Existing apartment load calculation to add new HVAC system

[AmpedUpElectric]

How Would I figure the existing load on a 1950s apartment/condo complex? The local inspector said why don't you get the demands from the local power company and they gave me what I believe is the KWH for a 30 day billing period. So how do I break that down into amperage to see if existing feeds to apartments is large enough to handle new loads? the service coming into building is 208 3 phase then the homes are 208/120 single phase so they use like a-b then b-c then c-a and so on.

I believe 540 KWH in 30 days

do I divide the 540,000 by 30 days and then that by 208?


or do i divide the 540,000 by 30 days and then divide that by 24 hours in a day then divided that by 208?

Please help me!!

 

[GoldDigger]

You really need to have peak demand figures (typically 15 minute interval) from POCO, not just total kWh for the billing period. And that usually is available only for commercial services for which there is a demand charge in the bill as well as a usage charge. The inspector is misguided.

That said, you need to divide kWh by h to get average kW.

That means dividing by (30 x 24) for a 30 day month. How you get from kW to amps (per line) for three phase service requires dividing by (208 x 1.732) to get the line current.

But that is futile in your case, since you have no idea of whether the load is 7x24 or 6x8 hour days, nor whether it has a lot of short time peak loads (such as all the heating equipment starting up in the morning.)

An apartment complex is less likely to be severely peaky than some factories, but it still will have a variable load profile over the course of the day.

 

[JFletcher]

Welcome. You need to know what kind of heating they have; electric heat is more of a draw in winter than AC is in the summer in most places. If every apt has electric baseboard heat, and it's 20* outside, that's when you'd have your highest loads. Spring/fall with moderate weather and little heating/cooling load, the KW-hrs used then will be substantially less.

looking at my last bill, my peak kw-hr was August and Sept with 1712 and 1581, lowest was Nov-Feb with around 850, though we have gas heat.

540KW-hr in a month is 18KW-hr/day, or 750W/hr. That may seem low, but work the math backward with a larger load; say you leave just a 3750W oven (5x the 750) on 24/7 all month. That's 3.75kw/hr x 720hr/month = 2700KW used, 5x the 540KW. At $0.12KW-hr, that would be a $324 electric bill, for just having 15.6A of constant, continuous load on a 240V service (15.625 x 240V = 3750W).

 

[AmpedUpElectric]

Just a little more information, at this time the heating system is provided from the owner as they have a boiler in the basement, hot water is also included from the same boiler.

thank you so much, I will talk to inspector on monday,

Have a great 4th guys!! thanks again

 

[Smart $]

Without peak demand from POCO for one year period, you must either do a regular calculation (standard or optional) or get a data logger, record current data for a 30-day period, and adjust accordingly per 220.87 Exception.

 

[Sahib]

What size the disconnecting means, feeder cable etc.,? What if oversized for apartment existing connected loads........?

 

[Smart $]

...What if oversized for apartment existing connected loads........?

This process is a part of that determination. The service will have to be oversized to connect additional load.

 

[Ingenieur]

Central heating
central HW
Laundry? Washer? Dryer? In apt or central room?

540 kwh seems low even with central htd
Assume avg peaking factor of 3
1.5 diurnal
2 summer winter elec clg vs gas heat (does not apply here, yet)

1.5 x 540000 / (24 x 30.4 x 208) = 5.3 A

 

[Smart $]

...
540 kwh seems low even with central htd
...

Sure does. Perhaps only for one unit. :blink:

 

[JFletcher]

Sure does. Perhaps only for one unit. :blink:

I figured the numbers were for one unit as well. OP did say that heating (and hot water) are taken care of by a boiler system.

Lowest month usage I had here last year was 788 for a 1200 sq ft house. We have a gas stove and fireplace but everything else (water, oven, dryer) is electric, and most lighting is still incandescent.

AmpedUp, as others have mentioned you still need a calculation of each unit, and w/e loads are on the house panels.

That said, if there is an existing HVAC system being replaced, a new one, even if larger, will likely use less power than the old due to being more efficient.

 

[Ingenieur]

Sure does. Perhaps only for one unit. :blink:

seen stuff close to that
sr citizens hi-rise apts
central heat, hw, laundry
a cafateria (although they have a small gas range)
no ac
only load is the fridge and some lighting, tv, etc
usually only 1 person per apt

Did the design for the rehab of a bunch of these
did site inspections
kind of sad, mostly elderly woman (they live longer)
warehousing old folk

 

[Sahib]

This process is a part of that determination. The service will have to be oversized to connect additional load.

Exactly. Minimum service size for residence 100A. Suppose maximum demand taken as connected load. Then what balnce capacity remains in 'oversized' design, if it actually present in OP case, to take care of proposed HVAC load?

 

[AmpedUpElectric]

Yes Central Laundry, problem is #4 tw to home and 70 MCP,

 

[kwired]

Exactly. Minimum service size for residence 100A. Suppose maximum demand taken as connected load. Then what balnce capacity remains in 'oversized' design, if it actually present in OP case, to take care of proposed HVAC load?

That is today's standard, plus OP said this is 1950's apartment complex. I can imagine it only having 60 or 100 amp service to 4-6 units easily if it has always had gas heating and gas water heating.

If same thing were built sometime after 1985 - 1995, probably would have a 200 amp service, even though load calculation may be under 100.