operating temperature tables

[Wire-Smith]

like look at 310.15 title

ampacities for conductors...



310.15(B) title

tables

(b7 used to have a table)



i see it as saying if you have a service of this size you cannot have less than this wire ampacity


i do not see it saying if you have a 200amp service you never need more than 4/0al no matter what the rest of the code says
(i know theres the part about you still have to derate, but thats the only other part of the code that people think they have to comply with on this)

 

[Wire-Smith]

310.15(B)(7)(1)...shall be permitted to have an ampacity not less than 83%.....

i don't see how this is an exception to 230.32(A) conductors shall have sufficient ampacity to carry the current for the load as calculated in accordance with article 220 and....



and i see how you might see 310.15(B)" as modified by (B)(1) through (B)(7) as saying b7 modifies table 310.15(B)(16) but i think that is a stretch because b7 is talking about service conductor ampacity not the ampacity of say 4/0al thhn, my explanation for this i would send you back to my reasoning in post 20, 110.14(C), 230.32(A) and that the old b7 table conductor sizes were not lower than what would be allowed without b7 at all because you can round up ocpd with feeders and services (240.4(B))

 

[Wire-Smith]

am i just way off? and there's no merit to my interpretation of this? i know most people (everyone i have ever discussed this with) interpret it the other way, but that alone is not enough reason for me to interpret it like that. it's not how i interpret the code on this. if that interpretation is correct why is it not in 220 as demand factor but in 310

 

[Wire-Smith]

NEC 310.15(B)(7) is intended for dwelling service sized per NEC 220 Load calcs.

If you dont like it, look for any required table adjustments, or load calc errors.

Any conduit on roof, >10% of raceway outdoors - subject to ambient > 30c, more than 3 ccc in raceway, or any pump/fan/motor loads they forgot to factor @ 1.25%. Any continuous loads not factored at 1.25% ?

i'm not an inspector or trying to convince someone to use the bigger wire, i will use the bigger wire no matter what the code says. i'm just trying to understand the code and i'm hung up on this one(compared to everyone else's interpretation)(my interpretation appears black and white to me). thanks for trying to explain it to me, i know i'm hard headed

 

[ActionDave]

am i just way off? and there's no merit to my interpretation of this? i know most people (everyone i have ever discussed this with) interpret it the other way, but that alone is not enough reason for me to interpret it like that. it's not how i interpret the code on this. if that interpretation is correct why is it not in 220 as demand factor but in 310

I think you are asking the code writers to hold your hand through every possible circumstance rather than doing the work to learn and understand electrical systems and why the code is written as it is.

 

[ramsy]

i'm just trying to understand the code and i'm hung up on this one(compared to everyone else's interpretation)(my interpretation appears black and white to me). thanks for trying to explain it to me, i know i'm hard headed

The old Table 310.15(B)(7), most familiar to old sparkys, was removed in recent code cycles. You can still see it in Annex D. Example D7.

To prove the 83% rule is the same as the old table, multiply old table service ratings by 83% to match its conductor in ampacity column.

 

[ramsy]

..to me this is more reason for the code to read the other way when you take into consideration trip curves

http://download.schneider-electric.c...ame=736-06.pdf
QOM FAMILY MOLDED CASE CIRCUIT BREAKERS CHARACTERISTIC TRIP CURVE NO. 736-6
175A breaker at 180 amps will take 16 minutes to 2.7 hours to trip at 40*c ambient temp cold start

218 amps(175 x 1.25) is 5.8 minutes to 2.7 hours

Don't know how trip curves were considered during NEC code adoption history, and not surprised if it was overlooked. In my experience, warm enclosures, or warmer termination temperatures regularly nuisance trip HVAC loads & Twin breakers, especially when those breakers lack space for air circulation.

Unfortunately, builders who exceed NEC / or other Specified requirements are likely punished by losing the bidding process, to someone that didn't care.

Rewards for exceeding Code or Specs can occur with the right knowledge of wiring methods that are most efficient, and perhaps during service work / re-work repairs.

Many new 208/120v installs requiring a plan check in my State are using #10 stranded on 1-Pole 20A breakers, probably due to Energy Codes. In these cases plan check forces everyone to bid by the same Spec, and the cheaper wiring methods are not allowed outside of residential-building zones.

 

[kwired]

like look at 310.15 title

ampacities for conductors...



310.15(B) title

tables

(b7 used to have a table)



i see it as saying if you have a service of this size you cannot have less than this wire ampacity


i do not see it saying if you have a 200amp service you never need more than 4/0al no matter what the rest of the code says
(i know theres the part about you still have to derate, but thats the only other part of the code that people think they have to comply with on this)

Before when they still had just the table - it wasn't all that clear if you were supposed to make adjustments for temp or number of conductors - the way it is now it says you must still make adjustments when those situations exist.

The 83% (I don't know how they come up with 83%) is a factor they have come up with that basically figures dwellings have a load diversity that never exceeds this level for long enough time to be that significant.

Think about typical dwelling - how often is every load ever on at same time, and even if they are it is never for very long at all. On top of that how often does one calculate load to be 190-200 amps and not go with more then 200 amps of service capacity just because of possible future loads being added? Even more common anymore is to install minimum of 200 amp services at dwellings regardless of actual load.

Outside of electric heat being involved I don't know I have ever measured more then 60-70 amps on a dwelling main supply at any given time, unless we were intentionally trying to load it up.

Unless you are cooking a big Thanksgiving day meal and doing laundry at same time - the load just doesn't rise all that much for very long. And if you are cooking that big Thanksgiving day meal - your cooking equipment is adding heat to the house - so heating system will have less demand - whether electric or not.

 

[Wire-Smith]

The old Table 310.15(B)(7), most familiar to old sparkys, was removed in recent code cycles. You can still see it in Annex D. Example D7.

To prove the 83% rule is the same as the old table, multiply old table service ratings by 83% to match its conductor in ampacity column.

240.4(B) gets you there without 310.15(B)(7) except for i think the last one on the table, i don't have my 11' book with me right now

 

[Wire-Smith]

Before when they still had just the table - it wasn't all that clear if you were supposed to make adjustments for temp or number of conductors - the way it is now it says you must still make adjustments when those situations exist.

The 83% (I don't know how they come up with 83%) is a factor they have come up with that basically figures dwellings have a load diversity that never exceeds this level for long enough time to be that significant.

Think about typical dwelling - how often is every load ever on at same time, and even if they are it is never for very long at all. On top of that how often does one calculate load to be 190-200 amps and not go with more then 200 amps of service capacity just because of possible future loads being added? Even more common anymore is to install minimum of 200 amp services at dwellings regardless of actual load.

Outside of electric heat being involved I don't know I have ever measured more then 60-70 amps on a dwelling main supply at any given time, unless we were intentionally trying to load it up.

Unless you are cooking a big Thanksgiving day meal and doing laundry at same time - the load just doesn't rise all that much for very long. And if you are cooking that big Thanksgiving day meal - your cooking equipment is adding heat to the house - so heating system will have less demand - whether electric or not.

i'm not saying the logic your using is wrong, i see how it could be that way.

but wouldn't that interpretation mean the code section dealing with this would be much more appropriate in 220 with demand factors?, like all the other diversification demand factors?

 

[Wire-Smith]

I think you are asking the code writers to hold your hand through every possible circumstance rather than doing the work to learn and understand electrical systems and why the code is written as it is.

i'm not asking this to be able to use 4/0 on a calculated load greater than its table 310.15(B)(16), i would use the bigger wire whether code required it or not. what i'm asking about is everyone else's interpretation of how that it is allowed to use 4/0 with 190A calculated load, i don't read the code to say that's allowed, everyone else seems to.

 

[kwired]

240.4(B) gets you there without 310.15(B)(7) except for i think the last one on the table, i don't have my 11' book with me right now

Not really. What 240.4(B) allows for is say for the 4/0 aluminum mostly been topic of discussion is that it can be protected at 200 amps because it's ampacity of 180 isn't a standard overcurrent device rating.

Art 210, 215 or 230 will still refer you to art 310 for minimum ampacity based on the load - so if you had a load of 181 amps - 4/0 aluminum is not large enough conductor.

What 310.15(B)(7) is doing is modifying the allowable conductor ampacity for conditions specified - which happens to be dwelling units with 120/240 single phase supply where the conductor in question is supplying the entire dwelling load. It can't be used for a subpanel within a dwelling.

Conventional method of determining minimum ampacity just happens to also give you 4/0 aluminum as long as your load is not over 180 amps. If you were using copper conductor 2/0 happens to have ampacity right at 175 - so there is no way of going up to next size when you are already at a standard size.

 

[Wire-Smith]

Not really. What 240.4(B) allows for is say for the 4/0 aluminum mostly been topic of discussion is that it can be protected at 200 amps because it's ampacity of 180 isn't a standard overcurrent device rating.

.

i don't mean 240.4(B) allows 190 on 4/0 i mean it allows 200A ocpd like the old table does

i don't believe you can use 4/0al on 190 calculated load if i was not clear on that(in normal situations)

 

[Wire-Smith]

Art 210, 215 or 230 will still refer you to art 310 for minimum ampacity based on the load - so if you had a load of 181 amps - 4/0 aluminum is not large enough conductor.

What 310.15(B)(7) is doing is modifying the allowable conductor ampacity for conditions specified - which happens to be dwelling units with 120/240 single phase supply where the conductor in question is supplying the entire dwelling load. It can't be used for a subpanel within a dwelling.

Conventional method of determining minimum ampacity just happens to also give you 4/0 aluminum as long as your load is not over 180 amps. If you were using copper conductor 2/0 happens to have ampacity right at 175 - so there is no way of going up to next size when you are already at a standard size.

this is where i think our interpretations really divide

i don't see b7 modifying the ampacities in (B)(16) that way, it is saying "shall be permitted to have an ampacity not less than 83% of the service rating" it doesn't say you can adjust (B)(16) values to 125%, it is giving you a minimum requirement not an exception or a up-rating(opposite of de-rating)

thanks for taking the time to try and explain this to me, i'm not trying to be a prick, just trying to figure out what i'm missing

 

[Wire-Smith]

it appears this was put in in 99' does that sound right to everyone? i'm going to try to find the panel discussions on it.

 

[kwired]

There are different things going on here

minimum conductor ampacity


maximum overcurrent protection

All (B)(7) is doing is letting you increase allowable ampacity of the conductor but only for specific situations mentioned.

It is an option when you meet the conditions mentioned - you can always use the regular method of selecting your conductors - and in some cases like with 4/0 aluminum you end up with same possible conductor size anyway.

Difference with the 4/0 aluminum case is with regular method that conductor has an ampacity of 180 amps and a 181-200 amp calculated load will require next larger conductor size even if still on a 200 amp OCPD.

With (B)(7) allowances you can use that 4/0 with a load calculation of 181-200 amps. They are saying the load diversity in such applications is such that it won't overheat the conductor.

 

[Wire-Smith]

With (B)(7) allowances you can use that 4/0 with a load calculation of 181-200 amps. They are saying the load diversity in such applications is such that it won't overheat the conductor.

what's your thoughts on what i said about that would be more appropriate in 220 as a demand factor then?

 

[Wire-Smith]

what i mean is in 220 allow residential(same as b7 sitaution) to reduce calculated load to 83%? then you would stay consistent with the rest of the code in my opinion.

 

[Wire-Smith]

i brought up trip curves in post 11 in this thread i'll post them again here. you could do the 83% in demand factors and if/when there is a "short" "spike" the ocpd still wouldn't trip. it doesn't make sense to me to allow using a conductor above it's ampacity when considering the calculated load. i know it will likely not get there that's why i think it should be dealt with in 220 rather than hacking the code up in 310. i appreciate other's thoughts on this if you care to share them.


http://download.schneider-electric.c...ame=736-06.pdf
QOM FAMILY MOLDED CASE CIRCUIT BREAKERS CHARACTERISTIC TRIP CURVE NO. 736-6
175A breaker at 180 amps will take 16 minutes to 2.7 hours to trip at 40*c ambient temp cold start

218 amps(175 x 1.25) is 5.8 minutes to 2.7 hours

and thats 175A ocpd if you didn't notice

 

[kwired]

Maybe need to search for ROP's back whenever this allowance was first introduced into code or for any times it has had changes to it to better understand why it is there. Yes they more recently removed the table that had been there for a long time and went to the 83% figure - but only thing that really did was made it more clear to applying adjustments like ambient temp or number of conductors in a raceway. without any adjustments you still run same size conductor you did using the old table.

What they have or more like how it applies it is not a demand factor. It also isn't an adjustment factor it is basically allowing us to use a different starting conductor ampacity in the situations described. How they come up with the 83% I don't know.