Minimum Conduit Length

[4me]

I cannot seem to find where the minimum conduit length is in NEC. Meaning, when you use the tables for open air vs in conduit. Is there any minimum length of conduit that is considered open air? if you have a 3 phase, 460V, motor with an open end conduit and length of 1 foot, to me, would be equivelent to open air. There would be minimal heat buildup in this short space. What NEC section states this?

 

[Dennis Alwon]

If the conduit is less than 24" then you do not have to de-rate. If, for instance, the amount of conduit is less than 10% of the run or 10' or less than you can use.

Here this says it better than I do

(2) Selection of Ampacity.

Where more than one ampacity applies for a given circuit length, the lowest value shall be used.
Exception:
Where different ampacities apply to portions of a circuit, the higher ampacity shall be permitted to be used if the total portion(s) of the circuit with lower ampacity does not exceed the lesser of 3.0 m (10 ft) or 10 percent of the total circuit.

 

[HEYDOG]

I cannot seem to find where the minimum conduit length is in NEC. Meaning, when you use the tables for open air vs in conduit. Is there any minimum length of conduit that is considered open air? if you have a 3 phase, 460V, motor with an open end conduit and length of 1 foot, to me, would be equivelent to open air. There would be minimal heat buildup in this short space. What NEC section states this?

If it is in a conduit it is not in open air. As far as conductors adjustment factors look at code article 310.15(B)3(A)2.

 

[LarryFine]

Welcome to the forum.

A one-foot length of conduit is considered a nipple, which has a greater fill capacity than one over two feet, due to less heat retention.

However, it is still a conduit. Free air normally refers to individual conductors suspended in the air, not inside cabinets and the like.

The one open end of your example does not, in my opinion, cause the entire length of the conductors to qualify as being in open air.

 

[infinity]

The one open end of your example does not, in my opinion, cause the entire length of the conductors to qualify as being in open air.

Couldn't the exception that Dennis posted apply to the 1' piece of conduit?

 

[4me]

I did see that exception, Dennis, but that is for multiple amperage rated circuits. This would not be, as it is one source, same amperage's. My first inclination is as Larry's (thank you for the welcome) but was hoping for an exception that I was not aware of.

I thank you all for the feedback.

 

[winnie]

Welcome @4me

Read the exception again. It isn't for 'multiple' circuits. It is for a single circuit where the wires pass through different regions that apply different derating requirements.

In general the lowest ampacity governs the entire circuit. For example you might have a circuit where a portion of the run is through a hot attic and subject to more derating then the rest of the circuit. But you can only use the lowest ampacity you calculate anywhere in the circuit.

The exception says that if the region with additional derating is short enough, you can ignore it.

-Jon

 

[4me]

Win, your right....my bad and understand it, now.

 

[Strathead]

I took a look at the code. I feel that the code section Dennis quoted applies. It is (2) as applied to general. Note one says "ass applies to the tables" So note 2 would also apply to the tables.

 

[LarryFine]

Couldn't the exception that Dennis posted apply to the 1' piece of conduit?

Maybe, if there are nine or more feet that are genuinely in free air, which to me is different than merely not being contained within the conduit, which is why I mentioned an enclosure above.