Service Size and Demand Load Relation

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Electriman

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Greetings,

I am looking at a project to justify the service size. Based on the NEC 230.30 (Size your service-drop conductors per Article 220 calculations). On the other hand, NEC 310.12 is saying that the conductor can be 83% of the service size therefor I can pick the OCPD 83% percent of the load. Now, I am confused, should I pick the demand load for service size or 83% of it.

Please advise.
 
I am going to assume you are referencing the 2020 NEC based on the references provided.

The process of sizing your service would typically be the following:
  1. Perform load calculation per article 220
  2. Determine service conductor minimum ampacity per 230.42 based on previous load calculation
  3. Select standard size OCPD that meets/exceeds service conductor minimum ampacity. This is your "service size".
  4. Size conductors based on service/OCPD size and article 310.
310.12 is part of step 4 and is applied after the service size from step 3 is determined. Table 310.12 provides conductor sizes with the 83% reduction already applied (assuming no adjustment factors are being used).

For example, assume my calculated load per article 220 for a one-family dwelling unit comes out to be 99A. Assume there are no continuous loads so the minimum ampacity per 230.42 is 99A. My service and OCPD size in this case would be 100A. Table 310.12 indicates that I may use #4 CU or #2 AL for my service conductors
 
Electriman said:
Greetings,

I am looking at a project to justify the service size. Based on the NEC 230.30 (Size your service-drop conductors per Article 220 calculations). On the other hand, NEC 310.12 is saying that the conductor can be 83% of the service size therefor I can pick the OCPD 83% percent of the load. Now, I am confused, should I pick the demand load for service size or 83% of it.

Please advise.
Click to expand...
230.30 deals with underground services while 310.12's 83% is for services rated 100 to 400A, single-phase, one-family dwelling services, and feeders. Otherwise, do the regular calculations.
 
PD1972 said:
I am going to assume you are referencing the 2020 NEC based on the references provided.

The process of sizing your service would typically be the following:
  1. Perform load calculation per article 220
  2. Determine service conductor minimum ampacity per 230.42 based on previous load calculation
  3. Select standard size OCPD that meets/exceeds service conductor minimum ampacity. This is your "service size".
  4. Size conductors based on service/OCPD size and article 310.
310.12 is part of step 4 and is applied after the service size from step 3 is determined. Table 310.12 provides conductor sizes with the 83% reduction already applied (assuming no adjustment factors are being used).

For example, assume my calculated load per article 220 for a one-family dwelling unit comes out to be 99A. Assume there are no continuous loads so the minimum ampacity per 230.42 is 99A. My service and OCPD size in this case would be 100A. Table 310.12 indicates that I may use #4 CU or #2 AL for my service conductors
Click to expand...

This has also been confusing to me

1 Does this 83% reduction optional rules affect both service entrance conductors and feeders?

2 after sizing OCPD and service conductor size based on calculated dwelling demand then reducing the service ampacity to 83% afterward while keeping the OCPD unchanged in proportion to OCPD 83% downsizing then you contradict and violate NEC code by not protecting conductors to their rated ampacity.
 
Jpflex said:
Besides feeders going to sub panels, when would feeders not carry entire load since they direct all source power to main panel?
Click to expand...
Notice Don's post was singular, he was not saying feeder(s). There would only be "one" Main Feeder" and for the 83% reduction that would be the only feeder that could use it.

Excerpt from 310.15(B)(7)
For application of this section,
the main power feeder shall be the feeder between the
main disconnect and the panelboard that supplies, either by
branch circuits or by feeders, or both, all loads that are part
or associated with the dwelling unit.
Click to expand...
 
roger said:
Notice Don's post was singular, he was not saying feeder(s). There would only be "one" Main Feeder" and for the 83% reduction that would be the only feeder that could use it.

Excerpt from 310.15(B)(7)
Click to expand...

The neutral is not included in 83% reduction nor ground right?
 
Jpflex said:
Besides feeders going to sub panels, when would feeders not carry entire load since they direct all source power to main panel?
Click to expand...
An example:

Service comes to a 200A breaker, then goes to a central subpanel. In the subpanel there are branch circuits and also a 125A subpanel breaker and a 60A subpanel breaker.

The service conductos and the feeder to the central sub get to use 310.12 because they supply the whole dwelling. The 125A subpanel circuit does not, because it does not supply the whole dwelling. The 80A doesn't, because it doesn't supply the whole dwelling and also because it is less than 100A.

Also no feeder needs to be larger than the service conductors if they use this rule.

So for another example:
A service comes into a 200A meter main which contains some branch breakers and also feed-through lugs to another 200A subpanel. Both the service conductors and the feeder to the subpanel get to use 310.12, because the service supplys the whole dwelling and the feeder conductors do not have to be larger than the service conductors.

One last example:
A 400A meter main contains two 200A main breakers to subpanels. The service conductors get to use 310.12 because they supply the whole dwelling. The 200A feeders do not, because they each only supply part of the dwelling and are smaller than the service conductors.
 
don_resqcapt19 said:
Only in the cases where the feeder carries 100% of the dwelling unit load.
Click to expand...
Or, because any downstream feeder need not be sized larger than the service conductors, any feeder rated the same as the service conductors/main feeder. E.g. any 100A feeder on a 100A residential service.

Cheers, Wayne
 
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