Why does the Optional Calculation provide a different result?

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bjhuffine

Member
I'm trying to learn all the intricate rules with calculating load. Since I'm adding new space to my home, I thought this would be ideal. Our local AHJ will let me complete the wiring since I'm a EE and the homeowner. Not only that, but I would like to take the contractors exam as well and this will be good practice.

I have run through the load calculations twice. Once using all the rules from 220, parts I through III and another using the Optional calculations defined in part IV. The results are pretty different which is very confusing. So the question here is why?

A little more info:

I know that I will have to change my load center and meter center out as a result, but the differences are so great that I'm not sure which is best or safe to use without understanding why the NEC allows such different approaches.

With Parts I-III, I get a service load (Single phase 120/240V) of 227A for a 1940 sq-ft space with a 260 sq-ft basement-type space to be added that will be a workshop one day. For the workshop space, I just added lights and receptacles as it will not be livable or conditioned space as its actuall located in a large crawl space type area.

With the Optional Calculation, I get 160A.

This means I will either be installing a 200A load center or a 300A one? But which is right?
 
B

bjhuffine

Member
Okay, I understand what you mean by 90.1 (B) and that makes sense since the NEC is to provide the minimum requirements. But why have they allowed an Optional calculation method?

And for this kind of square footage, which one of the values/methods would most professionals use? In other words would they install a 300A load center based on the calculation from parts I-III or a 200A load center from part IV? I.e. err on the conservative side or not?
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
For a dwelling unit I would have no worries at all about cutting it closer with the optional calculations.

The NEC load calculations for dwelling units result in services that are without a doubt over sized for the real load they will be subjected to.

IMO the NEC pretty much admits that fact because they allow a single family dwelling to use table 310.15(B)(6) which allows us to use smaller conductors than the overcurrent device size would normally require.
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
iwire said:
For a dwelling unit I would have no worries at all about cutting it closer with the optional calculations.

The NEC load calculations for dwelling units result in services that are without a doubt over sized for the real load they will be subjected to.

IMO the NEC pretty much admits that fact because they allow a single family dwelling to use table 310.15(B)(6) which allows us to use smaller conductors than the overcurrent device size would normally require.
Click to expand...

Sounds reasonable. Thanks.
 
B

bjhuffine

Member
And see with me being a newbie, when I see a difference of 100A, it's concerning. I would've thought the Optional would've been closer than that. Is it safe to say that if I go with a 200A service, the worst case would be nuissance tripping and the expense of upgrading the service again?

Also, for those who have worked with many dwelling units over the course of several years, does a 200A service sound adequate for a 1940 sq-ft home with an additional 260 sq-ft workshop space?
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
bjhuffine said:
And see with me being a newbie, when I see a difference of 100A, it's concerning. I would've thought the Optional would've been closer than that. Is it safe to say that if I go with a 200A service, the worst case would be nuissance tripping and the expense of upgrading the service again?

Also, for those who have worked with many dwelling units over the course of several years, does a 200A service sound adequate for a 1940 sq-ft home with an additional 260 sq-ft workshop space?
Click to expand...
This is a design issue. Without knowing your specific loads, it cannot be answered. What appliances are gas, heat and A/C are what, extras like hot tubs,....etc?

You may wish to be careful, you seem to be getting close to asking a DIY question.
 
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B

bjhuffine

Member
Thank you very, very much for your input. My point is not to ask a DIY question, but to be able to have a firm enough understanding that I can feel confident in my decisions using the code. And since I don't have a lot of experience in it, I was hoping to get a good feel for how a lot of the NEC veterans handle this. Or whether I seem so far off base that maybe I misapplied something (that was the reason for the last question).

I know all of my specific load and design criteria and had applied it to the best of my knowledge based on what I've been studying. So it just seemed frustrating that when I attempted to apply this to a real-world scenario that the two answers did not come out even relatively close.

I've worked through various questions out of different studying workbooks, to which none that I've seen so far cover a beginning to end realistic scenario that includes all the same kind of loads as this at one time. And since I would like to take my contractor's exam soon and hopefully take my PE exam in October, it was a little concerning to see the difference and not understand why. Not only that, but my learning is with a real situation that I am currently undergoing with the remodelling effort.

But if this is out of scope for this forum, no problem. I have a friend that is an electrician who is willing to look over my shoulder. I'll talk with him tomorrow or sometime this week and see what he thinks about my calculations.

Again, thank you for your input.
 
Chamuit

Chamuit

Grumpy Old Man
Location
Texas
Occupation
Electrician
Give us all your loads, ft? and any other applicable info. Without doing the work for you, we could see if we agree.

I agree with iwire on the using of the optional methed for residential.
 
B

bjhuffine

Member
Cool deal...

Square Footage: Living: 1940 sq-ft (per measurement of outside dimensions of space), +260 sq-ft of space in crawl space to be closed in for small workshop.

Bathrooms - 3
Laundry - 1
Attic Space - 1
Crawl Space with access - 1
Water Heater: 240V, 5.5kW
Range: 8kW (per Column C of T220.55)
Refrigerator: 11.2A, 120V... I used 12A (80% loading rule) since this item is replaceable. Though on an exam question, the 11.2 A would have to be used.
Dishwasher: 9.0A, 120V
Microwave/Range Hood combo unit: 1kW
Dryer: 240V/26A: 6.24kW
Garage Door Opener: All I could find on the rating was 1/2hp, so per T430.248: 9.8A @ 120V. And per 430.24, 1.25*9.8*120=1470VA

Receptacle Load beyond those Required in Article 210:
1 in garage + 1 run outside to garden + 8 in workshop area

Lighting Load beyond those Required in Article 210:
1 in Kitchen (non-linear rated 204 VA)
6 in crawlspace and workshop area

Additional Fixed Load:
3 bathroom ventilation fans @1.17A each, 120V
1 whirlpool tub whose motor is rated 7A for 120V, 840VA

HVAC:

Unit 1: Package Gas Furnace and A/C: Min Ckt Ampacity: 17.9A @240V
Unit 2: Split Unit Heat Pump:
Outdoor Unit: Min Ckt Ampacity: 14.1A @240V
Indoor Unit: Min Ckt Ampacity: 33.5A @240V for blower + 8kW auxilliary heat.

With the Artical 220 Parts I-III I get 227A, with Part IV (Optional) I get 160A. Though I didn't include the workspace in the sq-ft calc and am wondering if I should've.

Again... I'm not asking for a DIY so please don't feel like you have to do the calcs, but if you can tell me if I'm within the ballpark I'll feel more comfortable about my calculations. And if you say that the Optional has proven to be accurate, that too will help make me feel more comfortable as well. Even though I'm still baffled as to why an Optional is even offered, especially if there's such a difference in results between the two.

Thank you very, very much.
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
Sir, I did not mean to disparage you in any way. I was trying to explain that your question is not answerable in its current format.

When I said DIY, I meant that if you ask us "Is X amps okay for X ft sq., in general?", It cannot be answered, and if you ask " Will I be okay?" it becomes DIY.

I am not trying to be disrespectful by any means.
 
jumper

jumper

Senior Member
Location
3 Hr 2 Min from Winged Horses
bjhuffine said:
Cool deal...

Square Footage: Living: 1940 sq-ft (per measurement of outside dimensions of space), +260 sq-ft of space in crawl space to be closed in for small workshop.

Bathrooms - 3
Laundry - 1
Attic Space - 1
Crawl Space with access - 1
Water Heater: 240V, 5.5kW
Range: 8kW (per Column C of T220.55)
Refrigerator: 11.2A, 120V... I used 12A (80% loading rule) since this item is replaceable. Though on an exam question, the 11.2 A would have to be used.
Dishwasher: 9.0A, 120V
Microwave/Range Hood combo unit: 1kW
Dryer: 240V/26A: 6.24kW
Garage Door Opener: All I could find on the rating was 1/2hp, so per T430.248: 9.8A @ 120V. And per 430.24, 1.25*9.8*120=1470VA

Receptacle Load beyond those Required in Article 210:
1 in garage + 1 run outside to garden + 8 in workshop area

Lighting Load beyond those Required in Article 210:
1 in Kitchen (non-linear rated 204 VA)
6 in crawlspace and workshop area

Additional Fixed Load:
3 bathroom ventilation fans @1.17A each, 120V
1 whirlpool tub whose motor is rated 7A for 120V, 840VA

HVAC:

Unit 1: Package Gas Furnace and A/C: Min Ckt Ampacity: 17.9A @240V
Unit 2: Split Unit Heat Pump:
Outdoor Unit: Min Ckt Ampacity: 14.1A @240V
Indoor Unit: Min Ckt Ampacity: 33.5A @240V for blower + 8kW auxilliary heat.

With the Artical 220 Parts I-III I get 227A, with Part IV (Optional) I get 160A. Though I didn't include the workspace in the sq-ft calc and am wondering if I should've.

Again... I'm not asking for a DIY so please don't feel like you have to do the calcs, but if you can tell me if I'm within the ballpark I'll feel more comfortable about my calculations. And if you say that the Optional has proven to be accurate, that too will help make me feel more comfortable as well. Even though I'm still baffled as to why an Optional is even offered, especially if there's such a difference in results between the two.

Thank you very, very much.
Click to expand...

See, now you are cooking with gas.:)

Why is water heater 5.5kw?

Dryer seems a bit high?

Heater on whirlpool ?

We are here to help, please accept my apoligies if you are offended.
 
B

bjhuffine

Member
I never took you as being disrespectful and I totally understand. Since what sample questions I have worked through were so idealistic and since I don't have a lot of experience, I'm not sure how far off base I may be. And with the two methods providing such different results, I was concerned. Though this is a real problem, I really, really want to ensure I fully understand the methods before exam time as well. I'm hoping this will provide good solid practice with a real-world problem.

And you're definitely right about my wording. I should've been more careful. Sorry about that. At the time I figured maybe someone with more experience than me would see that and maybe say "dude you're way off base" or "sounds close...".
 
B

bjhuffine

Member
My bad, I think I posted on top of you...

5.5kW is the total connected watts on the nameplate
The Dryer nameplate shows 26A and its 240V
No heater on whirlpool.

And again thank you very, very much! :)
 
cadpoint

cadpoint

Senior Member
Location
Durham, NC
ok, I've read the answers...

For all it's worth here's my answer, This is how I was taught the Code.

You need to use the optipal method, you have an existing powered structure! (IE By Code)

I frankly think your mixing up the two, and incorrectly at that!

The optional method is for an addition to a structure.

The optional method is an acounting of the total load on the existing structure, with the consideration for all "additional"
to the construction. So your Basement is in! I'll even assume it will happen!

Take a deep breath, pay attention to the opening lines of each of the articles and and the sub-divisions.
 
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