Branch Circuit for Heaters

[Grouch]

The 2008 NEC, section 424.3(B), states the following:
Branch-Circuit Sizing. Fixed electric space-heating equipment shall be considered continuous load.

The 2020 NEC, section 424.4(B), states the following:
Branch-Circuit Sizing. The branch-circuit conductors for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125 percent of the load.

Why the change in language? Is it to give us flexibility to size the circuit breaker for a heater at 100%, if we know that the heater will be on for less than 3 hours / not a continuous load? But keep the wiring at 125%, to give us room to upsize the breaker later on in case the heater is then considered a continuous load?

 

[Grouch]

Just giving this a bump.

Curious on why the wording was changed from heaters being a continuous load, thus sizing the breaker and wiring to 125%, to saying only branch circuit conductors shall be sized at 125% with no mention of the breaker.

 

[Grouch]

I found this in another thread as an answer:

Wall Mounted Fan Forced Electric Heater

Someone asked me today about the branch circuit for a 10kw, 240 volt, thermostatically controlled electric garage heater. I immediately said 60 amp circuit. Then when I looked in the codebook I thought well maybe not so fast. 424.4(B) says the branch circuit conductors shall be sized at 125%...

forums.mikeholt.com

See Post #10.

 

[Dennis Alwon]

Perhaps the wording made it clearer I don't know. You would have to size the conductors at 125% but if they ever made a breaker that was rated 100% then you could use a breaker that was smaller than the conductors. Just a guess.

 

[infinity]

I found this in another thread as an answer:

If someone could dig up the actual PI that enabled this code change that might shed some light on why the wording is now different.

 

[kwired]

I never noticed this before. Kind of doesn't change what most of us do and the result is still in compliance.

The previous way which had been like it was for as long as I can remember stated it shall be considered a continuous load which automatically would mean 125% rated current be used for selecting both conductor ampacity and minimum overcurrent protection via other applicable code articles.

Comparing 422.22 in both 2017 and 2023 (2017 still says these are to be considered continuous loads) there doesn't seem to any straight out requirement to size overcurrent protection at 125%, but is certain conditions that allow 100%. I didn't look really hard but both seem to be about the same content in that section.

There is portions that state where heater is marked with minimum conductor size. But that isn't all that common in my experience with many unit heaters, baseboards, etc. and you often are stuck calculating rated current from unit voltage and kW ratings.

 

[garbo]

I'm old school and always went 125% of heater ampere to come up with the proper wire size for two reasons. 1) I have installed baseboard heaters in both primary & second homes where owners shut off heaters and turn off the water and pour something similar to antifreeze in traps & toilets. When they eventually turn heaters on during a cold winter day the heaters most likely will run continuously for over three hours. 2) voltage drop, for every 1% voltage drop on heaters you have have 2% less BTU'S produced.

 

[kwired]

voltage drop, for every 1% voltage drop on heaters you have have 2% less BTU'S produced.

If the branch circuit conductors are primarily within the space being heated, you didn't exactly have any losses.

 

[Grouch]

Comparing 422.22 in both 2017 and 2023 (2017 still says these are to be considered continuous loads) there doesn't seem to any straight out requirement to size overcurrent protection at 125%, but is certain conditions that allow 100%. I didn't look really hard but both seem to be about the same content in that section.

What does the 2023 version say? The same as 2020?

Where I bolded... it seems like they are giving you the flexibility to size the overcurrent protection at 100%... if it's not a continuous load. But the wiring however is strictly at 125%.

 

[kwired]

What does the 2023 version say? The same as 2020?

Where I bolded... it seems like they are giving you the flexibility to size the overcurrent protection at 100%... if it's not a continuous load. But the wiring however is strictly at 125%.

I was looking at NFPA free access versions. Without taking the time to see if they are word for word the same it appears as though all of 422.22 is nearly the same in 2017, 2020 and 2023. Nothing stood out as being much different when switching between them.

Note that with their on line free access you have no indications of what may have changed from previous edition like you normally have in printed copies. They disable printing though you still can use snip type applications to make copies of screen content.

 

[wwhitney]

If someone could dig up the actual PI that enabled this code change that might shed some light on why the wording is now different.

OK, this originated from Public Comment No. 695-NFPA 70-2018 on the 2020 NEC First Draft, which suggested:

424.4(B) The branch circuit and overcurrent protection for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125% of the load.

The substantiation:

This Comment is intended to make editorial improvements to the existing requirements without making substantive changes. If changed to the suggested 125% language, the conductors and overcurrent protection will remain unchanged. The change will better coordinate with load calculations in Article 220 where 220.51 states the feeder load for fixed electric space heating is 100%. The confusion among some users of the NEC is that since the branch circuit requirements in 424.3(B) are for a continuous load, the 125% value carries over into the load calculations for feeders and services. This Comment will maintain the required sizing of the branch circuit and remove the confusion.

The committee response:

Text revised for clarity related to conductor sizing. Text related to overcurrent protection not accepted as overcurrent protection requirements are covered in 424.22 (A).

But the committee seems to have overlooked the fact that 424.22(A) has no requirement for 125% sizing for OCPD, and so the change became substantive rather than just editorial, removing the 125% sizing requirement for branch circuit OCPD.

Cheers, Wayne

 

[kwired]

OK, this originated from Public Comment No. 695-NFPA 70-2018 on the 2020 NEC First Draft, which suggested:



The substantiation:



The committee response:



But the committee seems to have overlooked the fact that 424.22(A) has no requirement for 125% sizing for OCPD, and so the change became substantive rather than just editorial, removing the 125% sizing requirement for branch circuit OCPD.

Cheers, Wayne

125% for continuous loads was never a part of art 220 load calculations nor were you ever required to add an additional 25% to any results of art 220 load calculations. In fact via art 220 for service or feeder calculations you potentially can apply demand factors to some items that are otherwise treated as continuous load at the branch circuit level but must add the extra 25% for the branch circuit conductors and/or overcurrent protection.

 

[wwhitney]

125% for continuous loads was never a part of art 220 load calculations

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."

nor were you ever required to add an additional 25% to any results of art 220 load calculations.

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

 

[wwhitney]

Looking more closely at the 2017 language, it was:

Branch-Circuit Sizing. Fixed electric space-heating equipment and motors shall be considered continuous load

And the applicable definition:

Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

So the question is whether that "between" in the definition is inclusive or exclusive. I.e. is the "final overcurrent device" part of the branch circuit?

If the answer is no, the final overcurrent device isn't part of the branch circuit, then arguably 2017 NEC 424.3(B) doesn't require considering the load to be continuous when sizing the branch circuit OCPD. In which case the change in language for the 2020 NEC was just editorial, not substantive.

Cheers, Wayne

 

[Dennis Alwon]

Looking more closely at the 2017 language, it was:



And the applicable definition:



So the question is whether that "between" in the definition is inclusive or exclusive. I.e. is the "final overcurrent device" part of the branch circuit?

If the answer is no, the final overcurrent device isn't part of the branch circuit, then arguably 2017 NEC 424.3(B) doesn't require considering the load to be continuous when sizing the branch circuit OCPD. In which case the change in language for the 2020 NEC was just editorial, not substantive.

Cheers, Wayne

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

 

[wwhitney]

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

I guess you're right, under the 2017 NEC, even if 424.3(B) doesn't tell you directly to size the OCPD at 125%, 210.19(A)(1)(a) does.

But that's no longer true under the 2020 and later NECs.

Cheers, Wayne

 

[zbang]

voltage drop, for every 1% voltage drop on heaters you have have 2% less BTU'S produced.

by the heater.

The voltage drop is producing heat in the wiring - voltage drop always produces heat; if the wiring is inside the conditioned space, then that heat is somewhere useful.

 

[Eddie702]

To me if 2020 says "branch circuit sizing must be 125%" that is the breaker & the conductors. The rating of a circuit is the OCP protection not the conductor size.

 

[Dongbites]

OCPD protects the system for me, it's okay

 

[wwhitney]

To me if 2020 says "branch circuit sizing must be 125%" that is the breaker & the conductors.

2020 NEC 424.4(B) doesn't say that, it's the first NEC with the new wording "The branch-circuit conductors for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125 percent of the load."

The rating of a circuit is the OCP protection not the conductor size.

Sort of true per 210.18 (*), but 2017 NEC 422.3(B) doesn't refer to "rating". It refers to "branch-circuit sizing" and then says that "Fixed electric space-heating equipment and motors shall be considered continuous load."

Cheers, Wayne

(*) 2017 NEC 210.18 says in part "Branch circuits recognized by this article shall be rated in accordance with the maximum permitted ampere rating or setting of the overcurrent device." Note that it says "maximum permitted ampere rating" not "the actual ampere rating installed". Which, taken literally, if every other aspect of the install would be OK with a 50A OCPD (but not anything higher), and you choose to put in a 40A OCPD, it's still a 50A rated branch circuit. A bit odd.