Low, Medium & High Voltage Definition

[dbuckley]

I notice they don't list any voltage under 120 volts.

Apparently 24 VAC is 'no voltage'.

In that world there is a lower catergory than low voltage, namely Extra Low Voltage, which is up to 50V AC, and I cant remember what DC.

There is a further sub-categorisation, SELV, Safety Extra Low Voltage, the exact definition of which varies by country, but its essentially the same as ELV with restrictions on how the supply originates and various other regs to ensure that a SELV circuit doesn't through failure become a risk to life.

 

[charlie]

I wonder if electron volts at CERN (Organisation Europ?enne pour la Recherche Nucl?aire) would be of any value? :roll:

The comment that the ranges are determined by the Code you are working with makes the most sense. The range used by the NEC makes the most sense most of the time on this forum. However, when dealing with medium voltage and higher, you are in the IEEE realm and need to shift to their scale. In my opinion, that is when you are involved in the 15 kV class and above. :smile:

 

[weressl]

"lets not muddy the waters and say it is relative to what you do"
OK so now you have your IEEE voltage category limits. . What are you going to do with them ? . Will they help you understand why 250.180 lists over 1000v as high voltage ? . Will they help you understand why 490.2 lists over 600 as high voltage ? . Will the IEEE category limits help you understand the NEC installation requirements for something that is 700v or maybe 800v ?

I know that 700v equipment is categorized as high voltage and must follow the requirements of Article490. . I know that a 700v system is not categorized as high voltage by the NEC and does not have to follow the requirements of Article250 part 10.

Understanding NEC requirements is relative to what I do. . If you bring IEEE voltage categories into an NEC installation answers website, aren't you the one that will "muddy the waters" ?

You seem to think that there is only one answer to the low medium high voltage question. . I know there are many answers and none can claim to be the only right one. . Here on this website, the best answer is the NEC answer.

NEC is not an engineering document. Categorization and defintition of engineering parameters obviously belongs to an Engineering Standard, thus the IEEE/ANSI documents are mor authoritive than others.

What the best answer is defined by the nature of the question, not by the location of the discussion. You may point out that the NEC seems to be in conflict with engineering definitions accepted by ANSI, but it is an error on the part of NEC that should be corrected.

 

[dnem]

What the best answer is defined by the nature of the question, not by the location of the discussion. You may point out that the NEC seems to be in conflict with engineering definitions accepted by ANSI, but it is an error on the part of NEC that should be corrected.

You assume an NEC error but why should the NEC CMPs accept a category that spans from 0-600 ? . That span shows a bias in the IEEE toward what is applicable to utilities. . The safety differences between 1v and 600v can't be ignored. . Both sides would need to make changes to bring about a single standard.

I don't think the NEC CMPs have any desire to classify all voltages under 600 as "low". . IEEE would have to first add a new category for 0-49v. . The NEC would then have to replace their 1000v limit in their system classifications with 600v. . We would need to first know why 1000v was originally chosen.

SYSTEMS / DISTRIBUTION [NEC90.2(A)]
0-49 . . Low . . . . . . [250.20(A), 720]
50-1000 . . Medium . . . . . . [250.20(B)]
1000 & up . . High . . . . . . [250.20(C), examples: 250.180 + 410.140]

For equipment classifications, I don't think there currently is a conflict. . You can overlap IEEE and the NEC equipment categories. . All that is added is a subcategorizing of 601v-69,000v.

0-600 . . Low . . IEEE
. . 0-600 . . Standard . . NEC equipment [110.26 also see T310.13(A)]
601-69,000 . . Medium . . IEEE
. . 601-2000 . . Low ?High?/Low Industrial . . NEC equipment [110.30 + 490.2]
. . 2001-35,000 . . Medium ?High?/Medium Industrial . . NEC equipment [328.2 def, 328.10]
. . 35,001 & up . . High ?High?/High Industrial . . NEC equipment [example: 450.21(C)]
69,001-230,000 . . High . . IEEE
230,001-800,000 . . Extra High . . IEEE
800,000 & up . . Ultra High . . IEEE

It would also be helpful if IEEE would label the 0-49 range as "low" and use a different term for 50-600

There is a very real need to catagorize voltage levels relative to your type of work.
I often perform bench testing and want to alert people to a possible hazard. I often put out a sign that states " Caution - High Voltage Hazard" even when there may only be 120 -240Vac present. It is intended to make a point that the area is to be avoided.

Using the phrase "Caution - Extra Low voltage hazard" just does not have the same impact. :grin:

Finally, I don't see what would be accomplished by creating a category break at 120v. . We treat 50v-119v the same as we treat 120v-600v. . If we were in Europe, 120v would mean nothing at all. . Of course in the USA, 120v is the lowest level chosen for household power distribution, but I don't see any advantage in using 120v as a category limit.