Rule of Thumb for NEC 220.84

[steve66]

Each apartment currently has a existing 2-pole 60A breaker. So I would say my design increases their 60A capacity.

Do inspectors ask for these calculations? I just don’t want to shoot myself in the foot if I did it using rule of thumb and then they start asking for calculations

Yes sometimes they do ask. But even if they don't, as an engineer, you also need to CYA.

I wasn't sure if this was an existing building, or some new proposed new construction. Does the utility have a demand meter reading where they can give you the buildings historic peak demand? That's one way that the code allows.

 

[Tainted]

Yes sometimes they do ask. But even if they don't, as an engineer, you also need to CYA.

I wasn't sure if this was an existing building, or some new proposed new construction. Does the utility have a demand meter reading where they can give you the buildings historic peak demand? That's one way that the code allows.

Yes it's existing but we are upgrading the electrical infrastructure with new, this would renew life of the system and also provide extra capacity for future. The building is not single metered. Everyone has their own dedicated meter.

I should probably spec submeter on the load side of 1200A service switch so they can monitor it for every apartment upgrade that they do. Do POCOs let you install submeters on unmetered feeders?

 

[wwhitney]

Nameplates ratings of all appliances, if they dont know the range I use 8kw per unit.

Isn't that a bit low to use as a default? 220.55 doesn't apply when using 220.84, as I understand it.

Or maybe historically electric ranges were smaller, so the installed base average apartment range rating is less than what I typically see on the specs for a new apartments size range.

Cheers, Wayne

 

[Tainted]

Isn't that a bit low to use as a default? 220.55 doesn't apply when using 220.84, as I understand it.

Or maybe historically electric ranges were smaller, so the installed base average apartment range rating is less than what I typically see on the specs for a new apartments size range.

Cheers, Wayne

He's talking about the exception in 220.84 where you can pretend there is 8000VA range load if there isn't a range in the apartment.

 

[steve66]

Are you trying to do the calculation for the building service, or for the Power Company?

If there is an existing electrical service, say 1200 amps for example, and you are either replacing it or upgrading it, then I'm not sure why you would need a load calculation, unless you are just trying to verify the service was adequate to start with.

 

[augie47]

Taking your original post asking about red flags, if you want to look at these units becoming all electric (ranges, heat,dryers, etc), any way you play the numbers a 1200 service is woefully inadequate.

 

[drktmplr12]

I don't think you get the point. The apartments right now have 2-pole 60 amps. I can do the calculation based on the existing 60 amps. But what if people want to upgrade to 100 or 125 or 150 or 175 or 200 amps in the future? Surely I would need a service switch that can accommodate all of these extra amps. How will I know in the future how many amps each apartment would want so that the service switch can be sized appropriately and so that it is not oversized or undersized?

I don't think we should go to starbucks first, I think we should go to a gypsy with a crystal ball and they can let us know the future loads

you can check your math by working backwards to get the "max connected without demand factor", but nothing in 220.84 says to do calcs based on the existing feeder size.

the code is explicit:

1. 3 VA/ft^2 for lights and receptacles
2. 1,500 VA for each appliance circuit and 1,500 VA for each laundry circuit
3. Namplate of appliances
4. Namplate of motors
5. Larger of AC or heater

for each dwelling

then apply demand factor.

my view is, while using this method, if you do anything other than the above-prescribed calculation, you can be held iable if there is a problem.

do you have building plans available?? they should tell you 80% of what you need for the calculation.

You can then fill in the blanks with a site visit or two. You would be surprised how quickly you can visit 59 units to find two nameplates, stove and AC. Assuming a full 8-hour day and going door to door, you have 8 minutes to find two nameplates in each unit. Bonus if many of the units likely have identical hardware. You do as many as you can and if you are denied/unable to access you make a conservative assumption.

OR find the largest installed stove and AC units and assume that for all units.

you might have already spent as much time researching ways to get around doing the calc how it is prescribed.

 

[tortuga]

I'll admit would not visit all 58 units, I'd try to figure out the types of units and visit one or two of each type.
It helps to have dog treats.

 

[JoeStillman]

I don't think we should go to starbucks first, I think we should go to a gypsy with a crystal ball and they can let us know the future loads

Sometimes when I do a 220.84 calc I feel like I'm practicing voodoo.

Are these apartments or condominiums? I would think a condo owner would have some say over what loads he has but not an apartment renter.

 

[tortuga]

Article 220 calcs have nothing to do with future loads, in the beginning of the NEC it says its 'not a design manual'.
I use 220.84 just to come up with the minimum service required by the AHJ, and to give the utility a number.
Then its up to the estimator / owner to decide how much bigger than that they want for future expansion.
I have seen multifamly complexes value engineered within 2 amps of the calculated service size.

 

[Tainted]

Are you trying to do the calculation for the building service, or for the Power Company?

If there is an existing electrical service, say 1200 amps for example, and you are either replacing it or upgrading it, then I'm not sure why you would need a load calculation, unless you are just trying to verify the service was adequate to start with.

I am doing it for both building service and for power company. I've told the power company that I am not adding any load and that I am just upgrading equipment to have the ability to provide extra capacity in the future, they still wanted me to send them calcs.

Taking your original post asking about red flags, if you want to look at these units becoming all electric (ranges, heat,dryers, etc), any way you play the numbers a 1200 service is woefully inadequate.

How many KVA per unit would be acceptable for all electric?

Sometimes when I do a 220.84 calc I feel like I'm practicing voodoo.

Are these apartments or condominiums? I would think a condo owner would have some say over what loads he has but not an apartment renter.

It's a apartment owned by certain shareholders.

Article 220 calcs have nothing to do with future loads, in the beginning of the NEC it says its 'not a design manual'.
I use 220.84 just to come up with the minimum service required by the AHJ, and to give the utility a number.
Then its up to the estimator / owner to decide how much bigger than that they want for future expansion.
I have seen multifamly complexes value engineered within 2 amps of the calculated service size.

You still don't get it. These are really old apartments with 60 amps each. 7 apartments have renovated and have like 100-150 amps. We want to make sure the service that we are designing will be adequate for future renovations for the other 52 remaining apartments. So because of this, we need to use rule of thumb since we do not know what loads these 52 apartments will have in the future. Does 36kVA per apartment sound reasonable as a rule of thumb? The tree huggers in New York city are pushing for electrification of all buildings.

 

[tortuga]

You still don't get it. These are really old apartments with 60 amps each. 7 apartments have renovated and have like 100-150 amps. We want to make sure the service that we are designing will be adequate for future renovations for the other 52 remaining apartments. So because of this, we need to use rule of thumb since we do not know what loads these 52 apartments will have in the future. Does 36kVA per apartment sound reasonable as a rule of thumb? The tree huggers in New York city are pushing for electrification of all buildings.

Ahhh ok, so they are apartments not condo's, so the tenants may be adding random cord and plug connected loads not doing renovations themselves. The owners of the building have renovated 7 apartments the largest being a 150A feeder.
and they are not combining smaller apartments to make larger ones or anything?
What is the total square footage of all 59 apartments combined?
How many apartments have an electric range?
How are the units heated and cooled? Central system or individual ?

 

[HEYDOG]

I am doing it for both building service and for power company. I've told the power company that I am not adding any load and that I am just upgrading equipment to have the ability to provide extra capacity in the future, they still wanted me to send them calcs.



How many KVA per unit would be acceptable for all electric?



It's a apartment owned by certain shareholders.



You still don't get it. These are really old apartments with 60 amps each. 7 apartments have renovated and have like 100-150 amps. We want to make sure the service that we are designing will be adequate for future renovations for the other 52 remaining apartments. So because of this, we need to use rule of thumb since we do not know what loads these 52 apartments will have in the future. Does 36kVA per apartment sound reasonable as a rule of thumb? The tree huggers in New York city are pushing for electrification of all buildings.

I would think that the utility wants real numbers based on what the calculated load is now. The utility is not going to install a transformer that is only loaded to 50% based on the possibility that the load may increase in the future. If the loads increase in the future they will put in a bigger transformer to handle the additional load!

 

[wwhitney]

I am doing it for both building service and for power company.
. . .
These are really old apartments with 60 amps each. 7 apartments have renovated and have like 100-150 amps.

Sounds like what you need to do is a full 220.54 accounting for the 7 renovated apartments and for a representative sampling of the other 52 apartments.

Then you can get your future size rule of thumb from the 7 renovated apartments, and the customer can use that to size their new service equipment. While you can provide a current load calculation/enumeration to the power company.

Cheers, Wayne

 

[Tainted]

Ahhh ok, so they are apartments not condo's, so the tenants may be adding random cord and plug connected loads not doing renovations themselves. The owners of the building have renovated 7 apartments the largest being a 150A feeder.
and they are not combining smaller apartments to make larger ones or anything?
What is the total square footage of all 59 apartments combined?
How many apartments have an electric range?
How are the units heated and cooled? Central system or individual ?

Tenants may decide they want to upgrade their 60 amps in the future, we are preparing in case every single apartment will want to upgrade. I do not know future loads but we want to make sure the service switch will have the capacity for everything electric, including range, central heating, etc.

forget it, I know you're trying to help and thank you. But we're just going in circles, sizing a service switch such that it would be capable of being ready for the future renovations incase every apartment will upgrade is unheard of it seems.

 

[electrofelon]

forget it, I know you're trying to help and thank you. But we're just going in circles, sizing a service switch such that it would be capable of being ready for the future renovations incase every apartment will upgrade is unheard of it seems.

Why don't you just do a load calc and see what it comes out to? You say you don't know loads but seems pretty simple IMO. Figure each with electric range, electric dryer, 4500W, water heater.....heat? Ac? Just run some numbers and see what you get.

 

[Tainted]

Why don't you just do a load calc and see what it comes out to? You say you don't know loads but seems pretty simple IMO. Figure each with electric range, electric dryer, 4500W, water heater.....heat? Ac? Just run some numbers and see what you get.

I did the calc, 36kVA per apartment seems like a good rule of thumb. Keep in mind that not all apartments will have exactly 36kVA. If an apartment will have 30kVA, the remaining 6kVA can go to a different apartment. This means I need a 1600A service switch. But I will just break it up into 2 service switches since I do not want a freestanding switchboard. Will spec a 1200A service switch for (44apts) and a 600 service switch (15apts). 220.84 permits 36kVA per apartment with these service switch sizes

 

[augie47]

Using "wild" numbers (600 sq ft 8kw range, 5kw heat pump, waterheater,etc) I came up with a 1600 amp service but that may be quite low considering the hart required in NY if you go all electric
Be careful when you split the load into two mains as you loose that .24 diversity factor

 

[Tainted]

Using "wild" numbers (600 sq ft 8kw range, 5kw heat pump, waterheater,etc) I came up with a 1600 amp service but that may be quite low considering the hart required in NY if you go all electric
Be careful when you split the load into two mains as you loose that .24 diversity factor

That's why I used 0.27 factor for 44 apartments to be connected to 1200 amp service switch and .4 factor for 15 apartments connected to a 600 amps service switch. This is using 36kVA per aptartment as a rule of thumb which I think is more than enough. Keep in mind there is no way that every single apartment will have 36kVA exactly, some may have 25kVA, or 30kVA, or 40kVA. It is unlikely that 90% of the apartments will have more than 36kVA.

If I use a 1600A service switch, I would need a big freestanding switchboard which would not be feasible fitting in the room so I will separate it with 2 service switches.