UNDERSIZE FITTINGS

[maintenance man]

I have a installation where 3 #4/0 THHN cables were put into a condulet that is listed for 3 # 1/0 cables . condulet and conduit are both 2". We did not know there was a different capacity for both. Cables were tight but manage to get them in. We know what has to be done and will get to it.
Can someone point out what hazards exist with this install ?

 

[infinity]

If you installed the conductors without damaging them then there probably is no hazard. However you still have a violation.

Welcome to the Forum.

 

[wallypiper]

I thought conduit fill rules were related to heating. Squeezing conductors together like that would obviously affect the ability to dissipate heat. Doesn't that represent a hazard?

 

[kwired]

I thought conduit fill rules were related to heating. Squeezing conductors together like that would obviously affect the ability to dissipate heat. Doesn't that represent a hazard?

Ampacity adjustments and other ampacity selection rules have to do with heating, and primarily applies to current carrying conductors, there are rules for determining size of non current carrying conductors but ampacity adjustments do not directly apply to them.


Conduit fill rules have to do with ability to pull conductors without damaging the conductors and apply to all conductors whether they are current carrying or not.

 

[wallypiper]

Then why is there an ampacity derate for more than 3 current carrying conductors in a raceway?

 

[don_resqcapt19]

I thought conduit fill rules were related to heating. Squeezing conductors together like that would obviously affect the ability to dissipate heat. Doesn't that represent a hazard?

There is no less area in the conduit body for the dissipation of the heat then there is in the conduit itself. The issue is the bending space in the conduit body. You need enough space to be able to install the conductors into the conduit body without damaging them. The fill that is marked on the conduit body is less than the maximum permitted fill for a raceway of the same trade size as the conduit body.

 

[petersonra]

Unless there is conductor damage, i would not worry about it.

 

[kwired]

Then why is there an ampacity derate for more than 3 current carrying conductors in a raceway?

Because of heat. It is not just raceways it applies to, and it doesn't matter how close those conductors are to one another. 4-6 current carrying conductors needs 80% adjustment factor whether you have same conductors in a 1/2 inch raceway, an NM cable, or a 4 inch raceway. (6 - 12 AWG conductors have same ampacity in a 1/2 inch conduit as they have in a 4 inch conduit).

Conduit fill calculations have nothing to do with ampacity, adjustments to ampacity, or heat developed in the raceway. Conduit fill calculations are based on leaving a certain amount of free space in the raceway in order to prevent conductor damage when installing the conductors. You must determine what size conductors you need for your application then select a raceway with sufficient cross sectional area to contain them. Now you may be able to use less adjustment factor, possibly resulting in smaller conductors by putting them into more than one raceway.

 

[wallypiper]

(6 - 12 AWG conductors have same ampacity in a 1/2 inch conduit as they have in a 4 inch conduit).

Aha.
Thanks.

 

[GoldDigger]

(6 - 12 AWG conductors have same ampacity in a 1/2 inch conduit as they have in a 4 inch conduit).

But if you keep increasing the size of the conduit or raceway without limit, at some point you get very close the free air rating of the wires in practice, even though the NEC does not take that extreme into account. The surface area by which the heat can leave the conduit, as well as the space for convection or other air movement increases as you increase the size of the raceway.

 

[infinity]

But if you keep increasing the size of the conduit or raceway without limit, at some point you get very close the free air rating of the wires in practice, even though the NEC does not take that extreme into account. The surface area by which the heat can leave the conduit, as well as the space for convection or other air movement increases as you increase the size of the raceway.

Although this is completely true the NEC doesn't care.

 

[GoldDigger]

Although this is completely true the NEC doesn't care.

What else is new....

 

[maintenance man]

We were thinking after these cables being forced into a condulet being nicked would sooner or later short against the conduit. Cable will get warm and cold from the load where the damage will show at sometime. We really don't know what lies inside the pipe. Will the load on and off all day affect the cable break down at the nics ?

 

[kwired]

But if you keep increasing the size of the conduit or raceway without limit, at some point you get very close the free air rating of the wires in practice, even though the NEC does not take that extreme into account. The surface area by which the heat can leave the conduit, as well as the space for convection or other air movement increases as you increase the size of the raceway.

Running six #12's in a 4 inch raceway seems like an expensive way to get around having to derate the conductors anyway, but I guess if I were the inspector I may let it go

 

[kwired]

We were thinking after these cables being forced into a condulet being nicked would sooner or later short against the conduit. Cable will get warm and cold from the load where the damage will show at sometime. We really don't know what lies inside the pipe. Will the load on and off all day affect the cable break down at the nics ?

I think it really depends on the conditions of the application.

Non metallic raceway not in a wet location - maybe you will never have a problem.

Metal raceway surface abrasions on the insulation of conductors - maybe you have a weak point for a high voltage transient condition but otherwise never have a problem.

Large abrasion that exposes the conductor in a metal raceway - likely will have problem at some time.

Aluminum conductors underground whether direct buried or in raceway - if the abrasion exposes the conductor - it will likely totally fail within weeks, if it is only a surface abrasion - lightning events will usually find the weak spot at some point and that is where it will fail.