Residential load calcs 1

[Arp32]

Architect question for those of you that do residential projects...

When you guys do load calculations for a residential project (an addition or remodel, not a new home), how common is it for you to do them based on the previous 12 months of electrical bills?

I'm used to doing the math myself on smaller projects (3W per SF of livable, add fixed appliance loads in VA, take the reduction, etc...), and that's all I recall seeing on larger engineered projects previously.

Is there a reason why you'd use historical electrical bills instead? Working with a new engineer who uses historical utility bills and he says this is the normal way to do it. What's the standard of care with this from your perspective?

-Jason, Architect

 

[waross]

The Canadian Electrical code and I understand the NEC has similar provisions.

(9) Where additional loads are to be added to an existing service or feeder, the augmented load shall be permitted to be calculated by adding the sum of the additional loads, with demand factors as permitted by this Code to the maximum demand load of the existing installation as measured over the most recent 12-month period, but the new calculated load shall be subject to Rule 8-104(5) and (6).

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Arp32]

Thanks, I didn’t think to search “12 months” in the NEC.

Found it in 220.87

-Jason

 

[davidbeach]

I've never seen a residential rate schedule that included demand charges, and therefore there wouldn't be demand data in the bills.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[waross]

Good catch David.
However I have seen demand charges on a hotel. (But I have also seen a lot of hotels with no demand charges)
On industrial work we have used past power bills.
And back in the day before power factor control panels became common and power factor correction was more art than science, we used back power bills.
Actually power bills are still useful for power factor correction but only to see the highest month and order the appropriate PF correction panel.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[cranky108]

That said David, some electric companies can give you hour by hour usage for homes. This started as a load survey tool and has changed into the ability for time of use rates.
This extended to directional metering for customers with renewable generation, and I believe these meters are standard at some utilities.
But one would need to ask for the additional data, it is not given with a typical utility bill.

 

[davidbeach]

Hourly kW or just hourly kWh? If the AHJ is looking for 15 minute demand numbers, hourly kW may not be sufficient as it hides the peak values. Hourly kWh hide the true peaks even more so.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[Arp32]

This is what I saw. Just totals by month:

-Jason, Architect

 

[stevenal]

Arp32,
Not much to go on. I expect the zero in the kW column simply means this is not a demand customer. Take the kWh and divide by the number of hours since the last read, and you will have the average kW for that time period, with no information regarding the mins and maxes.
As Cranky indicated, many utilities can give you much more granular information if you ask for it.

 

[jghrist]

Some utilities provide more detail. I can go in the Duke Energy website, or use their mobile app, and download kWh readings in 30 minute intervals for my home energy meter. With this, you can calculate 30 minute demand values.

 

[resqcapt19]

I can't see my demand on my utility website...they only show me a kWH usage in 15 minute increments.

 

[jghrist]

I can't see my demand on my utility website...they only show me a kWH usage in 15 minute increments.

kW demand is the difference between consecutive 15 min kWh readings times 4.

If you have an average power use for 15 minutes of 1 kW, then the kWh usage for that period is 1 kW time 0.25 hr, or 0.25 kWh.