Feeder circuit for electric heaters

[brother]

I think the LOAD calculation is found in 220.51. It says that four heaters at 5kw shall be calculated as a load of 20kW (ie, 100% of connected) even if the heaters may not run at the same time, UNLESS, the AHJ grants you permission to use a lower load.

Article 215.2(A)(1) tells you the minimum feeder size for the LOAD calculated in Article 220.




Again, Art. 220 tells you how to calculate the load, not the feeder size. Article 424.3 deals with branch circuits (it doesn't mention feeders at all,) but how can a load be continuous on a branch circuit and non-continuous on the feeder feeding the branch circuit? It can't. If the load is continuous on a branch circuit, then it it continuous on the feeder, also.



And if you have 4 heaters that run at the same time for a 6 hour period, what reduction would you use then? It doesn't matter what the "reality" of how the installation will operate is, the installation must comply with code. .

I can see we can go round and round on this. Sorry to say but you have a gross misunderstanding of calculating a 'FEEDER' vs a branch circuit hopefully someone else can chime in and explain better that the rules for a FEEDER are different than the rules for branch circuit load calulation.

The Load caculated on this FEEDER is not the same way you calculate the load on a BRANCH circuit. I know its hard to understand this logic in the NEC at times, but just because a load is considered 'continuous' on a BRANCH circuit does NOT automatically make it continuous on a FEEDER for load calcuations. So when you go to 215.2(A)(1) this multiple heaters is NOT considered continuous loads on this feeder other wise there would not be any need for them to have section 220.51 to tell how to calculate the load on a feeder for heaters because one would automatically apply the 'branch circuit' rules to the feeder for heaters. Other loads would be continous on a feeder, like lighting on for more than 3hrs etc..

Another example of where a load is considered 'continous' on a branch circuit and NOT continuous on a feeder would be a water heater in a dwelling unit. You can use 100% of the Name plate on the water heater alone without having to add the 125% to the feeder load calculation NEC 220.80.

You should carefully think about the steps for calculating the feeder load and apply them keeping them seperated from the rules for a branch circuit.

1) what will be supplied/fed by this FEEDER
2) what is considered 'continous' and 'non continous' load on a FEEDER
3) caculate the loads under the FEEDER rules
4) then size the FEEDER that can handle the amps that was calculated and use 310.16 ampacity tables.

Your big issue/ERROR is that you are missing #2. Because you automatically think that a load that is continuous on a BRANCH circuits is continous on a FEEDER when doing a load calculation. The nec constantly applies demand factors both spoken and unspoken when dealing with feeders and services.

4 heaters at 5kw is 20,000 at a 100%. 20,000/240 =84 amps . So the 84 amps is ok because this is NOT calculated as a continous load on FEEDER. This installation WOULD apply with Code!!

 

[david luchini]

You should carefully think about the steps for calculating the feeder load and apply them keeping them seperated from the rules for a branch circuit.

1) what will be supplied/fed by this FEEDER
2) what is considered 'continous' and 'non continous' load on a FEEDER
3) caculate the loads under the FEEDER rules
4) then size the FEEDER that can handle the amps that was calculated and use 310.16 ampacity tables.

Your big issue/ERROR is that you are missing #2. Because you automatically think that a load that is continuous on a BRANCH circuits is continous on a FEEDER when doing a load calculation. The nec constantly applies demand factors both spoken and unspoken when dealing with feeders and services.

4 heaters at 5kw is 20,000 at a 100%. 20,000/240 =84 amps . So the 84 amps is ok because this is NOT calculated as a continous load on FEEDER. This installation WOULD apply with Code!!

I'm not missing #2, you are. The load is either continuous or it is non-continuous. It can't be both. Article 424 tells us that it is continuous.

Following your logic, I could have a heater which is 4800W, 120V with a 40A load current. I can feed the heater with #6AWG from a disconnect fused at 50A (assuming 60deg terms), because the branch circuit needs to be rated at 125% of load.

I could then feed the fused disconnect switch from a panel with #8Awg from a 40A c/b, because the feeder only needs to be rated at 100%.:-?

Why would the code require me to supply the heater branch circuit at 50A, but permit me to supply the feeder circuit at 40A?

It wouldn't, and it doesn't. If the heater load is considered continuous on the branch circuit, then it is continuous on the feeder.

(And I'm wondering what are the "unspoken" demand factors that the NEC allows for feeders? Of course, it really doesn't matter, because demand factors have nothing to do with whether a load is continuous or non-continuous, only how much of the load will be running concurrently. And 220 tells us to use a 100% demand factor for electric heat, unless permitted to do otherwise by the AHJ.)

 

[Dennis Alwon]

Dennis, I read this as four separate heaters, each with its own branch circuit, being fed from a panel. The question is what size must the feeder be for the panel.

So what are these 4 units that they are on together-- I assume not a furnace.

 

[brother]

I'm not missing #2, you are. The load is either continuous or it is non-continuous. It can't be both. Article 424 tells us that it is continuous.

No, your missing #2, and lack understanding on HOW to read the NEC. For the last time, 424.3 deals with BRANCH circuits for the heaters, not the FEEDERS!!Its written in BOLD the lists (A) and (B) under that section, and since they under that section, it for the BRANCH circuits.

Following your logic, I could have a heater which is 4800W, 120V with a 40A load current. I can feed the heater with #6AWG from a disconnect fused at 50A (assuming 60deg terms), because the branch circuit needs to be rated at 125% of load. could then feed the fused disconnect switch from a panel with #8Awg from a 40A c/b, because the feeder only needs to be rated at 100%.:-?

Why would the code require me to supply the heater branch circuit at 50A, but permit me to supply the feeder circuit at 40A?It wouldn't, and it doesn't.

This is not my logic and definitely not what I have posted or said, you've missed it all together! This shows you lack comprehension and reading of section 220.51 and what I have posted. The latter part of 220.51 clearly says, "....However in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied." So your question is moot and already answered if you read 220.51 correctly. Your feeder in that example you gave would have to be at least 50amp. Because it cannot be smaller than the largest branch circuit (notice this is SINGULAR NOT PLURAL,)and since this is the only heater you mention it has to be at least that size since its the 'largest branch circuit'. This would probably change if you had more heaters the feeder was feeding.

If the heater load is considered continuous on the branch circuit, then it is continuous on the feeder.

Again, you are wrong the NEC clearly shows you are!! 424.3 is for branch circuits for heaters (125%,) 220.51 is for feeders for heaters 100
%. I noticed you never responded to the example I gave about water heaters. According to YOUR logic, since the water heaters are considered a continuous load on the branch circuit, they have to be continuous loads on feeders. This would not be case, you would be installing an overpriced and and unecessary large wire for a job. This practice of 'demand factors' is done all the time on multifamily housing jobs.

(And I'm wondering what are the "unspoken" demand factors that the NEC allows for feeders? Of course, it really doesn't matter, because demand factors have nothing to do with whether a load is continuous or non-continuous, only how much of the load will be running concurrently. And 220 tells us to use a 100% demand factor for electric heat, unless permitted to do otherwise by the AHJ.)

The Demand factors would have ALOT to do with calculating the load on feeders and services. Following your logic, all these multi family apartment complexes and other buildings are not code compliant because they used the DEMAND factors for sizing the load to get the ampacity of what wires to install. This includes the 100% load of the feeders for heaters.

I will give you ONE more example, in hopes that you comprehend!!

Floor plans for a small office are specifying the installation of seven wall heaters; each heater is rated 3,000 watts at 240 volts. How much load will these heaters add to a 240-volt, single-phase service? In accordance with 220.51, calculate the heaters at 100 percent (7 ? 3,000 = 21,000 watts). Since the service voltage is known, the total current draw of the heaters can be calculated. Simply divide the total watts by 240 volts (21,000 ? 240 = 87.5 = 88 amperes). The calculated load in amperes for seven 3,000 watt, 240-volt heaters on a 240-volt, single-phase service is 88 amperes.

This is from ECM click the link!! It might help I suggest reading it. http://www.ecmag.com/?fa=article&articleID=7781

 

[david luchini]

No, your missing #2, and lack understanding on HOW to read the NEC. For the last time, 424.3 deals with BRANCH circuits for the heaters, not the FEEDERS!!Its written in BOLD the lists (A) and (B) under that section, and since they under that section, it for the BRANCH circuits.

Again, I am NOT missing your issue #2. You are. You say for #2 "What is considered continuous and non-continuous on a FEEDER." How in the world can 20A on a branch circuit be CONTINUOUS, but the same 20A be NON-CONTINUOUS on the feeder that feeder that branch circuit? It CANNOT be. If the 20A on the branch circuit continues for 3 hours or more, then that 20A on the feeder continues for 3 hours or more. The load on the feeder is CONTINUOUS if the load on the branch circuit is CONTINUOUS.

This is not my logic and definitely not what I have posted or said, you've missed it all together! This shows you lack comprehension and reading of section 220.51 and what I have posted. The latter part of 220.51 clearly says, "....However in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied." So your question is moot and already answered if you read 220.51 correctly. Your feeder in that example you gave would have to be at least 50amp. Because it cannot be smaller than the largest branch circuit (notice this is SINGULAR NOT PLURAL,)and since this is the only heater you mention it has to be at least that size since its the 'largest branch circuit'. This would probably change if you had more heaters the feeder was feeding.

You're right - bad example, but it doesn't change the point above that continuous load on the branch circuit equals continuous load on the feeder. You seem to be confusing NON-CONTINUOUS load with DEMAND FACTOR. If I understand your argument, the load is NON-CONTINUOUS on the feeder, because all of the heaters on the feeder may not be operating "at the same time." Whether the loads on the feeder are operating at the same time has nothing to do with whether the load is CONTINUOUS or not.

Again, you are wrong the NEC clearly shows you are!! 424.3 is for branch circuits for heaters (125%,) 220.51 is for feeders for heaters 100%. I noticed you never responded to the example I gave about water heaters. According to YOUR logic, since the water heaters are considered a continuous load on the branch circuit, they have to be continuous loads on feeders. This would not be case, you would be installing an overpriced and and unecessary large wire for a job. This practice of 'demand factors' is done all the time on multifamily housing jobs.

220.51 is for determining the LOAD on the feeder, not for determining the SIZE of the feeder (the feeder size is determined by 215.2.) Again, a load CANNOT be continuous on a branch circuit, but non-continuous on the feeder feeding that branch circuit. I didn't see your example regarding water heaters, but yes, the load is continuous on the feeder if it is continuous on the branch circuit. If the water heater is 30A (for more than 3 hrs continuously) on the branch circuit, then the load for the water heater will be 30A (for more than 3hrs continuously) on the feeder. How will the current for the water heater magically stop flowing on the feeder when it is still flowing on the branch circuit? The practice of "demand factors" is done when/where the code says they can be done, not just any place you feel like providing a demand factor. And again, this is clearly where you are going wrong, you are confusing "demand factor" with "non-continuous load."

The Demand factors would have ALOT to do with calculating the load on feeders and services. Following your logic, all these multi family apartment complexes and other buildings are not code compliant because they used the DEMAND factors for sizing the load to get the ampacity of what wires to install. This includes the 100% load of the feeders for heaters.

Where did I suggest that demand factors don't have anything to do with calculating loads on feeders and services? Now it is really clear to me that you are confusing "demand factor" with "non-continuous load" because I said that demand factors have nothing to do with whether a load is CONTINUOUS OR NOT. These are completely different things.

Let me give you an example that hopefully will help you comprehend that you are confusing "demand factor" and "Non-continuous load."

You are suggesting that 220.51 says that the feeder should be size for 100% of the heater load, that is to say, that the load is "non-continuous" on the feeder, even though it is continuous on the branch circuit. 220.51 is under section III. Feeder & Service Load Calculation, so lets look at another load in calc in that section. How about 220.42 for lighting. Imagine a 3000sf store. The branch circuit loads will be 3va/sf, so the total lighting load on the feeder for the store will be 9000VA. Section 220.42 tells us that the lighting load for the store should be calculated at 100% (just as 220.51 says that heaters should be calculated at 100%.)

By your logic, the feeder load for the lighting (on a 240V feeder) will be 37.5A. If a feeder was feeding only this lighting, a #8awg (60deg term) would be allowed? However, take a look at example D3 in the Annex D. You will see that the 9000VA for the lighting is calculated as continuous load @ 125% for the feeder. Why? Section 220.42 says to figure it at 100%? Because the load, calculated at 100% in Article 220 is continuous on the feeder for sizing the feeder per Article 215.


Now lets look at the OP again. If there are four 5kW, 240V heaters connected to the same feeder, each heater has a load current of 21A. 424.3 says that the load for the branch circuit shall be considered continuous load, so by 210.19(A)(1), each branch circuit must have an ampacity of at least 26.25A. Let's for the sake of argument say that the heaters will run for 4 hours continuously, that would make them "continuous load." Section 220.51 then tells us that the heating load shall be calculated at 100% of connected load, that is to say that all of the heaters are operating at the same time. 100% of 4x21A=84A. Now imagine that these four heaters turn on at 8AM and off at noon. Each branch circuit sees 21A continuous load (as per 424.3) and the feeder sees 84A of continuous load (per 220.51.) The size of the feeder per 215.2 shall have an ampacity of at least 125% of the continuous load, or 105A.


If we say, well one heater will come on at 8AM and run for 4 hours, and one will come on at 9AM, one at 10AM and one at 11AM, so that the feeder never sees the full load continuously, then we would be violating section 220.51. We are calculating the load on the feeder at LESS THAN 100%.


Now imagine, in the OP, that the four heaters, each in its separate conex, are running overnight when the outside temperature drops to below zero. Since it is so cold, the heaters run all night. But by using the 100% size for the feeder, the heater are connected by a #3 feeder to a 90A c/b (using 60 deg terms.) How is that feeder and circuit breaker going to respond when it is trying to carry the full 84Amps for 12 straight hours?

 

[david luchini]

So what are these 4 units that they are on together-- I assume not a furnace.

It appears that they are four separate electric heaters, each in a separate conex storage unit.

 

[iwire]

David, I think brother is correct, the branch at 125% and the feeder at 100%.

Things do not have to make sense to be code.

 

[david luchini]

David, I think brother is correct, the branch at 125% and the feeder at 100%.

Things do not have to make sense to be code.

Bob, how can a load be continuous on a branch circuit but the same load be non-continuous on the feeder feeding that branch circuit? It is impossible. And nowhere does the code say that the heater load is 'non-continuous' on the feeder.

215.2 says the feeder should be sized for 100% of the non-continuous load plus 125% of the continuous load. The heater loads, being continuous, must be calculated at 125% when sizing the feeder.

 

[Dennis Alwon]

David if it were one heater then the feeder cannot be less then the branch circuit but otherwise I have to agree that 220.51 seems to be clear which makes no sense if all the loads are on at the same time and are continuous.

 

[iwire]

Bob, how can a load be continuous on a branch circuit but the same load be non-continuous on the feeder feeding that branch circuit?

David first off please take a deep breath, both you and Brother seem to have really gotten into this.

Here is another NEC oddity

How can a conductor that carries current be called a non-current carrying conductor?

Because the NEC tells us so.

Back to the heaters

Did you look at the link Brother provided?

Here is an image from it

 

[david luchini]

David first off please take a deep breath, both you and Brother seem to have really gotten into this.

Here is another NEC oddity

How can a conductor that carries current be called a non-current carrying conductor?

Because the NEC tells us so.

Back to the heaters

Did you look at the link Brother provided?

The code doesn't tell us that the heater loads are non-continuous on the feeder. That's the point.

I did look at the link. Again it doesn't tell us that the load is non-continuous. It tells us that the load must be calculated as 100% of the connected load of the heater. IE, that the demand factor for electric heater is 100% - all heaters must be calculated as running at the same time. That's what I've been saying all along. The heater load is calculated at 100% demand factor per 220, and is continuous load per 424. Therefore the feeder in the OP sees 84A amps of continuous load, must be size to at least 105A.

Just in the same way that 220 says that the lighting load for an office building must be calculated at 100%, and is continuous load. So 84A of lighting load in an office building would need a feeder of at least 105A.

 

[iwire]

The code doesn't tell us that the heater loads are non-continuous on the feeder. That's the point.

IMO it clearly tells us that for feeders and services they will be treated as non-continuous loads,

220.51 Fixed Electric Space Heating. Fixed electric
space-heating loads shall be calculated at 100 percent of the
total connected load. However, in no case shall a feeder or
service load current rating be less than the rating of the
largest branch circuit supplied.

Not 125% of the total connected load, not 125% of the largest heater, just 100% of the total connected load.

Obviously you do not see it that way.

IMPO you are mistaken and have dug your heels in so firmly that nothing I can post will change your mind. That being the case I am out of this one.

 

[david luchini]

IMO it clearly tells us that for feeders and services they will be treated as non-continuous loads,

Not 125% of the total connected load, not 125% of the largest heater, just 100% of the total connected load.

Obviously you do not see it that way.

IMPO you are mistaken and have dug your heels in so firmly that nothing I can post will change your mind. That being the case I am out of this one.

Well, one last try....I don't see the words non-continuous anywhere in 220.51. I hope you would agree that "non-continuous load" and "demand factor" are not the same thing?

220.40 says "the calculated load of a feeder shall not be less than the sum of the loads of the branch circuits, after any applicable demand factors permitted by part III....have been applied."

220.51 is an article in part III, as referenced in 220.40, and the 100% noted in 220.51 is the demand factor for the fixed electric space heating.

220.42 says to calculate the lighting load for an office (or other) building at 100%. Would you consider that lighting to be non-continuous because the calculated load for the lighting is at 100%?

The load for four 5kW heaters on the same feeder is 20kW per 220.51. Whether the load is continuous or non-continuous, the load is still 20kW. The load for 20kW of connected lighting load in a 6500sf store, is 20kW, whether the load is continuous or non-continuous. The demand factor for both of these loads on a feeder or service is 100%, per section III of Art. 220.

The size of the feeder that must carry these loads will change depending on whether these loads are continuous or non-continuous, but the load doesn't change, it is 20kW in both cases. If either or both of these loads is non-continuous, the feeder would need to have an ampacity of at least 84A on a 240V feeder. If either or both of these loads is continuous, the feeder would need to have an ampacity of at least 105A on a 240V feeder.

220.51 only tells us how to calculate the load of the electric heaters, not whether or not the load is continuous, just as 220.42 tells us how to calculate the load for the general lighting. Both sections tell us what demand factor to apply to calculate that load. Whether the load is continuous or not is determined outside of article 220, either by expected operation of the equipment, such as with the lighting, or by other code section, such as in 424 for the electric heaters.

I'll ask it again, and it seems like a simple concept, but if all 4 heaters in the OP are calculated to be running at the same time (100% of the connected load per 220.51) and all 4 heaters are running for 3 or more hours continuously (continuous load per 424.3) how can the 84Amps on the feeder (100% of connected) flowing for 3 or more hours continuously be considered non-continuous load?

 

[iwire]

Well, one last try....I don't see the words non-continuous anywhere in 220.51.

Neither do I

But I do see

shall be calculated at 100 percent of the
total connected load

That as it is written actually makes it an NEC violation to even voluntarily decide to use 125%.

90.5 Mandatory Rules, Permissive Rules, and Explanatory
Material.

(A) Mandatory Rules. Mandatory rules of this Code are
those that identify actions that are specifically required or
prohibited and are characterized by the use of the terms
shall or shall not.

I hope you would agree that "non-continuous load" and "demand factor" are not the same thing?

I do, but see no relevance in it.

I'll ask it again, and it seems like a simple concept, but if all 4 heaters in the OP are calculated to be running at the same time (100% of the connected load per 220.51) and all 4 heaters are running for 3 or more hours continuously (continuous load per 424.3) how can the 84Amps on the feeder (100% of connected) flowing for 3 or more hours continuously be considered non-continuous load?

Because the 220.51 says differently.

My guess is they do not feel there is a likelihood of all the branch circuits all running continuously for the same 3 hour period.

Continuous Load. A load where the maximum current is expected to continue for 3 hours or more.

All it takes is one of the multiple heaters to cycle off and the feeder is no longer subject to its 'maximum current' and is not subject to the rules of a continuous load.

Again, I see your logic, I just don't feel that it applies in this case.

IMO 220.51 prohibits calculating the feeder load at anything but 100% which is also illogical.

 

[david luchini]

Neither do I

But I do see

shall be calculated at 100 percent of the
total connected load

That as it is written actually makes it an NEC violation to even voluntarily decide to use 125%.

What I see is

[COLOR=Red][B]loa[/B][B]ds[/B][/COLOR] shall be calculated at 100 percent of the
total connected load

It appears that you are trying to substitute "feeder size" for "loads." The load on the feeder is not calculated at 125%, only the feeder size is calculated at 125%.

The load for the 4 heaters is 84Amps. If the heater load is noncontinuous, then the feeder size must have a minimum ampacity of 84Amps. If the heater load is continuous, then the feeder must have a minimum ampacity of 105Amps. In either case, the load is still only 84Amps. Whether the load is continuous or not does not change the calculated load on the circuit, and 220.51 only serves to tell us how to calculate the load for the heaters.

Art 220 doesn't tell you what size to make your feeder, Art 215 does that. Read 215.2(A)(1).

Feeder conductors shall have an ampacity not less than required
to supply the [COLOR=Red]load[/COLOR] as calculated in Parts III ([COLOR=Red]see 220.51[/COLOR]), IV, and V of
Article 220.  The minimum feeder-circuit conductor size...shall have an
allowable ampacity not less than the noncontinuous load plus 125% of 
the continuous load.

My guess is they do not feel there is a likelihood of all the branch circuits all running continuously for the same 3 hour period.

All it takes is one of the multiple heaters to cycle off and the feeder is no longer subject to its 'maximum current' and is not subject to the rules of a continuous load.

If one of the heaters cycled off, then the branch circuits are not seeing "continuous" load per 424.

The exception to 220.51 specifically covers allowing you to reduce the feeder size for duty-cycle intermittent operation, or units not all operating at the same time.

If you are not applying the exception, then it stands to reason that the 220.51 expects that all of the units are operating at the same time, and "continuously."

IMO 220.51 prohibits calculating the feeder load at anything but 100% which is also illogical.

Feeder load yes, but not feeder size. Following this logic, it would also be a violation to calculate the size of a lighting feeder at 125% for an office or school building, because 220.42 says the those lighting loads shall be calculated at 100% also. But we know this is not true.

 

[charlie b]

I have lost track of who is supporting which point of view, and I strongly dislike the statements I have read that convey such messages as, ?you lack comprehension.? It is OK to say, ?I disagree? or ?I believe you are wrong.? It is NOT OK to say, ?you don?t understand what you are talking about.? Let us stay civilized, or find someplace else to debate.

That said, I will, without looking back to any earlier statements or citing anyone else?s previous comments, simply give my views. Take them or leave them, as you choose.

1. To start with, I infer that we are discussing a single feeder that serves a panel that, in turn, has four branch circuits, each serving a single heater, and that no other loads are to be served by that feeder.
2. I submit that the calculated load on each branch circuit is 20.8 amps (which I will join the others in rounding to 21), and not 26 amps. Basis: Part II of 220 does not require us to add 25% to continuous loads.
3. I submit that the minimum ampacity of the conductors in each branch circuit is 26 amps. Basis: 210.19(A)(1) tells us to size branch circuit conductors for 125% of continuous load, and 424.3(B) tells us that, for the purposes of sizing branch circuits, a space heater is a continuous load.
4. I submit that the calculated load on the feeder is 84 amps, and not 89 amps (i.e., 125% of the first heater and 100% of the other three), nor is it 105 amps (i.e., 125% of the total for four heaters). Basis: 220.40 tells us the feeder load is not less than the sum of the branch circuit loads, after demand factors are applied, and the rest of 220 does not give us any demand factors for heaters.
5. I submit that the minimum ampacity of the conductors in the feeder is 84 amps, not 105 amps. Basis: 215.2(A)(1) tells us to size feeder conductors for 125% of continuous load, and nothing, not even 424.3(B), tells us that, for the purposes of sizing feeders, a space heater is a continuous load.

Despite all thoughts and expectations we may have concerning lunacy versus consistency, a heater is considered a continuous branch circuit load, and a non-continuous feeder load.

 

[iwire]

David I stand by all I have said.

What I see when I read the post above is someone desperately trying to stretch things to fit their logical position. I have asked the other mods if they could jump in here with a fresh perspective.

Here is what the CMP has to say about it in the 2007 ROP and IMO it proves Brother correct.

2-314 Log #1042 NEC-P02 Final Action: Reject
(220.51)

Submitter: Daniel Leaf, Seneca, SC

Recommendation: Change 100 percent to 125 percent in the text and exception.

Substantiation: 424.3(B) states electric space heating loads are continuous, and requires conductor ampacity no less than 125 percent of the load. 215.2(A)(1) requires feeder ampacity not less than 125 percent of continuous load. Since load may determine minimum ampacity of conductors, a calculated 100 percent can result in conductor ampacity less than 125 percent.

Panel Meeting Action: Reject

Panel Statement: Section 220.51 is written for the purposes of determining a load contribution from electric space heaters used in determining the overall feeder and service load. It is not intended to specify the conductor size of the branch circuit feeding an electric space heater. The submitter has not substantiated that these loads be added into the feeder and service calculations at 125%.

Number Eligible to Vote: 12

Ballot Results: Affirmative: 12

 

[jim dungar]

There is absolutely no doubt that the NEC calculate loads for electric space heating differently than it does for general circuits.

220.51 specifically says to use 100% of the total connected load. The exception to 220.51 allows the feeder to "have an ampacity" less than 100% if the loading is actually 'non-continuous' (it uses words like, duty-cycle and intermittently).
So, assuming the space heating devices are thermostat controlled their feeder ampacity could possibly be chosen as 80% of 100% of the connected load.

While 215.2(A)(1) says to size feeder conductor ampacity based on 125% of continuous load (as determined in 220), the exception to 220.51 allows the feeder ampacity to be less than the continuous load.

 

[iwire]

2. I submit that the calculated load on each branch circuit is 20.8 amps (which I will join the others in rounding to 21), and not 26 amps. Basis: Part II of 220 does not require us to add 25% to continuous loads.

3. I submit that the minimum ampacity of the conductors in each branch circuit is 26 amps. Basis: 210.19(A)(1) tells us to size branch circuit conductors for 125% of continuous load, and 424.3(B) tells us that, for the purposes of sizing branch circuits, a space heater is a continuous load.

IMO I think between 2 and 3 you cleared things up.

We do not increase the 'load' 25%, we size the conductors for 125%, a subtle difference that I think is important.

 

[Dennis Alwon]

Again I agree with the calculated load for the feeder being 84 amps. Here is the problem I see.

If the disconnect at the heater has ocpd then the wire from the panel to the disco is a feeder. And the wire from the disco to the unit is a branch circuit-- right there I have issues accepting that-- I know what the code states.

Now, if these 4 units, which has been stated to run at the same time, were one 20 kw unit instead of 4 individuals heaters, then the wire to the unit would be calculated at 104 amps but the feeder could be 83 amps. This makes no sense and I doubt there would be an inspector around that would pass that install.