NEC Discrepancy in conduit sizing.

[noxx]

Ran into an interesting contradiction while sizing conduit.

Situation: 3 conductors of 300kcm each in EMT w/o a grounding conductor. Each conductor (THWN) has an area of .4608 square inches. The total area for all three is then 1.3824 Sq in.

If you plug that total into the table for EMT in chapter 9, you find 2.5" EMT is required at 40% fill. However, if you check in Annex C (as all the wires are the same size) It allows 3 conductors in 2" EMT, altho the combined area clearly exceeds the limit for 40% fill.

Obviously I would choose to err on the side of the larger conduit, for a variety of reasons (cost not being one of them) but I found the conflicting information interesting.

Thoughts?

 

[websparky]

Re: NEC Discrepancy in conduit sizing.

Noxx,

The total area for all three is then 1.3824 Sq in.

I think you forgot about this note?

(7) When calculating the maximum number of conductors permitted in a conduit or tubing, all of the same size (total cross-sectional area including insulation), the next higher whole number shall be used to determine the maximum number of conductors permitted when the calculation results in a decimal of 0.8 or larger.

 

[noxx]

Re: NEC Discrepancy in conduit sizing.

You are correct sir!

 

[a.wayne3@verizon.net]

Re: NEC Discrepancy in conduit sizing.

Thats why I like this site,It1s ok to be humble

 

[stew]

Re: NEC Discrepancy in conduit sizing.

But doesnt the calculation result in a decimal of 1.38? If .8 is where a round up is done then wouldnt you need to hasve a resulting calculation of say 1.8 something to round up to the next whole number of 2?or am i missing something here?

never mind I found note 7 thanks. still confused by above however??

[ March 23, 2004, 12:01 AM: Message edited by: stew ]

 

[charlie b]

Re: NEC Discrepancy in conduit sizing.

I think the math works this way.

Step 1: From Chapter 9, Table 4, looking at the row for a 2? conduit, the column for 40% of the internal area gives you 1.342 square inches.

Step 2: From Chapter 9, Table 4, the cross-sectional area of a single 300 MCM THWN is 0.4608 square inches.

Step 3: 1.342 divided by 0.4608 gives 2.91. This means that you can install 2.91 of these 300 MCM conductors in the 2? EMT. Now we ask, do we round this down to 2 conductors, or round it up to 3 conductors?

Step 4: We observe that the decimal ?.91? is bigger than the decimal ?.8.? From Chapter 9 Note (7), we conclude that we can round up the number of allowable conductors from 2.91 to 3.

 

[steve66]

Re: NEC Discrepancy in conduit sizing.

Question for the electricians:

How difficult is it to actually try to squeeze 3x 300KCM's in a 2" conduit? How about for a short run? A long run?

As an engineer, I can do the calculations per the NEC, but I have no idea how realistic it is to actually try this.

I never round the conductor up as allowed. And I often specify conduit that is one size larger than required by the NEC for long runs. Do you think this is unnecessary, or a good idea?

 

[dave81]

Re: NEC Discrepancy in conduit sizing.

That is a good idea. And getting 3 300kcmil into a 2 inch would be kind of a bear, short and deffinitly a long run.

 

[stew]

Re: NEC Discrepancy in conduit sizing.

Thanks for the clarification charlie

 

[noxx]

Re: NEC Discrepancy in conduit sizing.

Originally posted by charlie b:
I think the math works this way.

Step 1: From Chapter 9, Table 4, looking at the row for a 2” conduit, the column for 40% of the internal area gives you 1.342 square inches.

Step 2: From Chapter 9, Table 4, the cross-sectional area of a single 300 MCM THWN is 0.4608 square inches.

Step 3: 1.342 divided by 0.4608 gives 2.91. This means that you can install 2.91 of these 300 MCM conductors in the 2” EMT. Now we ask, do we round this down to 2 conductors, or round it up to 3 conductors?

Step 4: We observe that the decimal “.91” is bigger than the decimal “.8.” From Chapter 9 Note (7), we conclude that we can round up the number of allowable conductors from 2.91 to 3.

And this is of course why the noted exception shot by me. I always take the sum of all the conductors, then check my 40% fill list to for conduit size, and either it fits or it doesn't. If you do the math you can find this "fudging" of 0.8 of a conductor causes more mayhem at larger conductor sizes, for instance, using this I can legally put 3 900kcm conductors in a 3" conduit.

Eek?

 

[iwire]

Re: NEC Discrepancy in conduit sizing.

Originally posted by steve66:
Question for the electricians:

How difficult is it to actually try to squeeze 3x 300KCM's in a 2" conduit? How about for a short run? A long run?

I do not remember doing that combination but it sounds pretty tough, we have the equipment I would get it in. The real problems would be LBs, I doubt three 300s are allowed in a 2" LB and if they are I would not want to be the one getting them in.

I do four 4/0s in 2" quite often and even that can be tough.

If we get plans that the raceways seem undersized we just go bigger on our own, some of the engineers we get prints from obviously go right to the letter of the code and that is fine we just chose to go bigger when it seems worth while.

Originally posted by steve66:
I often specify conduit that is one size larger than required by the NEC for long runs. Do you think this is unnecessary, or a good idea?

From my side yes it is a great idea, however you may save the client money by specifying the minimum we bid to that and eat the cost of up sizing when we think it is needed.