Rule of Thumb for NEC 220.84

[Tainted]

Without having to go to every single apartment to gather all the loads, what would be a good rule of thumb to calculate the service switch needed to feed 59 apartments using NEC article 220.84? Let's pretend everyone will have electric stoves and electric dryers as well as electric cooling and heating. I am going to feed 59 apartments with a 1200A service switch. Does this set off any red flags to any of you? Building voltage is 208V 3 phase

 

[Buck Parrish]

Without having to go to every single apartment to gather all the loads, what would be a good rule of thumb to calculate the service switch needed to feed 59 apartments using NEC article 220.84? Let's pretend everyone will have electric stoves and electric dryers as well as electric cooling and heating. I am going to feed 59 apartments with a 1200A service switch. Does this set off any red flags to any of you? Building voltage is 208V 3 phase

How many bedrooms? Sq ft?

 

[Tainted]

How many bedrooms? Sq ft?

It wouldn't be a rule of thumb if we start counting bedrooms and sq ft though... and I don't know how many bedrooms and sqft each apartment is.

 

[letgomywago]

Seems light to have only 20 amps available on Thanksgiving dinner time but may be enough

 

[electrofelon]

Seems light to have only 20 amps available on Thanksgiving dinner time but may be enough

35 amps single phase 208. I'm sure that's plenty. Half the people will be having Thanksgiving elsewhere.

Is be curious what POCOs figure per unit for apt buildings, they likely have very good data.

 

[letgomywago]

35 amps single phase 208. I'm sure that's plenty. Half the people will be having Thanksgiving elsewhere.

Is be curious what POCOs figure per unit for apt buildings, they likely have very good data.

Nvm I didn't notice the 3 phase. 35 at 208 I think would hold even with some ovens dryer and heating going on in each unit at the same time

 

[drktmplr12]

my take after reviewing 220.84

apartments buildings typically have 3 or 4 floorplans. one for "middle units" one for corners, and maybe a couple with an extra bedroom. get with building owner to see how many, look at one of each type and calculate according to 220.84, then multiply by the number of units.

last apply the demand factor based on # of units.

at least that's where i would start

 

[Tainted]

35 amps single phase 208. I'm sure that's plenty. Half the people will be having Thanksgiving elsewhere.

Is be curious what POCOs figure per unit for apt buildings, they likely have very good data.

35 amps single phase 208. I'm sure that's plenty. Half the people will be having Thanksgiving elsewhere.

Is be curious what POCOs figure per unit for apt buildings, they likely have very good data.

I did the calc too and got this:

1200amps x 360 = 432000VA

432000VA/0.24 = 1800000VA of total connected load available to distribute to 59 apartments without demand factor.

0.24 came from table 220.84

That means each apartment has a available total connected load give or take 30508VA without demand factor

 

[LarryFine]

So, about 125a per?

 

[Tainted]

So, about 125a per?

That’s worst case scenario. Since 30508VA is 146 amps at 208 single phase, some people might require less than 30508VA and that would give more capacity to other apartments.

For example if apartment A only uses 25508VA of total connected load, then that 5000VA of extra capacity can go to apartment B

 

[tortuga]

Is be curious what POCOs figure per unit for apt buildings, they likely have very good data.

Years ago when utility transformer blew on a recently renovated apartment building, the utility went after the building owner for damages. The owners insurance took over then went after the EE and their insurance for errors and omissions.

 

[electrofelon]

Years ago when utility transformer blew on a recently renovated apartment building, the utility went after the building owner for damages. The owners insurance took over then went after the EE and their insurance for errors and omissions.

You are supposed to inform POCO of any change in load for sure. I was just saying I would like to know what the actual max load per unit is when I get into dozens of units, considering all the diversity. I bet POCOs have good proven data on this that they use to size their equipment.

 

[steve66]

t

Without having to go to every single apartment to gather all the loads, what would be a good rule of thumb to calculate the service switch needed to feed 59 apartments using NEC article 220.84? Let's pretend everyone will have electric stoves and electric dryers as well as electric cooling and heating. I am going to feed 59 apartments with a 1200A service switch. Does this set off any red flags to any of you? Building voltage is 208V 3 phase

There is no rule of thumb. Someone needs to do the load calculation.

Let's pretend everyone will have electric stoves and electric dryers as well as electric cooling and heating.

You will more info on those loads at the very least.

and I don't know how many bedrooms and sqft each apartment is.

Your gonna need that too.

 

[steve66]

You are supposed to inform POCO of any change in load for sure. I was just saying I would like to know what the actual max load per unit is when I get into dozens of units, considering all the diversity. I bet POCOs have good proven data on this that they use to size their equipment.

Yes, probably so. And they can use that data all day to size their stuff, but you can't use that data to size the electrical service for the building.

 

[Tainted]

t

There is no rule of thumb. Someone needs to do the load calculation.



You will more info on those loads at the very least.


Your gonna need that too.

See this is what I don’t really understand. I do not have a crystal ball, I do not know what loads these apartments will have in the future, so this is why I chose to do rule of thumb. There’s absolutely no way to do this legitimately without knowing future loads for each single unit.

 

[electrofelon]

Yes, probably so. And they can use that data all day to size their stuff, but you can't use that data to size the electrical service for the building.

Correct, and I never said you could use their data for NEC purposes.

 

[electrofelon]

See this is what I don’t really understand. I do not have a crystal ball, I do not know what loads these apartments will have in the future, so this is why I chose to do rule of thumb. There’s absolutely no way to do this legitimately without knowing future loads for each single unit.

I would say you size it for the current design, and ask the owner if he's willing to pay for extra capacity in case things are added.

 

[Tainted]

I would say you size it for the current design, and ask the owner if he's willing to pay for extra capacity in case things are added.

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

 

[tortuga]

There’s absolutely no way to do this legitimately without knowing future loads for each single unit.

How so? 220.84 does not require you to anticipate the future, they do require I leave the office though.
I do 220.84 calcs like so;
First I hit Starbucks,
Next go over (A) make sure you meet the criteria.
Then do a standard calc for the house loads for (B)
Then (C)

1. 3 VA X SQFT of all dwelling space.
2. 1500VA X each kitchen and laundry circuit in each unit
3. Nameplates ratings of all appliances, if they dont know the range I use 8kw per unit.
4. Motors
5. Larger of Heating or Cooling (Nameplate)

Then apply the table 220.84(B)
Done

 

[Tainted]

How so? 220.84 does not require you to anticipate the future, they do require I leave the office though.
I do 220.84 calcs like so;
First I hit Starbucks,
Next go over (A) make sure you meet the criteria.
Then do a standard calc for the house loads for (B)
Then (C)

1. 3 VA X SQFT of all dwelling space.
2. 1500VA X each kitchen and laundry circuit in each unit
3. Nameplates ratings of all appliances, if they dont know the range I use 8kw per unit.
4. Motors
5. Larger of Heating or Cooling (Nameplate)

Then apply the table 220.84(B)
Done

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