12 kv clearances

[augie47]

I have a contractor that wishes to install a 12kv open conductor span (customer owned) over a metal building (industrial complex). From what I read NEC refers me to NESC and I don't have a copy of that.
It it permissable to install open 12kv over a metal roof building and what clearance must be maintained?
Thanks

 

[bob]

augie
You do need to refer to the NESC and you local utility probably has a copy. If the roof is not accessible the is a minimum ht that is required,
I think, is 12 ft but don't take that figure without checking. You also need to be concerned with equipment, such as a crane, if this is an industrial area.

Check here http://www.pnm.com/esg/drawings/ds-13-2_0p2.pdf

 

[jtester]

It is even more complicated than that. For example you must apply the vertical clearances at the temperature and conductor loading conditions that produce the most sag. You must apply the horizontal clearances with the conductor displaced from rest with a certain wind force depending on your location.
Unfortunately the NESC is not a document that says simply "you must have 12' of clearance."

Jim T

 

[coulter]

Guys and girls -

Educate me a little.

Q1. Where in the NEC does it say you have to use the NESC? I thought you could use the NEC if you wanted to. The operative word here is "thought" - I don't know.

Q2. Would this also apply to a private, non-industrial site. In particular, I'm thinking of where a house is located several hundred feed from the utility feed and the power company wants a bundle to run the poles. So, one person I know of put the service at the utility, bought two 50kVA(or so), 1ph, 4160/240 xfmrs (dry type w/weather shields) ran 5kV direct buried to the house. Didn't seem to bother the power company. And I don't know about any AHJ conversations.

carl

 

[bob]

Unfortunately the NESC is not a document that says simply "you must have 12' of clearance."

I'm not sure what you mean but the NESC does not provide the engineering but it does provide clearances

Q1. Where in the NEC does it say you have to use the NESC? I thought you could use the NEC if you wanted to. The operative word here is "thought" - I don't know.

The NEC does not provide much help with voltages over 600 volts. I'm
not aware of where this info is in the NEC.

Question 2. I think this is done often where the utility is requiring a high price to extend there primary lines. The NEC provide required information necessary to bury cable.

 

[jtester]

Bob

I'm not sure what you mean but the NESC does not provide the engineering but it does provide clearances

You said it best. The NESC does not provide the engineering, whereas with the NEC you look up the answer. The NESC will fail you if you just look in the tables, and not read the extremely specific conditions that the clearances apply.

For example if you measured 12.5' of clearance over a roof this afternoon, there is no simple way to compare it with the requirement in NESC Table 234-1, which requires 12.5'.

Secondly to Coulter's question

Q1. Where in the NEC does it say you have to use the NESC? I thought you could use the NEC if you wanted to. The operative word here is "thought" - I don't know.

The NEC Handbook refers the reader to the NESC in 4 locations in 225.60 and 61. The clearance values in Tables 225.60 and 225.61 require a set of standard conditions such as what the NESC offers.

Jim T

 

[coulter]

...The NEC Handbook refers the reader to the NESC in 4 locations in 225.60 and 61....

Yeah, I saw that. But two of those are blue print and the other two are FPN. None of which is regulatory.

... The clearance values in Tables 225.60 and 225.61 require a set of standard conditions such as what the NESC offers.

Absolutely one should use all available resources to do a job right. I just can't find where the NEC mandates the use of the NESC.

Where I live, one could install per 225.61 in December and have the conductor around your ankles come July. A 140F temp differential can wreak a lot of havoc with most installations.

JAO: I think it would be reasonable for an AHJ to say, "Yeah, you were at 12.5 feet when inspected in December. You got an OSHA issue now. It's off till you fix it. If you don't want to use the NESC as a guideline, pick something that works or your staying off."

carl

 

[bob]

Where I live, one could install per 225.61 in December and have the conductor around your ankles come July. A 140F temp differential can wreak a lot of havoc with most installations

Good point. So if the code requires a minimum of 12 ft, you build it so you have 16 or 18 ft.

 

[jtester]

Good point. So if the code requires a minimum of 12 ft, you build it so you have 16 or 18 ft.

The proper way to do it would perform the calculations, determine the maximum sag, and then apply the 12' clearance. It is the only way to be sure you have met clearance requirements. The real kicker is performing the calculations.

This becomes a real specialty area, not all engineers, and even fewer contractors, can perform the calculations.

Jim T