Article 220 Part III vs. Part IV

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LibertyEngineering

Senior Member
Location
Allentown, PA
I ran the calcs for a single family dwelling unit using both of these sections. The Part III calcs indicate a service load of 146A while the Part IV calcs indicate a service load of 105A. I was surprised to see that much of a difference. I triple checked to make sure both were correct. Part IV is a much easier calculation of course but seems to offer much more 'forgiveness' from a demand factor. 220.82 (B) this applies a demand factor to everything but the heat or AC whichever is larger while the Part III demand factor only applies to the General lighting/receptacle loads small appliance load and laundry load. I understand things like ranges and dryers etc. get their own demand factors applied as Part III calcs but in the situation where there is only one of each appliance no demand factor is applied.
I supposed this post is more of a statement than a question but wondering if anyone else experienced the same.

Thanks!
 

Npstewart

Senior Member
Yes, the optional method is always substantially less than the standard method. The standard method is significantly inflated.


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Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Yes, the optional method is always substantially less than the standard method. The standard method is significantly inflated.


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There are situations were the optional is higher than the standard but that is unusual. Interesting that many exam questions seem to use the standard calc instead of the optional.

Corrected my statement
 
Last edited:

bwat

EE
Location
NC
Occupation
EE
Posts #2 and #3 disagree.

#2: optional << standard.... always
#3: optional < standard.... unusual

Which is it? As someone who does next to zero residential work, I'm just curious.
 

LibertyEngineering

Senior Member
Location
Allentown, PA
Posts #2 and #3 disagree.

#2: optional << standard.... always
#3: optional < standard.... unusual

Which is it? As someone who does next to zero residential work, I'm just curious.
I am guessing Dennis meant to say......standard is lower than optional.
I am not doubting this but would be interested to see why, my guess is several ranges/cooktops , Dryers in that dwelling unit taking advantage of the larger demand factors.
 

suemarkp

Senior Member
Location
Kent, WA
Occupation
Retired Engineer
Want to post your standard calc so people can critique it? Usually it is somewhat larger than the optional, but usually not by a huge amount. You do know that a typical electric range would go in at 8KW if it is a 12KW or smaller range don't you, so you get some relief there. And you get some demand factors in 220.53. That's one where it can help to add a garage door opener or two as many forget those and it helps to get the fastened-in-place equipment count to 4 or more so you can apply the 75% factor. There are also questions as to what is an appliance and what is a motor (e.g. are garage door openers and garbage disposals motors or appliances)? I'd consider them appliances and assume any 125% factor is already in their nameplate.
 

LibertyEngineering

Senior Member
Location
Allentown, PA
Want to post your standard calc so people can critique it? Usually it is somewhat larger than the optional, but usually not by a huge amount. You do know that a typical electric range would go in at 8KW if it is a 12KW or smaller range don't you, so you get some relief there. And you get some demand factors in 220.53. That's one where it can help to add a garage door opener or two as many forget those and it helps to get the fastened-in-place equipment count to 4 or more so you can apply the 75% factor. There are also questions as to what is an appliance and what is a motor (e.g. are garage door openers and garbage disposals motors or appliances)? I'd consider them appliances and assume any 125% factor is already in their nameplate.
No need to have it critiqued I've done that enough on my own thanks. The difference in the 2 calcs for the same dwelling unit was 41A so yes not a huge amount but enough to make it the next standard size feed if I went with the standard calcs (125 to a 150.) I was aware of the range demand, although that table is 'fun' all on its own to follow. 220.53 allows for the 75% demand on appliances but than they list those appliances which are not permitted and in my case, other than a water heater & microwave, those listed are the only appliances in the dwelling unit. This is only a 754 sq. ft. 1 bedroom apartment in a dwelling unit. I agree garage doors and garbage disposals are appliances.

NEC definition;
Appliance. Utilization equipment, generally other than industrial, that is normally built in standardized sizes or types and is installed or connected as a unit to perform one or more functions such as clothes washing, air-conditioning, food mixing, deep frying, and so forth.
 

JoeStillman

Senior Member
Location
West Chester, PA
Part IV usually beats Part III for a single dwelling. For multi-family, Part III only wins when yo use all natural gas for water heat, cooking, heating and dryers and even then, only up to about 30 dwelling units.

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LibertyEngineering

Senior Member
Location
Allentown, PA
Thank You fellow Pennsylvanian! I noticed that too. Must be because to you must have (or include) electric cooking equipment in the optional calculation for the multifamily service.
 

josi-dasilva

Member
Location
USA
Occupation
Retired
I ran the calcs for a single family dwelling unit using both of these sections. The Part III calcs indicate a service load of 146A while the Part IV calcs indicate a service load of 105A. I was surprised to see that much of a difference. I triple checked to make sure both were correct. Part IV is a much easier calculation of course but seems to offer much more 'forgiveness' from a demand factor. 220.82 (B) this applies a demand factor to everything but the heat or AC whichever is larger while the Part III demand factor only applies to the General lighting/receptacle loads small appliance load and laundry load. I understand things like ranges and dryers etc. get their own demand factors applied as Part III calcs but in the situation where there is only one of each appliance no demand factor is applied.
I supposed this post is more of a statement than a question but wondering if anyone else experienced the same.

Thanks!
If you would like to try it on a spreadsheet: https://docs.google.com/spreadsheets/d/1wv_exMq--9rFEXG_JhADlTNqg78zZF0CGat8Oqi6qrg/edit?usp=sharin
 
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