83%percent rule for multi family dwellings

Dsg319

Member
Location
West Virginia
Occupation
Apprentice Electrician
I’m stumped on this one. It say in the code book. (For a service rated 100-400amperes, the service conductors supplying the entire the entire load associated with a one family dwelling, or the service conductors supplying the entire load associated with an individual dwelling unit in a two family or multi family dwelling,shall be permitted to have an ampacity not less than 83 percent of the service rating.) I know I can take advantage of the 83% on a single family dwelling but I’m not understand for the multi family. Can the main service conductors coming to a meter stack of an apartment complex use the 83%rule and also the feeders going to the individual apartment unit?
 

Dsg319

Member
Location
West Virginia
Occupation
Apprentice Electrician
Please ignore my smelling I just re read the whole thing and seen all my mess ups and not sure how to go back and edit lol
 

Carultch

Senior Member
Location
Massachusetts
The 83% rule applies after you diverge the conductors to feed each dwelling unit. Upstream of this point where the conductors branch to each dwelling, or for the circuit feeding the common loads (also called the "house loads"), the 83% part of the rule doesn't applies. You would treat the "master" service conductors just like you would treat service conductors in a commercial application.

As an example, suppose we had a 3-family building, with a collective 800A service, that feeds a 200A sub-service to each dwelling unit, and a 200A house loads sub-service. For simplicity, in this example, I'm assuming that the 800A collective service is the total of the 200A individual services. I'm not familiar with whether or not there is a more nuanced rule, that allows load diversity to reduce the total.

Each 200A dwelling unit, is what could be served by just 166A of conductor ampacity, according to the 83% rule, and still be allowed to be protected by the 200A service breaker.

The 83% rule would not be allowed to apply for either the 800A collective service conductors, or the 200A house loads service, because neither of these circuits feed just one dwelling unit. You would not be able to put in 664A and 166A worth of conductors on these circuits. Depending on the calculated load, you may be able to take credit for 240.4(B), the "next size up rule", but in any case, the 83% rule and the table associated with it, do not apply.
 

kwired

Electron manager
Location
NE Nebraska
The rule applies to a conductor (service or feeder) that supplies all the load to one dwelling unit. A feeder within said dwelling unit can not use the reduced conductor size allowed by this section, it has to be something supplying the entire dwelling load.

Now when calculating a service or feeder conductor supplying multifamily applications you can't use this rule for selecting conductor size but at same time just because you have say 4- 100 amp feeders in a 4 family unit doesn't mean you need 400 amp supply either, if art 220 load calculations only come up to 180 amps you could have 180 amp supply conductor to this four family complex and still feed each unit with 100 amp feeders.
 

Carultch

Senior Member
Location
Massachusetts
But at same time just because you have say 4- 100 amp feeders in a 4 family unit doesn't mean you need 400 amp supply either, if art 220 load calculations only come up to 180 amps you could have 180 amp supply conductor to this four family complex and still feed each unit with 100 amp feeders.
If this is the case, does this also mean that you need a master OCPD to protect the collective service conductors? In your example, you have 4x100A services to each dwelling unit, and 180A worth of service conductors to the collective of dwelling units.

Does this mean that you would have to have no larger than a 200A breaker protecting the 180A conductors, upstream of the meter bank and the four dwelling unit main disconnects? If not, what prevents the 4 dwelling unit services from temporarily adding up to 300A, thus overloading the collective service conductors? There is the possibility that all four dwelling units could draw 75A at the same time, even if it is statistically unlikely according to the Article 220 load calculations.
 

kwired

Electron manager
Location
NE Nebraska
If this is the case, does this also mean that you need a master OCPD to protect the collective service conductors? In your example, you have 4x100A services to each dwelling unit, and 180A worth of service conductors to the collective of dwelling units.

Does this mean that you would have to have no larger than a 200A breaker protecting the 180A conductors, upstream of the meter bank and the four dwelling unit main disconnects? If not, what prevents the 4 dwelling unit services from temporarily adding up to 300A, thus overloading the collective service conductors? There is the possibility that all four dwelling units could draw 75A at the same time, even if it is statistically unlikely according to the Article 220 load calculations.
Reality is the scenarios you mention could result in more than 180 or even 200 amps being drawn.

230.90(A) exception 3 however says you only need service conductors sized per art 220 load calculations in this situation though.

Keep in mind the general rules for overcurrent protection for service conductors is conductors to a single overcurrent device must have overcurrent protection that corresponds to the conductor ampacity - so a 4/0 aluminum common supply conductor for this recently mentioned scenario would be fine for 180 amps or less of art 220 calculation. You could land that 4/0 in a gutter and tap off to 4 - 100 amp individually enclosed breakers or fused switches with the 83% rule to each and use #2 aluminum to each - if they are supplying dwelling units.
 

kwired

Electron manager
Location
NE Nebraska
Also keep in mind (still talking the 4 - 100 amp feeders to dwelling units here)

you might come up with load calculation for an individual dwelling of say 50 to 75 amps, but that calculation likely has just one range, one dryer, etc. on that particular feeder. A service calculation may be able to utilize demand factors that would put you less on the service than the sum of all four feeders comes out to be.
 

Dsg319

Member
Location
West Virginia
Occupation
Apprentice Electrician
The 83% rule applies after you diverge the conductors to feed each dwelling unit. Upstream of this point where the conductors branch to each dwelling, or for the circuit feeding the common loads (also called the "house loads"), the 83% part of the rule doesn't applies. You would treat the "master" service conductors just like you would treat service conductors in a commercial application.

As an example, suppose we had a 3-family building, with a collective 800A service, that feeds a 200A sub-service to each dwelling unit, and a 200A house loads sub-service. For simplicity, in this example, I'm assuming that the 800A collective service is the total of the 200A individual services. I'm not familiar with whether or not there is a more nuanced rule, that allows load diversity to reduce the total.

Each 200A dwelling unit, is what could be served by just 166A of conductor ampacity, according to the 83% rule, and still be allowed to be protected by the 200A service breaker.

The 83% rule would not be allowed to apply for either the 800A collective service conductors, or the 200A house loads service, because neither of these circuits feed just one dwelling unit. You would not be able to put in 664A and 166A worth of conductors on these circuits. Depending on the calculated load, you may be able to take credit for 240.4(B), the "next size up rule", but in any case, the 83% rule and the table associated with it, do not apply.
Also in example say I have a two story home with a living space in the basement and the whole house has a 200amp service. And I place a new 100 sub panel in the basement for the new living space(seeing that load calculations permit to have room for this). Since the panel is going to supply the whole new living space it would be correct to use the 83% rule for the conductors feeding the sub panel?
 

Carultch

Senior Member
Location
Massachusetts
Also in example say I have a two story home with a living space in the basement and the whole house has a 200amp service. And I place a new 100 sub panel in the basement for the new living space(seeing that load calculations permit to have room for this). Since the panel is going to supply the whole new living space it would be correct to use the 83% rule for the conductors feeding the sub panel?
Good question.

The answer is that the 83% rule and associated table would not apply. The scope of where it can apply, is a narrow window: single dwelling unit, and its entire load only. It doesn't apply for the collective service before diverging the current among multiple dwelling units, it doesn't apply if the application isn't a residence, and once you diverge circuits within a dwelling unit, it no longer applies. The rules for feeder conductor sizing, would apply instead.

Consider the following example, with each device numbered:
1. The service point of a 200A service for a single family home
2. A hot sequence self-contained meter on the outside wall
3. The main service disconnect the outside wall. Could be either a fused switch, or a breaker. In any case, all current going in this device, will go outward to device #4.
4. A 200A main panel is inside on the first floor
5. A 100A subpanel in the basement, fed from a branch circuit of device #4

Between devices 1 and 2, as well as devices 2 and 3, the conductors classify as service conductors. They only feed a single family dwelling unit, and carry the complete load. The 83% rule applies.

Between devices 3 and 4, the conductors cease to be classified as service conductors, and now classify as a feeder. Due to the fact this circuit is directly in series with the service conductors, without any brached paths yet, it feeds a single family dwelling unit, and carries the complete load. The 83% rule still applies.

Between devices 4 and 5, the conductors classify as a feeder. However, because the main panel has local loads within it, this feeder no longer carries the complete load of the dwelling unit. It only carries a portion of the dwelling unit's load, that which is specific to the basement. The 83% rule does not apply.
 

kwired

Electron manager
Location
NE Nebraska
Good question.

The answer is that the 83% rule and associated table would not apply. The scope of where it can apply, is a narrow window: single dwelling unit, and its entire load only. It doesn't apply for the collective service before diverging the current among multiple dwelling units, it doesn't apply if the application isn't a residence, and once you diverge circuits within a dwelling unit, it no longer applies. The rules for feeder conductor sizing, would apply instead.

Consider the following example, with each device numbered:
1. The service point of a 200A service for a single family home
2. A hot sequence self-contained meter on the outside wall
3. The main service disconnect the outside wall. Could be either a fused switch, or a breaker. In any case, all current going in this device, will go outward to device #4.
4. A 200A main panel is inside on the first floor
5. A 100A subpanel in the basement, fed from a branch circuit of device #4

Between devices 1 and 2, as well as devices 2 and 3, the conductors classify as service conductors. They only feed a single family dwelling unit, and carry the complete load. The 83% rule applies.

Between devices 3 and 4, the conductors cease to be classified as service conductors, and now classify as a feeder. Due to the fact this circuit is directly in series with the service conductors, without any brached paths yet, it feeds a single family dwelling unit, and carries the complete load. The 83% rule still applies.

Between devices 4 and 5, the conductors classify as a feeder. However, because the main panel has local loads within it, this feeder no longer carries the complete load of the dwelling unit. It only carries a portion of the dwelling unit's load, that which is specific to the basement. The 83% rule does not apply.
I don't agree with everything you mentioned.

The 100 amp feeder can use the 83% rule IF it is supplying the entire load of a NEC art 100 defined dwelling unit, whether it be within what is otherwise a dwelling building or even for something like a guest unit in other type of occupancy that still meets the definition of a dwelling unit.

The 83% rule can no longer apply to the main supply conductor in the example as it is no longer the main supply to a single dwelling unit.
Now if the main supply conductor is 4/0 aluminum and the load calculation is still below 180 amps you still can use the 4/0 aluminum on 200 amp breaker because of 240.4(B). If using 2/0 copper ampacity is 175 amps and that is already a standard OCPD size.
 

Dsg319

Member
Location
West Virginia
Occupation
Apprentice Electrician
This doesn’t make since to me. Please correct me if I’m wrong but this study book shows for a general purpose demand load (Optional method) for a multi family Dwelling and they sum it up and take the 40% off. I know for single family dwelling for using the optional method you take the first 10’000 at 100% than the remainder at 40%.....which is correct way for multi-family dwelling?
 

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kwired

Electron manager
Location
NE Nebraska
This doesn’t make since to me. Please correct me if I’m wrong but this study book shows for a general purpose demand load (Optional method) for a multi family Dwelling and they sum it up and take the 40% off. I know for single family dwelling for using the optional method you take the first 10’000 at 100% than the remainder at 40%.....which is correct way for multi-family dwelling?
Didn't verify, but pretty certain you are right and that study book is wrong.

I know it is a certain value at 100% and then the remainder at a lesser % and not just a general reduction to the net.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
This doesn’t make since to me. Please correct me if I’m wrong but this study book shows for a general purpose demand load (Optional method) for a multi family Dwelling and they sum it up and take the 40% off. I know for single family dwelling for using the optional method you take the first 10’000 at 100% than the remainder at 40%.....which is correct way for multi-family dwelling?
There must be more to that question in the Study Book. What does the rest of the example say?
 

Dsg319

Member
Location
West Virginia
Occupation
Apprentice Electrician
The answer seems correct for the given question.
I believe I found the confusion. They used this table of demand factors and for some reason in my head I was still stuck on table 220.42 demand factors. So I assume for the the table 220.42 demand factors for dwelling units are single family dwelling only and am suppose to use table 220.84 for multi family dwelling demand factors.
 

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kwired

Electron manager
Location
NE Nebraska
The "question" doesn't tell us how many SABC's or laundry circuits there are.

The presuming there is 2 SABC's and a laundry circuit per unit like the answer key is indicating - there should be total of 115,500 VA like it shows.

First 10,000 @ 100% = 10,000

remainder is 105,500
40 % of remainder 105,500 is 42,200.

First 10,000 plus 40% of remainder (42,200) equals 52,200. That value is not even in the multiple choice answer list.

No heating or AC loads were mentioned, but those would also need to be added at any applicable factor in accordance with 220.82(C).
 
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