Service wire size with 83% rule
- Thread starter 2xcheck
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- Tennessee NEC:2017
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- Semi-Retired Electrician
No
That would not make any sense to give you the allowance then require you to change the breaker.
The rule is for dwellings and the panel/service has to supply all the loads in the dwelling. The code makers know that not all the loads will be on at the same time so they made an allowance to allow smaller wire than the ampacity tables show.
That would not make any sense to give you the allowance then require you to change the breaker.
The rule is for dwellings and the panel/service has to supply all the loads in the dwelling. The code makers know that not all the loads will be on at the same time so they made an allowance to allow smaller wire than the ampacity tables show.
- Location
- Tennessee NEC:2017
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- Semi-Retired Electrician
No, 83% of 200A is 166A. 1/0 copper is only good for 150A. 3/0 is what it would be without an allowance. So 1/0 is definitely too small.
The 3/0 Al is also too small.
So for the in between values they do not apply becuase without the 83% adjustment 4/0 al is 200 amp ok . But with 83 %
So 1/0 copper is rated 175 a, but not up to the next value say 175 to 199 ? All of the in between values which you get with 83% rule ?
- Location
- New Jersey
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- Journeyman Electrician
#1/0 copper is rated for150 amps at 75° C not 175 amps.
Take the size of the service OCPD and multiply by 83% then go to Table 310.15(B)(16) and find a conductor that has at least that value. That would be the minimum conductor size.
For example 300 amp service:
(300*83%=249 amps), then Table310.15(B)(16) so either 250 kcmil copper (255 amps) or 350 kcmil aluminum (250 amps).
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
This has nothing to do with the 'next size up' rule.
In most situations you must use the 60 or 75 degree temperature rating for building wiring; that 170 value for 1/0Cu is their to tease you,using most common materials in most situations you end up using 150A as the ampacity for 1/0Cu.
Jon
kwired
Electron manager
- Location
- NE Nebraska
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- EC
The 170 amp value for 1/0 copper is for 90 degrees C, you can use it for starting point for ampacity adjustments (presuming you have 90 C insulation)
when needed but you still can not use a conductor smaller than 60 or 75 C columns after adjustments because of termination ratings.
All the 83% rule does is allow you to use an 83 amp conductor with 100 amp overcurrent protection in specific situations (or whatever value is 83 percent for other than 100 amp overcurrent device).
When they took the table away a few years back they wanted us to apply ampacity adjustments before coming up with a final conductor size, the old table didn't factor any adjustments but the values in the table were 83%. Now 2020 brings the table back but look at the table - it is posted in post 5, it has a note at bottom saying those values only apply when there are no adjustments necessary. I didn't look at 2020 content, but am guessing if adjustments are necessary they still want you to use the 83% value after adjustments are made.
Correct. In Phoenix, when I was doing services, you had to use 3/0 CU for 200A because of the temperature correction. Here in Texas, you can follow that table with no adjustments needed.
Eddie702
Licensed Electrician
- Location
- Western Massachusetts
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- Electrician
related to this I recall that if you used XHHW or SE cable containing XHHW for the service drop you do not have to derate for temperature under any conditions. I recall reading this in an older code book (2017?) but can't find it in 2020. Am I nuts?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
That applied to derating for conductors on a roof.
Carultch
Senior Member
- Location
- Massachusetts
Is that because Texas is on a different code cycle? Or is that because Phoenix has a hotter design temperature? I think of both those places as hot, and would expect similar results with required temperature corrections.
flashlight
Senior Member
- Location
- NY, NY
- Occupation
- Electrician, semi-retired
So for structures that are not dwellings, such as barns or commercial buildings, the 83% rule does not apply ?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
That's correct. 310.15(B)(7) is specifically for "Single-Phase Dwelling Services and Feeders".
I'm doing this from memory. I think the design temperature was 104F.
200A*83%/.91CF = 182A
One year, some ding-dong SRP engineer was going to make a name for himself and we had to use 4/0 for 200A services. This went on for about 6 months before someone got it changed back.
acin
Senior Member
- Location
- pacific grove california
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- general building contractor est.1984 . C 10 elec. lic.as of 8 / 7/ 2020
I saw that table in annex d pg. 1231 2017 handbook
flashlight
Senior Member
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- NY, NY
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- Electrician, semi-retired
4/0 Copper? That's some overkill, I would be grumbling about that guy as I tried to manipulate it.
Little Ames
Member
- Location
- 33401
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- Architect
Can you confirm that you take the size of the overcurrent protection device, and not the load demand calc on a panel to apply the 83% rule. I have an existing 150 amp panel with a 2/0 AL. wire. Appliances were added and the new load demand on the service is 140 amps. So do you apply it to 150 x .83= 124.5 amps or 140 x .83= 116.2 amps. I am trying to identify If I can keep the 150 amp panel with a 2/0 AL. wire or I need to upgrade to 175 amp 3/0 AL wire
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
You would use 150 amps not 140..
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