First some background. I am not an electrician. I am an “energy manager”. I analyze energy consumption for a school district and try to figure out ways to save the district either energy usage or cost. Sometimes things I suggest to the district entails adding or modifying circuits, so I joined this forum to ask questions.
But, my 2nd job was a rules enforcer. My junior year in college I began umpiring to help pay tuition. I found I really enjoyed it. Until arthritis took its toll I had been a certified referee/umpire in softball, baseball, basketball, volleyball, and soccer, officiating sports for 45 years. I took it seriously, re-reading the rulebooks weekly during the season, and with each reading I would try to envision more and more exotic scenarios and figure out how to make the call. I actually “have a rule”.
Imagine my excitement when I discovered this forum and the NEC; a book of rules 1000 pages thick and a forum with its main focus of trying to interpret those rules when new scenarios arise.
Each time I read a post I look up the code on my pdf copy of the NEC, then I do a search of related terms and see what other rules may apply. Then, I do an internet search of Mike Holt videos to try to gain more insight, but... Sometimes that leads me to wondering, “what if...?”
Recent posts about derating ampacity because of bundling ccc in conduit or raceway led me to that wondering.
Moderators, I realize this is designed as a nuts and bolts forum to help practitioners with actual events in the field. As this is a hypothetical for my own clarification please feel free to shut it down if you think it violates the TOS.
The relevant codes
310.15(B)(3)(a)
Adjustment Factors.
(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a)
From the table: 4-6 ccc 80%
So this says to adjust the ampacity to 80% if the bundling is more than 24”. It seems pretty clear and they made a point of saying as little as 24” was enough to adjust the ampacity. But,
310.15(A)(2)Exception: Where two different ampacities apply to adja-
cent portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a dis-
tance equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less.
So if someone wanted to bundle direct buried conductors to bring them into a building....
Given: You are running a sub panel to an outbuilding. You would like to have a 3-way switch in the house to turn on an electric heater in the outbuilding 30 minutes before you enter. The cables for these would exit the main building from different locations, and be direct burial in their own trench. They would meet at the outbuilding at a single location. For a neater installation and only one penetration into the outbuilding you would like to exit and return the switch loop wiring at the outbuilding through the conduit with the mhf. The conduit begins 18” under ground, extends up 66” along the outbuilding, then enters the building through a 6” wall.
The total conduit length at the outbuilding is 90”
Feeder is Aluminum 2-2-2-4 mhf = 2 ccc
Switch loop is 12awg copper uf = 2 ccc.
Total ccc =4, total length bundled in conduit = 90”
Scenario 1
Both the feeder distance and the switch loop distance to the outbuilding are greater than 100’. (see the diagram)
What are the ampacities of the cables?
Scenario 2
The distance for the mhf from the main panel to the outbuilding is 70’.
The distance from the 3-way switch to the outbuilding is greater than 100’.
What are the ampacities of the cables?
On my ipad the thumbnail only pulls up part of the diagram. I am inserting a full image in hopes it is complete. ... No go you may have to look at the thumbnail to see the bottom half.
But, my 2nd job was a rules enforcer. My junior year in college I began umpiring to help pay tuition. I found I really enjoyed it. Until arthritis took its toll I had been a certified referee/umpire in softball, baseball, basketball, volleyball, and soccer, officiating sports for 45 years. I took it seriously, re-reading the rulebooks weekly during the season, and with each reading I would try to envision more and more exotic scenarios and figure out how to make the call. I actually “have a rule”.
Imagine my excitement when I discovered this forum and the NEC; a book of rules 1000 pages thick and a forum with its main focus of trying to interpret those rules when new scenarios arise.
Each time I read a post I look up the code on my pdf copy of the NEC, then I do a search of related terms and see what other rules may apply. Then, I do an internet search of Mike Holt videos to try to gain more insight, but... Sometimes that leads me to wondering, “what if...?”
Recent posts about derating ampacity because of bundling ccc in conduit or raceway led me to that wondering.
Moderators, I realize this is designed as a nuts and bolts forum to help practitioners with actual events in the field. As this is a hypothetical for my own clarification please feel free to shut it down if you think it violates the TOS.
The relevant codes
310.15(B)(3)(a)
Adjustment Factors.
(a) More Than Three Current-Carrying Conductors in a Raceway or Cable. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multiconductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a)
From the table: 4-6 ccc 80%
So this says to adjust the ampacity to 80% if the bundling is more than 24”. It seems pretty clear and they made a point of saying as little as 24” was enough to adjust the ampacity. But,
310.15(A)(2)Exception: Where two different ampacities apply to adja-
cent portions of a circuit, the higher ampacity shall be
permitted to be used beyond the point of transition, a dis-
tance equal to 3.0 m (10 ft) or 10 percent of the circuit
length figured at the higher ampacity, whichever is less.
So if someone wanted to bundle direct buried conductors to bring them into a building....
Given: You are running a sub panel to an outbuilding. You would like to have a 3-way switch in the house to turn on an electric heater in the outbuilding 30 minutes before you enter. The cables for these would exit the main building from different locations, and be direct burial in their own trench. They would meet at the outbuilding at a single location. For a neater installation and only one penetration into the outbuilding you would like to exit and return the switch loop wiring at the outbuilding through the conduit with the mhf. The conduit begins 18” under ground, extends up 66” along the outbuilding, then enters the building through a 6” wall.
The total conduit length at the outbuilding is 90”
Feeder is Aluminum 2-2-2-4 mhf = 2 ccc
Switch loop is 12awg copper uf = 2 ccc.
Total ccc =4, total length bundled in conduit = 90”
Scenario 1
Both the feeder distance and the switch loop distance to the outbuilding are greater than 100’. (see the diagram)
What are the ampacities of the cables?
Scenario 2
The distance for the mhf from the main panel to the outbuilding is 70’.
The distance from the 3-way switch to the outbuilding is greater than 100’.
What are the ampacities of the cables?
On my ipad the thumbnail only pulls up part of the diagram. I am inserting a full image in hopes it is complete. ... No go you may have to look at the thumbnail to see the bottom half.
Last edited:
