Need Help with this Project Question;

[schicco]

I am preparing for my state licensing exam and need help with this project question;

A conductor contains 8 wires consisting of two 3-phase, 4 wire branch circuits serving fluorescent lighting that will operate for more than 3 hours. These circuits run thru an attic at a temperature of 120F. If each circuit is protected by a 20A OCPD Device, what size conduit, wire and insulation type is required?

It looks like without the actual load rating, I need to work backwards to the 20A OCPD. How do I go about doing that?
Thanks

Steve

 

[MarineTech]

Using NEC 2011

This is my interpretation:

Greater than 3 Hour = Continuous Load. 20A OCPD, Example Lighting.

20A/125% = 16A Load

Assume 90 Degree THHN Insulation.

Derating:

120 Degree F Temperature correction = 0.82

Assume balanced (linear loads) 4 Wire 3 Phase Grounded System, Neutral not counted in bundling.

Bundling (4-6) = 0.80

Conductors sized 125% = 20A.

10 AWG THHN = 40A, After derate = 26.24A > 20A

Assume EMT Raceway having 8 10 AWG THHN Conductors. Trade size 3/4 inch minimum.

Could someone else please check my work, Thanks.

 

[augie47]

My opinion:
With THHN insulation, you can use the 90? ampacity, For 120?F you would have a derating factor of .82 ans, unless it's a non-linear load, you would have a conduit fill factor of 0.8.
So your ampacity multiplier would be .82 x .8 or .656.
A 20 amp OCP device would be designed for 16 amps of continuous load so you need a 90? rating of 24.4 amps or #12 THHN.
The question, however, does not qualiy the type light and it's likely to be a non-linear load which would then be 8 current-carrying conductors for a fill deratng factor of 0.7 so your multiplier would be .82 x .7 or .574 which would cause yoiu to need a insulation rating of 27.8. #12 THHN would still be good.

 

[infinity]

The question, however, does not qualiy the type light and it's likely to be a non-linear load which would then be 8 current-carrying conductors for a fill deratng factor of 0.7 so your multiplier would be .82 x .7 or .574 which would cause yoiu to need a insulation rating of 27.8. #12 THHN would still be good.

Disregarding continuous if you're assuming non-linear then I agree that you derating factor is 70% but how do you get to 27.8 amps for #12 THHN conductor, and are you going up to the next standard size?

30 amps * .70 * .82 = 17.22 amps, next standard size good for a 20 amp OCPD.

 

[MarineTech]

As augie47 pointed out, I really should have included the calc for non-linear loads, inclusion of the Nuetral for bundling (7-9) at 70%, as stated in the question as Fluorescent Lighting.

 

[Volta]

I am preparing for my state licensing exam and need help with this project question;

A conductor contains 8 wires consisting of two 3-phase, 4 wire branch circuits serving fluorescent lighting that will operate for more than 3 hours. These circuits run thru an attic at a temperature of 120F. If each circuit is protected by a 20A OCPD Device, what size conduit, wire and insulation type is required?

It looks like without the actual load rating, I need to work backwards to the 20A OCPD. How do I go about doing that?
Thanks

Steve

Steve, welcome to the forum!

Note that the different code years lead to different answers.

I believe that they are considering these circuits as non-linear, as evidenced by the fact they called it out as fluorescent.

So the neutrals would then be considered current-carrying, making a total of 8 current-carrying conductors (CCC). That brings the derating to 70% of the highest rating of the wire.

Assuming a conductor with a 90 deg C insulation like THHN, the temperature of 120 deg F derates the wire by another 89% (which I believe to be an error in the book, but that is what it says in 2011) per Table 310.15(B)(2)(b). The product of 70% and 89% equals .623.

As the load is said to be continuous, we will not exceed the OCPD by 80%, so 16 amp load max. A #12 THHN is rated 30 amps at 90 deg C, so 30 amps derated by 62.3% is 18.69 amps, and good for that load.

If figured by the 2008 NEC, the temperature derating would be 82%. Then the combined rating would be 57.4%. For a 30 amp wire, it would then only be worth 17.22 amps. But as Roger said, and still large enough.

 

[Smart $]

....

Assuming a conductor with a 90 deg C insulation like THHN, the temperature of 120 deg F derates the wire by another 89% (which I believe to be an error in the book, but that is what it says in 2011) per Table 310.15(B)(2)(b). The product of 70% and 89% equals .623.

...

If figured by the 2008 NEC, the temperature derating would be 82%. Then the combined rating would be 57.4%. For a 30 amp wire, it would then only be worth 17.22 amps. But as Roger said, and still large enough.

In 2011 NEC, you should use Table 310.15(B)(2)(a). It has the same adjustment factors as Table 310.16 in 2008 NEC, and should be used for any ampacity table based on an Ambient Temperature of 30?C (86?F). You only use Table 310.15(B)(2)(b) for ampacity tables based on an Ambient Temperature of 40?C (104?F).

 

[Volta]

In 2011 NEC, you should use Table 310.15(B)(2)(a). ....

Ah, thanks. Knew something was off, turns out it was me. Always good to slow down and check. :blink:

 

[CONDUIT]

Here is how I see it. 16 amp load continous load 16 x1.25 = 20 Since it is feeding fluorescent lighting it would be a non linear load so ther would be 8 current carrying conductors 20/.7 =28.57
Adjustment for ambient temperature of 120 F 28.57/.82 = 34.8 amps. Conductor would need to be a minimum of a #10 THHN. Before you reply look at 210.19(A)(1).

 

[CONDUIT]

Actually after finding some other examples on the internet I am not sure that I am correct. I know that you typically upsize the conductors on a continous load to match the overcurrent device at 1.25%. But in this case since the overcurrent device is 20 amps do you only need to make the correction for ambient temperature and number of current carrying conductors. If you read 210.19(A)(1) it sounds like you increase it by 1.25% before applying the correction factors. What are your thoughts?

 

[schicco]

Thanks guys for the input. These is such an amazing wealth of information here.

 

[GoldDigger]

Thanks guys for the input. These is such an amazing wealth of information here.

Much of it correct!