Adding or upgrading a dwelling unit to an existing multifamily dwelling

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Tainted

Tainted

Senior Member
Location
New York
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Engineer (PE)
Suppose there's an existing 30 unit apartment building and we would like to add an additional unit or upgrade a unit to the existing apartment building.

The existing 30 units is supplied by a existing 500A fuse.

How will i know whether or not the 500A fuse has the capacity to add or upgrade a unit?

would utilizing NEC 220.87 at the 500A fuse be appropriate for this?

220.84 Cannot be used since all the loads are unknown.

What are my options in this situation?
 
A 30 unit building served by a 500 A fuse? That is not even 17 Amps per unit.

I guess you could use 220.87, but I would be looking real close at what is actually going on there.
 
petersonra said:
A 30 unit building served by a 500 A fuse? That is not even 17 Amps per unit.

I guess you could use 220.87, but I would be looking real close at what is actually going on there.
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Why is the 500A fuse a concern? What is the rule of thumb for the right fuse size?
 
Dennis Alwon said:
If it is an existing building then the loads should be known. Is there electric ranges or gas, electric heat or gas, a/c units, water heaters,etc
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no electric cooking equipment, I think boiler is fed by the building public meter, there are windows ac units. There is no way I will have access to all 30 apartments just to get every load to do calculations using 220.84. Why can't I just use 220.87?
 
Tainted said:
no electric cooking equipment. There is no way I will have access to all 30 apartments just to get every load to do calculations using 220.84. Why can't I just use 220.87?
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you can. just get a years worth of data from the electric company, or install a recording meter to get the data.

Incidentally, you may not need access to all of the apartments to do the load calculations. If they are all the same you can do it by looking at a single unit.

But again, 17 Amps per unit seems very low.
 
petersonra said:
There is no rule of thumb. You have to do the calculations. But 17 Amps per unit on average sounds very low to me. That would not even work for a motel.
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I know, I just wanted to know for estimating purposes the fuse size to expect.
 
Tainted said:
no electric cooking equipment, I think boiler is fed by the building public meter, there are windows ac units. There is no way I will have access to all 30 apartments just to get every load to do calculations using 220.84. Why can't I just use 220.87?
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You can if you have that data
 
Tainted said:
no electric cooking equipment, I think boiler is fed by the building public meter, there are windows ac units. There is no way I will have access to all 30 apartments just to get every load to do calculations using 220.84. Why can't I just use 220.87?
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There is no way that all the juice required to provide 30 units worth of window A/C and whatever else needs to be run simultaneously can be supplied by a 500 A fuse. Someone has given you bad information, IMO.
 
petersonra said:
There is no way that all the juice required to provide 30 units worth of window A/C and whatever else needs to be run simultaneously can be supplied by a 500 A fuse. Someone has given you bad information, IMO.
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I've seen 400A fuse with 20 units, but maybe you're right I'll verify when I go there
 
petersonra said:
you can. just get a years worth of data from the electric company, or install a recording meter to get the data.

Incidentally, you may not need access to all of the apartments to do the load calculations. If they are all the same you can do it by looking at a single unit.

But again, 17 Amps per unit seems very low.
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tell me how many amps per unit is appropriate for estimating purposes
 
Tainted said:
tell me how many amps per unit is appropriate for estimating purposes
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It will depend on how large the units are and how much equipment is electric. I would suggest you perform a load calc for a typical unit with whatever assumptions you need to make and then use that as a basis for your multi-unit calculation. It won't be exact, but will provide a value for your estimating purposes.
 
Dennis Alwon said:
If these units are one or two bedroom, everything gas, no a/c unit then it could potentially work at 17 amps
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Built in the 1920's that was probably the norm. Betcha the apartments only get 120V not 120/240.

petersonra said:
In an earlier post he said the units have window air conditioners.
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Anybody can plug anything in anywhere.

I would say that you are going to have a problem adding or splitting up an apartment. Dunno, but I suspect at least that apartment is going to trigger a new service which means getting rid of that 500A fuse service.

-Hal
 
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