Reason for conduit fill rules?

[MD88]

My situation is the client had verified an existing duct bank to have 5" ducts, and we based our ccable size on that (3c 15kV cable). Construction is just about to start (cables bought) and the contractor has discovered they are really 4" and the new cable doesn't fit the conduit fill rule. The client admits responsibilty but I would like to know the reasoning behind the rules before i tell him we need to build a new duct bank. Reason being it's a short run (200') and straight as an arrow, and I know they'd have no trouble pulling it.

Thanks!

 

[hardworkingstiff]

What size conductors in that cable?

 

[hardworkingstiff]

Who has jurisdiction, the PoCo or the local AHJ? I would at least see if they would accept a letter from an engineer stating they reviewed the project and agreed the cable can safely be installed. (Then you just have to find an engineer to say that).

 

[templdl]

My situation is the client had verified an existing duct bank to have 5" ducts, and we based our ccable size on that (3c 15kV cable). Construction is just about to start (cables bought) and the contractor has discovered they are really 4" and the new cable doesn't fit the conduit fill rule. The client admits responsibilty but I would like to know the reasoning behind the rules before i tell him we need to build a new duct bank. Reason being it's a short run (200') and straight as an arrow, and I know they'd have no trouble pulling it.

Thanks!

The conductors need to dissipate heat. My son learned that then he used my retracting drop light that had an outlet with the light on the end. He pulled off just enough cord that he need and plugged in a 1500w heat gun. The next time I tried to use it wouldn't extend out. The cord melted together. I was very fortunate that he didn?t start it on fire.
What I'm trying to get at is that those conductors within the conduit do generate heat, maybe not enough that you can easily feel, but never the less there is some heat generated.
As such you must pay very close attention to conduit fill such that the cable within the conduit stays within the temperature limits of the insulation of the cable. If you start to overfill the conduit (raceway) there is a risk that the cable will exceed the temperature rating of the insulation of the wire.
There is the possibility that you could consider derating the wire but I doubt that it would be practical to for an existing installation.

 

[hardworkingstiff]

Who has jurisdiction, the PoCo or the local AHJ? I would at least see if they would accept a letter from an engineer stating they reviewed the project and agreed the cable can safely be installed. (Then you just have to find an engineer to say that).

Showing my ignorance again, I just checked, and you are an engineer, so sorry.

 

[MD88]

There are no inspectors, the site is under its own rule, but still governed by NEC (FM global insured). That being said, we do have some flexibility and if we stamp the design it'll be ok. I need to understand the reason (is it really heat dissipation? It doesn't seem like it is... Or just some "well it'll be really hard to pull otherwise so this is best practice" type rule). Just wondering if anyone knows the true intent.

Thanks

 

[m sleem]

The conductors need to dissipate heat. My son learned that then he used my retracting drop light that had an outlet with the light on the end. He pulled off just enough cord that he need and plugged in a 1500w heat gun. The next time I tried to use it wouldn't extend out. The cord melted together. I was very fortunate that he didn?t start it on fire.
What I'm trying to get at is that those conductors within the conduit do generate heat, maybe not enough that you can easily feel, but never the less there is some heat generated.
As such you must pay very close attention to conduit fill such that the cable within the conduit stays within the temperature limits of the insulation of the cable. If you start to overfill the conduit (raceway) there is a risk that the cable will exceed the temperature rating of the insulation of the wire.
There is the possibility that you could consider derating the wire but I doubt that it would be practical to for an existing installation.

templdl,
Is that the main reason? why the filling ratio for two wires is only 31% while it is 40% for more than two wires. From your opinion, the increasing of no of wires the increasing of heat dissipation the decreasing of filling ratio which doesn't comply with code.

 

[infinity]

IMO conduit fill limits have nothing to do with heat, they are requirements used to ensure that the conductors can be installed without damage.

 

[Smart $]

There are no inspectors, the site is under its own rule, but still governed by NEC (FM global insured). That being said, we do have some flexibility and if we stamp the design it'll be ok. I need to understand the reason (is it really heat dissipation? It doesn't seem like it is... Or just some "well it'll be really hard to pull otherwise so this is best practice" type rule). Just wondering if anyone knows the true intent.

I certainly don't know the true intent, but I believe it to be more mechanical than electrical. FWIW, the ratios of conduit I.D. to a circle circumscribing max' sized conductors are respectively...

1.3736
1.2700
1.2710
1.3099
​

...for the following depiction.

Something you may also want to consider is Notes to Tables (2) on first page of Chapter 9 Tables.

 

[Dennis Alwon]

IMO conduit fill limits have nothing to do with heat, they are requirements used to ensure that the conductors can be installed without damage.

I have to agree with that but certainly the number of conductors in a raceway does relate to heat. I would think a cable tray does not have the same issues as an enclose raceway.

 

[hardworkingstiff]

I know my opinion doesn't count, but I want to play.

I am also dubious about the conduit fill being about heat. It just seems there is not enough air movement in a conduit for the air around the conductors to make a difference on heat dissipation. Also the point brought up about the 2 conductor fill vs the 3 or more conductor fill seems to validate the opinion the rule is more about installing the conductor/s without damage.

 

[masterinbama]

How much over allowed fill is it?

 

[growler]

I would like to know the reasoning behind the rules before i tell him we need to build a new duct bank. Reason being it's a short run (200') and straight as an arrow, and I know they'd have no trouble pulling it.

Thanks!

I need to understand the reason (is it really heat dissipation? It doesn't seem like it is... Or just some "well it'll be really hard to pull otherwise so this is best practice" type rule). Just wondering if anyone knows the true intent.

Thanks

I think heat was part of the reason for conduit fill rules. These rules cover all different types of conduit and different sizes of cables. Stress on cables during installation was another consideration.

Most cables are not loaded to their full potential but they are allowed to be and in those cases there is more heat.

This is just my opinion: So long as you comply with the NEC rules you can't get into trouble but if you decide to engineer around said rules then you need to be right because if there are ever any problems then it's on you. An inch smaller conduit is much smaller than it sounds.

 

[Gregg Harris]

My situation is the client had verified an existing duct bank to have 5" ducts, and we based our ccable size on that (3c 15kV cable). Construction is just about to start (cables bought) and the contractor has discovered they are really 4" and the new cable doesn't fit the conduit fill rule. The client admits responsibilty but I would like to know the reasoning behind the rules before i tell him we need to build a new duct bank. Reason being it's a short run (200') and straight as an arrow, and I know they'd have no trouble pulling it.

Thanks!

300.17 Number and Size of Conductors in Raceway. The number and size of conductors in any raceway shall not be more than will permit dissipation of the heat and ready installation or withdrawal of the conductors without damage to the conductors or to their insulation.

 

[iwire]

The fill rules have to do with preventing physical damage of the conductors.

The derating rules deal with the heat issue.

 

[north star]

& & &

iwire,

Aren't both conditions to be considered in this application?
Damage from too small of a raceway & damage from lack
of heat dissipation capabilities [ i.e. - the First Law of
Thermodynamics ] ?

& & &

 

[iwire]

& & &

iwire,

Aren't both conditions to be considered in this application?
Damage from too small of a raceway & damage from lack
of heat dissipation capabilities [ i.e. - the First Law of
Thermodynamics ] ?

& & &

I think I was pretty clear about my opinion.

 

[don_resqcapt19]

& & &

iwire,

Aren't both conditions to be considered in this application?
Damage from too small of a raceway & damage from lack
of heat dissipation capabilities [ i.e. - the First Law of
Thermodynamics ] ?

& & &

You can make the case that the conductors in the smaller conduit will have more area in physical contact with the raceway and would better dissipate the heat than the conductors in the larger conduit.
I don't believe that the conduit fill rules have anything to do with heat...it is just a mechanical issue.

 

[Smart $]

You can make the case that the conductors in the smaller conduit will have more area in physical contact with the raceway and would better dissipate the heat than the conductors in the larger conduit.
...

I don't believe your case would stand under scutiny. The counter argument is the conductors would be in closer proximity to a larger conduit, and given the conduit material is a substantially better thermal conductor than air, the larger conduit provides a greater surface area for faster heat dissipation.

The following depicts #2THHN in 1" and 1-1/2" EMT...

 

[rbalex]

The NEC is an eclectic mix of hard science, various experimental and empirical observations, special interest protectionism, and sometimes wishful thinking.

The Neher/McGrath based ampacities generally reflected in Section 310.15 and its associated Tables are actually conservative; which is why common ampacities are based on ??not more than three conductors in <fill in the blank>?? The size of a raceway relative to the fill has virtually no practical effect on the rated ampacity.

Most other fill rules are primarily based on the ability to install conductors without damaging them at the time of installation. Most of those rules were developed empirically. In the ?old days,? when many of these rules were developed, they were based on the ability to visually inspect, rather than rigorously test the installation.

I have seen installations that were well over 20 years old and non-compliant even at the time they were installed; nevertheless, there was no indication that the conductors had been damaged or any damage was imminent. There was no reasonable purpose served by ?bringing the installation up to Code.?

Assuming a reasonable pull calculation indicates no over stress, and proper follow-up testing indicates no installation damage, I see no reason not to install as stated in the OP.