83% rule and next size up rule....

Status
Not open for further replies.
Dsg319

Dsg319

Senior Member
Location
West Virginia
Occupation
Wv Master “lectrician”
So have run into a situation in a work book that have never seen nor been in before.(120/240volt single phase) (single family dwelling)

Here it is..... there’s a demand load of 56,000va. And it wants to know maximum size OCPD and minimum size Aluminum conductors.

56,000va/240volt=210.8amps
(83percent rule) 210.8x83%=175amps

Leaves you with 4/0 aluminum conductors.
BUT the work book also uses of OCPD of 225amps. Are you allowed to use the 83%rule and next size up in a situation like this? Even if so, it is something I don’t believe I would ever do.
 
You apply the 83% factor to the rating of the service. The rating of the service will be the OCPD rating of the service disconnecting means. If you have to round up from 210.8A to 225A, then you take 83% of 225A, rather than 83% of 210.8A.

This assumes your utility company will allow a service of 225A, as opposed to requiring you to pick from a limited list of standard service ratings. As an example, if you have to round up to a 300A service to meet utility standards, then you would use 83% of 300A, rather than the 225A service you could otherwise use.
 
Thanks I’ll double check the work book and post pic tomorrow lord willing. Evidently the author was under the impression that 83% could be applied to the calculated load not the “service rating-OCPD”.... either that or I I mistakenly read it wrong. Will post picture tonight or in the morning.

I thought it seemed funky....
 
Dsg319 said:
So have run into a situation in a work book that have never seen nor been in before.(120/240volt single phase) (single family dwelling)

Here it is..... there’s a demand load of 56,000va. And it wants to know maximum size OCPD and minimum size Aluminum conductors.

56,000va/240volt=210.8amps
(83percent rule) 210.8x83%=175amps

Leaves you with 4/0 aluminum conductors.
BUT the work book also uses of OCPD of 225amps. Are you allowed to use the 83%rule and next size up in a situation like this? Even if so, it is something I don’t believe I would ever do.
Click to expand...
My calculator says that 56000va/240v = 233.33 not 210.8
 
Little Bill said:
My calculator says that 56000va/240v = 233.33 not 210.8
Click to expand...
Thank you I just now double checked my math too once you said that. Once I get home I’ll double check the question.....its either really jacked up or I have really messed up lol. Post pics soon.
 
They are also incorrect on the maximum circuit breaker size. Just because the demand load is only 211A doesn't mean that I can't use a 5000A service as long as I provided large enough conductors. The question is poorly written.
 
d0nut said:
They are also incorrect on the maximum circuit breaker size. Just because the demand load is only 211A doesn't mean that I can't use a 5000A service as long as I provided large enough conductors. The question is poorly written.
Click to expand...
Good point
 
So the correct answer is not a choice. 83%, as someone mentioned, is multiplied by the service breaker not the load. 83% of 225 = 187 amps so 4/0 isn't any good. 250 KCM & 225 amp breaker is the correct answer
 
Dennis Alwon said:
So the correct answer is not a choice. 83%, as someone mentioned, is multiplied by the service breaker not the load. 83% of 225 = 187 amps so 4/0 isn't any good. 250 KCM & 225 amp breaker is the correct answer
Click to expand...
Yep! Everybody makes a boo boo here and there:p. Just surprise a reviewer or somebody never caught it.
 
Dsg319 said:
Yep! Everybody makes a boo boo here and there:p. Just surprise a reviewer or somebody never caught it.
Click to expand...
Who's material is the question from?
If it is Mike Holt's material, they acknowledge there are mistakes and even offer a list of mistakes and their corrections.
 
Little Bill said:
Who's material is the question from?
If it is Mike Holt's material, they acknowledge there are mistakes and even offer a list of mistakes and their corrections.
Click to expand...
This one was written by Ray Holder(NOT bashing his material as I do like it also)Maybe I should check into it and see if they have any idea about it. Have to look about contact info maybe tomorrow.
We’re all human at the end of the day( Not perfect).
 
Last edited:
Status
Not open for further replies.
Top