Table 230.51(C)

[jwelectric]

Table 230.51 (C) has two listings for maximum 600 volts. Each has a different measurement. What is the difference between the two and where are we told this?

The last 600 volts where exposed to weather I understand.

 

[infinity]

Here is the errata to the 2005 NEC. I don't see the table you've listed as on it. Could it be that no one (until now) has seen an error?


http://www.nfpa.org/assets/files/PDF/CodesStandards/TIAErrataFI/Errata70-05-1.pdf

 

[Dennis Alwon]

Mike I just got off the phone with the state inspector in Raleigh and he was scratching his head on this one. He thinks it's an errata.

His guess was the first one should be 300 then 600 then 300*, and then 600*

That doesn't make sense either because why would you need to support it closer on indoor--- I am assuming outdoor supports should be closer because of ice etc. but that may not be correct assessment of why the supports are given this way.

Ron said he would call me as soon as he finds out.

 

[jwelectric]

If it is some kind of mistake it is one that has been published from 1975 to present.

Sorry to ask a question that even Chilton can?t answer :grin: :grin: :grin:

 

[infinity]

I just looked in the oldest NEC codebook that I have and that table has remained unchanged since 1987. My guess is that you need to look at the entire line horizontally. If you have 600 volts or less and the conductors only 6" apart then you can use a maximum spacing of 9'. If you were to increase your spacing to 12" apart you increase your support span to 15' for the same voltage.


Probably has to do with the conductors hitting each other in between their supports when they're installed closer together.

 

[jwelectric]

I just looked in the oldest NEC codebook that I have and that table has remained unchanged since 1987. My guess is that you need to look at the entire line horizontally. If you have 600 volts or less and the conductors only 6" apart then you can use a maximum spacing of 9'. If you were to increase your spacing to 12" apart you increase your support span to 15' for the same voltage.


Probably has to do with the conductors hitting each other in between their supports when they're installed closer together.

I do believe that you are right.

 

[Dennis Alwon]

This table is rarely used according to Ron perhaps that's is why no one has questioned it. He was determined to find the answer so perhaps we will know something tomorrow.

 

[infinity]

Since that table has been unchanged for more than 20 years it's my guess that it is written as intended and is correct. It's the way that it's being read that is incorrect.

 

[jwelectric]

Since that table has been unchanged for more than 20 years it's my guess that it is written as intended and is correct. It's the way that it's being read that is incorrect.

You got it Mr. Trevor


Edited to add:

When I was asked earlier this morning I just couldn't get my head to wrap around it.

 

[Dennis Alwon]

I just looked in the oldest NEC codebook that I have and that table has remained unchanged since 1987. My guess is that you need to look at the entire line horizontally. If you have 600 volts or less and the conductors only 6" apart then you can use a maximum spacing of 9'. If you were to increase your spacing to 12" apart you increase your support span to 15' for the same voltage.


Probably has to do with the conductors hitting each other in between their supports when they're installed closer together.

Trevor, apparently I missed your statement first time around. I agree--- good catch. The only question I do have is why there is not a 300 Volts when not exposed to weather. Perhaps because it doesn't matter and it's the same for 300 exposed to weather or not.

 

[infinity]

Trevor, apparently I missed your statement first time around. I agree--- good catch. The only question I do have is why there is not a 300 Volts when not exposed to weather. Perhaps because it doesn't matter and it's the same for 300 exposed to weather or not.

After opening the 1987, 1996, & 2005 NEC and looking at the same table in each, I assumed that I must be reading the thing incorrectly. As you mentioned earlier this is a little used table. Until today I don't believe that I've ever looked at it.