Branch Circuit for Heaters

[wwhitney]

But you still cannot load an overcurrent protective device at 100% for continuous loads so in effect the 125% rule is still there.

Bear with me, I got a little confused and have to retract post #16 (which should have referenced 210.20 rather than 210.19).

424.3 (2017 NEC) or 424.4 (2020 NEC and later) could just say "fixed electric space-heating equipment shall be considered continuous loads." If it did, that would mean considered a continuous load for all purposes--service conductor and OCPD sizing, feeder conductor and OCPD sizing, and branch cirucit conductor and OCPD sizing.

But 2017 NEC 424.3 doesn't say that. It says that for "branch-circuit sizing," they shall be considered continuous load. That clearly means that for service conductor and OCPD sizing, and for feeder conductor and OCPD sizing, you don't have to consider the load continuous. And for branch-circuit conductor sizing you do have to consider the load continuous, as branch-circuit conductors are part of the branch circuit.

What about the branch circuit OCPD, is it part of the branch-circuit or not? If the answer is no, because in the definition of branch circuit the word "between" is not inclusive of that endpoint, then 2017 NEC 424.3 on "branch circuit sizing" does not apply, as what you are sizing is not part of the branch circuit. Which means when we apply 210.20 on sizing for OCPD protecting the branch circuit, we don't have to consider the load continuous.

And that's clearly the result under the new wording for the 2020 NEC. Given that the committee statement asserted that the change from the 2017 NEC to the 2020 NEC was editorial, that was apparently the intention for the 2017 NEC as well.

A distinction that's usually of no consequence, but occassionally could be. For example, in Article 705 on interconnected power sources, one of the rules for determining if the busbar of a panelboard with multiple sources of supply is properly protected or not counts the actual rating of the OCPDs installed in that panelboard. So sizing the OCPD for fixed electric space heating at the load, rather than at 125% of the load, could make a difference between complying with that portion of 705 or not.

Cheers, Wayne

 

[tortuga]

Interesting discussion, I would not bet on any residential grade inverse time breaker holding up well loaded to 100% continuously even with 125% wires connected.
Also all electric resistance heat is not a continuous load but some is if its expected to run 3-hrs or more.
A cafe with a infrared patio heaters for outdoor seating, its basically a infrared lighting load on for 12 hrs while the cafe is open.
On a feeder / service load calc those would be a continuous load.

 

[kwired]

Mostly true, but the 2020 NEC added a note to Table 220.12 on lighting loads that says "Note: The 125 percent multiplier for a continuous load as specified in 210.20(A) is included when using the unit loads in this table for calculating the minimum lighting load for a specified occupancy."


Articles 210, 215, 230 and several others do require you to use an additional 25% for continuous loads when not using 100% rated OCPD.

Cheers, Wayne

That note basically just means you don't have to add an additional 25%. That table is a general lighting by occupancy type and is based on square footage and in general will give you higher VA than what you actually install in most cases.

other articles I already covered in what I said, I don't think there is any place in art 220 that makes you add an additional 25% for continuous load reasons, especially in the parts specifically addressing service and feeder calculations. those are the areas where possible demand factors are most commonly used or are sort of built into any methods used there. Demand factors in other articles are mostly limited to types of items that have a duty cycle of some kind whether by some design or just natural operational characteristics of said item.

And yes the additional 25% is normally due to not using 100% OCPD but never really comes up when using art 220 to do the calculations.