ECN Forum Forums Code Related Discussion NEC & other Code issues conductor sizing for load and OCP

After reviewing this topic, I am looking for some input. It seems that conductor loading falls under one criteria and overcurrent protection falls under another, and they may not necessarily correspond with each other.

If I use THW wires (75-deg C) and have a non-continuous connected load of 290A on a 300A breaker, does the next higher breaker rule allow me to use 300mcm (285A) or do I need to use 350MCM (310A) since I have to size the conductors for the actual load? Please assume no use of 90-deg C conductors.

This is from the '11 NEC, and yes, it is not Code, but information:
Informational Note: Table 310.15(B)(16) through Table
310.15(B)(19) are application tables for use in determining
conductor sizes on loads calculated in accordance with Article 220.
Allowable ampacities result from consideration of one or more of the following:
(1) Temperature compatibility with connected equipment,
especially the connection points.
(2) Coordination with circuit and system overcurrent protection.
(3) Compliance with the requirements of product listings
or certifications. See 110.3(B).
(4) Preservation of the safety benefits of established industry practices and standardized procedures.

IMHO, you size the conductor to the load. 350Kcmil Copper within the 75 degree col. insulations on a 300 amp OCP is compliant. (No derating factored in) I believe in the real world, the 'calc load' would 'loose' 5-10 amps.

I agree with John. Conductors are sized to the load. O/C devices can be the next bigger size when it falls between rating but the main purpose of the O/C device is to deal with faults, not overloads. In "small conductors" typically serving receptacles, where the user is easily going to be able to add load until the breaker trips, we specify a safer size breaker with 240.4(D).

Start with the load.

In your example, you state a load of 290A. Therefore, your wire - after taking into account all derating factors (temperature, number of conductors in the raceway, etc.) must calculate out to at least 290A. Keep in mind that different types of loads, such as welders, have specific factors you need to consider. In the end, this is a MINIMUM wire size. There is no issue with using a larger wire.

Likewise, your fuse or breaker is selected based upon both your specific application, and your load. It also needs to be at least 290A. As before, different types of loads will have specific ways to calculate the breaker to use.

Unlike with the wire, the breaker or fuse you calculate is the MAXIMUM size breaker or fuse. You can always go smaller.

For situations where the circuit is designed for a specific load, there is no code issue with the wire rating being less than the rating of the fuse or breaker. We put 40-amp breakers on #12 (25-amp) wire all the time, when we're powering air conditioners.

At the other extreme, there's no code rule against protecting a 500MCM wire with a 10-amp fuse.

We tend to get confused on this point simply because we usually think in terms of 'convenience receptacle' circuits, which are sized in a unique manner.

Thank you for your responses, you've been very helpful.