Basic Calc Outlines

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horsegoer

Senior Member
Location
NJ
Looking for some general guidance on OCPS/feeder/load info.

  1. Can the OCPD protecting the feeder and serving the load be rated higher that the wire ampacity? Shouldn't I believe.
  2. The wire ampacity should be at least that of the OCPD. Question #1 answers this by default.
  3. The ampacity of the wire feeding the load should not be smaller than the load in amps?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Start with #3, yes ! The conductor must be sized to carry the load (210.19, 215..2)
#1 & 2, generally the OCPD is slected to protect the conductor, however, 240.4 allows for "the next size up" (800 amps or less), 240.4G) also allows larger OCPD (motors & HVAC) and 240.21 allows larger for taps.
 

horsegoer

Senior Member
Location
NJ
So taking a panel as an example. So i guess you would size the conductor to the MCB rating and not the calculated load as those are only present loads and there may be spare breakers for future loads?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The "normal" steps would be (a) calculate the load; (b) select a conductor to carry that load; (c) select an overcurrent device to protect the cable.
That said, when a panel is involved, often one calculates the load but then selects a standard size panel and, as you noted, adds provisions for future loads if you wish and make your selection on that number.
 
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