece of equipment, at a splicing method, or at a point where the conditions of use change ((ambient temperature and/or number of current carrying conductors in a single cable or conduit (CCCs)).

Each such feeder segment can be labeled with all the info needed to calculate its ampacity: the insulation temperature (usually 90C), wire material (copper or aluminum), wire size, number of parallel sets, ambient temperature, and number of CCCs (by which I mean in a single cable or conduit). Also, Article 220 of the NEC gives us a continuous and noncontinuous load for each feeder segment.

Then to calculate the ampacity for a feeder segment, we start with the insulation temperature, wire material and wire size, and get a base ampacity from the appropriate table in Article 310 (usually 310.16). Then we multiply that base ampacity by the number of sets, the adjustment factor for number of CCCs, and the correction factor for ambient temperature.

Each end of a feeder segment incident at an equipment or splicing method also gets a termination ampacity. This is based on the lesser of the insulation temperature and the temperature rating of the equipment (usually 75C) or splice (usually at least 90C), respectively. Then the termination ampacity is the Table 310.16 value for that temperature rating multiplied by the number of sets, without any adjustment or correction for number of CCCs or ambient temperature.

The NEC requires the following checks on these ampacity numbers:

(1) The ampacity of each segment needs to be at least equal to the sum of the continuous and non-continuous loads on that segment. (Ampacity of a conductor is a continuous rating, so no need for a 125% factor on the continuous load for this check).

(2) Each segment needs to be protected by an OCPD in accordance with Article 240 (OCPD size may also be restricted by other articles). E.g. for the case of OCPD of 800A or less, the ampacity has to be greater than the next smaller standard size OCPD per 240.4(B).

(3) The termination ampacity of any feeder segment at a piece of equipment not rated for continuous operation at 100% of its rating (e.g. most OCPD) needs to be at least 125% of the continuous load plus 100% of the non-continuous load. Other feeder segment termination ampacities need only be equal to the sum of the continuous and non-continuous loads.

Cheers, Wayne

Well you just took this and ran with it, didn't you? You've certainly dug into this one.

 

[jar546]

Exception No. 2. This exception addresses installations where feeders are installed in multiple segments. Feeders installed under this scenario comprise three segments: two segments at the terminations — supply and load — and an intervening segment. The allowable ampacity of the intervening segment is permitted to be calculated without applying 125 percent to the continuous load.

The two segments at the terminations of the feeder are sized to accommodate the effects of continuous loading on the termination devices. Their ampacities are selected and coordinated to not exceed the lowest temperature rating of any connected termination, conductor, or device, as covered in 110.14(C)(1).

The intervening segment must terminate in enclosures at the feeder source and destination. This segment is sized in accordance with the ampacity requirements for the conductor specifically to provide wiring that will accommodate the maximum current, whether or not any portion of that current is continuous. The ampacity of this intervening segment is limited only by the ampacity parameters that apply over the length of the intervening segment and the ampacity at the listed and identified temperature rating of the pressure connector, as covered in 110.14(C)(2).

 

[wwhitney]

Exception No. 2.

Sounds like you are quoting from something, in which case an attribution would be nice. If it's the Handbook, that's just a commentary and is not definitive, although I don't disagree with anything you quoted.

You've certainly dug into this one.

Yeah, my initial interest was in why the 125% factor for continuous loads exists and what its proper applicability should be. It's a bit annoying that to synthesize the procedure I outlined in my first post, you need to look at different sections spread across Chapters 1, 2, and 3 of the NEC. It's highly non-obvious to the first time reader of the NEC.

And even my post above is not comprehensive, as I didn't mention the 10% or 10ft rule. Probably something else I forgot as well.

Cheers, Wayne

 

[jar546]

Sounds like you are quoting from something, in which case an attribution would be nice. If it's the Handbook, that's just a commentary and is not definitive, although I don't disagree with anything you quoted.


Yeah, my initial interest was in why the 125% factor for continuous loads exists and what its proper applicability should be. It's a bit annoying that to synthesize the procedure I outlined in my first post, you need to look at different sections spread across Chapters 1, 2, and 3 of the NEC. It's highly non-obvious to the first time reader of the NEC.

And even my post above is not comprehensive, as I didn't mention the 10% or 10ft rule. Probably something else I forgot as well.

Cheers, Wayne

Your input is appreciated more than you know. Thank you for posting.