Continuous Duty - Residential Dwelling

[jaggedben]

How often does the initial ramp of the temperature cycle actually require full current for more than 3 hours? I.e. the target rate of temperature rise, plus the heat loss rate, exceeds what the heater can puts out, so it stays at full current?

Cheers, Wayne

What I said was based on my recollection from having seen consumption data for a house with a kiln. The data for several hour intervals in the middle of the kiln cycle was roughly the nameplate kW of the kiln above the otherwise normal usage. I could be wrong but as you ask these questions and I try to recollect it again, I have the impression that the peak temperature would actually be the equilibrium of the heat loss at continuous full power. Which seems plausible given that as I look up kiln temps, they're several times the max temp of an electric kitchen oven for the same size circuit. Also it seems plausible they want the temperature to be a smooth curve, not go up and down with cycle on and off. The firing cycle is like 20hr or something.

 

[tortuga]

Service Entrance Conductors are required to be sized with a 125% continuous use factor, see 230.42.

The first part of 230.42 says to determine the loads according to article 220, parts 3,4 or 5.
Then for a dwelling unit we can choose to use;

1. The standard calc as 220.40,
2. The optional as per 220.80 if its only one dwelling unit,
3. Or the existing dwelling unit in 220.8,
4. Two dwelling calc if its a duplex 220.85,
5. Multi family if its 3 or more dwelling units 220.84.

I dont think any of those calcs include a continuous factor, so the amount of continuous load is zero.
And correct me if I am wrong but the last four out of the five will actually apply a demand factor to continuous loads such as a EV charger or water heater.

 

[ramsy]

So if our heater rating is less then 8138W, then yes, it would take over 3 hours.

2.5kW water heater element in Annex D, Example D2(a) would also be continuous load.

2500W cold start would take 3.5 hrs for typical residential 40 Gallon heater with water @ 50 Deg.F

Water Heating Calculator for Time, Energy, and Power

The calculators on this page compute how long it takes to heat water, how much energy is consumed, and how much heating power is required.

gettopics.com

Since water heater tanks maintain temperature, cold starts require power failures, so actual use should be considered.
* Average shower is 16 Gallons of water in 8 minutes @ 75% hot = 12gal x 2 people = 24gal
* Average washing machine is 7 to 20 Gallons per load @ 50% hot = 10gal
* Average dishwasher is 4 to 6 Gallons per load @ 80% hot = 5gal

Miser shower heads & efficient appliances use less water, but dwellings may take multiple showers in the morning, and some take 20 minute showers. Standard 4800W elements handle ~60gal in 3 hrs, per above calculator link.

Example D2(a) assumes <=35gal to get <=3 hrs with 2500W elements, not related to later applied demand factor in 220.82(B).

 

[wwhitney]

The first part of 230.42 says to determine the loads according to article 220, parts 3,4 or 5.
. . .
I dont think any of those calcs include a continuous factor

Article 220 never mentions a continuous use factor (or maybe there's a small exception to that), it's just other articles that care about what fraction of the load is continuous. So whenever you apply Article 220 you have to keep track through the summations and demand factor what part of the total load is continuous. It would be better if Article 220 were more explicit about that.

In the particular case of the optional method 220.82, we have no guidance about how to do that tracking with respect to the "first 10 kVA at 100%, rest at 40%" modifier. The conservative approach is to apply the continuous load preferentially to the 10 kVA bucket. But you could argue for applying it proportionally, i.e. if 20% of the total load before this 100%/40% modifier is continuous, then 20% after the modifier is continuous.

So yes, if your residence has an EVSE, you definitely have a continuous load that affects the SEC sizing under 230.42.

Cheers, Wayne

 

[ramsy]

So yes, if your residence has an EVSE, you definitely have a continuous load (per) 230.42

Appendix D examples are long overdue for a dwelling car charger calculation.

No doubt its figured at 125%, before demand factors. Thanks for sharing 230.42

 

[tortuga]

Article 220 never mentions a continuous use factor (or maybe there's a small exception to that), it's just other articles that care about what fraction of the load is continuous. So whenever you apply Article 220 you have to keep track through the summations and demand factor what part of the total load is continuous. It would be better if Article 220 were more explicit about that.

In the particular case of the optional method 220.82, we have no guidance about how to do that tracking with respect to the "first 10 kVA at 100%, rest at 40%" modifier. The conservative approach is to apply the continuous load preferentially to the 10 kVA bucket. But you could argue for applying it proportionally, i.e. if 20% of the total load before this 100%/40% modifier is continuous, then 20% after the modifier is continuous.

So yes, if your residence has an EVSE, you definitely have a continuous load that affects the SEC sizing under 230.42.

Cheers, Wayne

There have been continuous loads in dwellings long before EVSE, take a pool pump or a dusk till dawn yard light pole for an example, I did a quick search and find no examples in new or old NEC editions, Mike Holt's, Tom Henry's or James Stallcup's books that depict any continuous load in a dwelling service or feeder calculation. They all use 100% not 125%. They all add 125% for commercial continuous loads.

 

[ramsy]

They all use 100% not 125%.

220.82(C)(6) is HVAC equipment calculated from nameplate, never subject to 125% or Article 430.

625.41 ESVE, and 424.3(B) fixed electric heating "shall be considered a continuous load", by 125% 210.19(A)(1) and 210.20(A).

 

[Fred B]

Residential does look at factors such as continuous loads and demand factoring, but also has diversity allowances (eg: existing dwelling without adding any new heating or cooling loads, first 8,000 VA at 100% then the balance counted at 40%). This does allow for a lower service calculation. For purpose of the OP's inspector question the use of the calculations in 220.83 should give him the answer.

 

[wwhitney]

There have been continuous loads in dwellings long before EVSE, take a pool pump or a dusk till dawn yard light pole for an example, I did a quick search and find no examples in new or old NEC editions, Mike Holt's, Tom Henry's or James Stallcup's books that depict any continuous load in a dwelling service or feeder calculation. They all use 100% not 125%. They all add 125% for commercial continuous loads.

Yet there is no wording in the NEC to support any such distinction between commercial and residential. If your residential service has continuous loads, when sizing the SECs under 230.42, you need to consider what fraction of the total load is continuous. 220.82 is silent on this aspect, but so is almost all of Part II and Part III of Article 220.

The situation is even clearer for feeders in a dwelling unit not supplying the entire load. Then 220.82 does not apply, and one uses Part II and Part III as usual, exactly the same as "commercial."

Cheers, Wayne

 

[jaggedben]

For some time now I have believed that one is permitted to use 220 to override 215.2 and 230.42. Because if not, then why does 220 even exist? Why not just delete Article 220 in its entirety? It has bothered me, ever since I realized it, that I can't point to any specific code section to firmly support this argument, other than the basic reasonableness of it. But there you go. I believe this is the intent of Article 220 even if they never came out and said so explicitly.

 

[jaggedben]

Another point is that the 'sum of non-continuous plus 125% of continuous' formulation is overly simplistic. As various sections make clear, the 125% factor is to help overcurrent devices function properly at full continuous load. If a sufficient amount of the load is non-continuous (and arguably depending on an appropriate diversity factor for that non-continuous load), then including the 125% factor for continuous load should not be required. Because 100% of the combined load will never be continuous.

 

[junkhound]

figuring what the Continuous Duty might be on a residential electrical service without doing any power monitoring

Most poco with smart meters do that for you. Simply look at typical days that the poco provides.
Here is my house from PSE for a cold day last week - I'm on TOU billing, so I have tstat set back from 7-10 AM and 5-8 PM. I think this particular day it got down to 11 F, my house is sized for the gshp for 12F.


LH scale is kW-hrs per 15 min intervals, so X4 for peak load in kW.
When using 50 A electric pressure washer (but NOT on an 11F day!), wife baking with both dryers running on a Saturday morning, have hit higher peaks. Tstat setback ONLY on weekdays, for instance on TOU, house cools down from 70F at 5 PM and does not drop below 63F till lower rates again at 8PM- big massive house
Data downloads from PSE in Excell

 

[wwhitney]

For some time now I have believed that one is permitted to use 220 to override 215.2 and 230.42. Because if not, then why does 220 even exist? Why not just delete Article 220 in its entirety?

You've completely lost me. The scope of 220 is given in 220.1 as "This article provides requirements for calculating branch-circuit, feeder, and service loads." While, for example, the first sentence of 215.2(A)(1) says "Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220."

So there is zero conflict between the two articles and nothing for one to override in the other. Article 220 tells you the value of the "load" in the context of sizing conductors and OCPD (versus when "load" is used in the sense of utilization equipment). And 215.2 explicitly refers to the Article 220 calculations.

If you deleted Article 220, we'd have no procedure for calculating the load values referenced by other Articles. At best we might default to what the NEC refers to as the "total connected load," i.e. the simple sum of the ratings of the pieces of utilization equipment connected. With no demand factors or other reductions.

Cheers, Wayne

 

[jaggedben]

You've completely lost me. The scope of 220 is given in 220.1 as "This article provides requirements for calculating branch-circuit, feeder, and service loads." While, for example, the first sentence of 215.2(A)(1) says "Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220."

No, you've lost me. That is not the first sentence of 215.2(A)(1). Did you make that up or get confused with a different section? 215.2 starts out this way.

215.2 Minimum Rating and Size.

(A) General. Feeder conductors shall have an ampacity not less than the larger of 215.2(A) (1) or (A) (2) and shall comply with 110.14(C).

(1) Continuous and Noncontinuous Loads. Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

230.42 references Article 220, but in my opinion it still kind of contradicts itself by doing that and then demanding a load calc of its own in 230.42(A)(1) using the same language as above.

...

If you deleted Article 220, we'd have no procedure for calculating the load values referenced by other Articles. At best we might default to what the NEC refers to as the "total connected load," i.e. the simple sum of the ratings of the pieces of utilization equipment connected. With no demand factors or other reductions.

Exactly my argument. Deleting Art 220 would be ridiculous, and leave us without some answers as to how to calculate the load for certain items (receptacles, for example). So it should be permitted to be used. But 215.2 and 230.42(A)(1) lay requirements on conductors that will usually result in a larger conductor than required by 220, and I'm not aware of any code section that settles the contradiction.

If 215.2 referenced 220 like you are incorrectly claiming it does, and if Article 220 had some blanket language at the beginning along the lines of "Where a load is continuous, 125% of the nameplate shall be used in all calculations required by this article.", then the code would be a lot closer to saying what you seem to think it says. As it is, 220 contains no 125% requirements (with the small exception that doesn't bother us) so if we're permitted to use article 220 I say we're permitted to ignore it.

 

[david luchini]

so if we're permitted to use article 220 I say we're permitted to ignore it.

I don't see how you would be permitted to ignore something that is required.

 

[wwhitney]

No, you've lost me. That is not the first sentence of 215.2(A)(1). Did you make that up or get confused with a different section? 215.2 starts out this way.

Ah, that's a 2017 vs 2020 NEC difference. Nonetheless, in 2020 NEC 215.2(A)(1), when the first sentence uses the word "load", it means the value calculated according to Article 220. The verbiage I quoted from the 2017 NEC was redundant, so in the 2020 NEC they just deleted it

230.42 references Article 220, but in my opinion it still kind of contradicts itself by doing that and then demanding a load calc of its own in 230.42(A)(1) using the same language as above.

230.42 just still has the redundant language that was deleted from 215.2(A). The further calculation mentioned is not a load calculation. It is a simple linear combination of two different Article 220 load calculations (or perhaps one enhance load calculation).

But 215.2 and 230.42(A)(1) lay requirements on conductors that will usually result in a larger conductor than required by 220, and I'm not aware of any code section that settles the contradiction.

This is where you've lost me. The requirements that 215.2 and 230.42(A)(1) put on conductors are entirely mediated through the Article 220 load calculations that are inputs to the further simple calculations mentioned in those sections. You are speaking as if there are two different numbers that occur in the context of 215.2(A)(1)(a), and making a comparison between them, but I see only one number that occurs from 215.2(A)(1)(a), and hence don't understand what you mean by "larger conductor than required by 220".

220 doesn't tell you how to size conductors. It just tells you a piece data, called the load (or rather two pieces of data, since we have to track continuous loads separately), attached to any conductor. Then 215.2, 230.42, or 210.19 tells you how to size the conductor based on that data.

Cheers, Wayne

 

[jaggedben]

I don't see how you would be permitted to ignore something that is required.

Right, I'm not saying that part makes sense. But again then, what's the point of Article 220. I'm pointing out that the code is badly written IMO.

 

[david luchini]

Right, I'm not saying that part makes sense. But again then, what's the point of Article 220. I'm pointing out that the code is badly written IMO.

The point of Article 220 is how to calculate load. Seems like an important step.

 

[tortuga]

220.82(C)(6) is HVAC equipment calculated from nameplate, never subject to 125% or Article 430.

625.41 ESVE, and 424.3(B) fixed electric heating "shall be considered a continuous load", by 125% 210.19(A)(1) and 210.20(A).

What Wayne is suggesting is Article 230.42(A)(1) is requiring a second calc in addition to the load calc, call it a continuous load calc, which I say is unenforcable since there is no guidance in 220.

220.14 says "(A) Specific Appliances or Loads. An outlet for a specific
appliance or other load not covered in 220.14(B) through (L)
shall be calculated based on the ampere rating of the appliance
or load served."
So my take on that is I use the nampleate of the EVSE,water heater etc, same as the optional calc.

Article 422 long has declared a waterheater continuous just like EVSE and there are no examples of applying a continuous factor to a waterheater.

"422.13 Storage-Type Water Heaters. A fixed storage-type
water heater that has a capacity of 450 L (120 gal) or less shall
be considered a continuous load for the purposes of sizing branch
circuits."

 

[jaggedben]

Ah, that's a 2017 vs 2020 NEC difference. Nonetheless, in 2020 NEC 215.2(A)(1), when the first sentence uses the word "load", it means the value calculated according to Article 220. The verbiage I quoted from the 2017 NEC was redundant, so in the 2020 NEC they just deleted it

In my strong opinion it was not redundant and shouldn't have been deleted. I read "the minimum feeder conductor size shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load" as a simple instruction to add up the load, without demand factors. If I'm allowed to use demand factors and everything in Article 220, that needs to be explicit. That deletion was a bad mistake.

What is still unclear in all of it is how I'm supposed to account for the 125% factor when using Article 220. Since article 220 doesn't mention it, I don't do it. Not to mention the point I made in post #31, which would make a 125% factor inappropriate for pretty much all the resi load calcs I do.

230.42 just still has the redundant language that was deleted from 215.2(A). The further calculation mentioned is not a load calculation. It is a simple linear combination of two different Article 220 load calculations (or perhaps one enhance load calculation).

Nowhere is this spelled out. It strikes me as a very in-between-the-lines interpretation.

This is where you've lost me. The requirements that 215.2 and 230.42(A)(1) put on conductors are entirely mediated through the Article 220

Where is this spelled out?

load calculations that are inputs to the further simple calculations mentioned in those sections. You are speaking as if there are two different numbers that occur in the context of 215.2(A)(1)(a), and making a comparison between them, but I see only one number that occurs from 215.2(A)(1)(a), and hence don't understand what you mean by "larger conductor than required by 220".

...

See first response above.