Basic Calc Outlines

[Alwayslearningelec]

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]

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.

 

[Alwayslearningelec]

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]

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.