Electric Demand for Feeder Circuit Sizing 1

[Mstrvb19]

I have been doing quite a bit of research looking for an applicable source to determine good values for coincident/non-coincident peak residential load values for sizing of a future distribution feeder. I know that many utilities use different assumed values for future residential customers, but I am not a utility employee per-se, nor does my project's confidentiality allow me to contact the utilities at this time. I need something that has a good logical basis and can be related to size of home/size of service drop. Searches in government public domain data are not helping either. Someone out there must have a good data source with a reasonable basis for determining a home's expected coincident/non-coincident electric requirements. Thanks in advance, I'm stuck. Based on NEC requirements and what we expect the homes to have, I would say the electric service drop will be 400A.

 

[saladhawks]

Take a look at thread238-157887.

 

[jghrist]

You must have some very large homes. Most 120/240 volt residential services are 200A and the load is considerably lower. I'd consider 3Ø services if the load is really that large.

 

[Mstrvb19]

Thanks, Saladhawks. This was helpful, although most of the homes in the development are off the chart. This is very difficult to define when one doesn't have access to customer loading data.

jghrist, you are correct, these homes will be quite large, at least by middle-class standards, at 7,500 square feet. In all likelihood, one of these homes could use upwards of 40 kW with the amenities they are likely to have. The 400A service drops are becoming fairly common in my area. Nonetheless, my scope is to determine the construction cost of the off-site utilities. The actual on-site electric system will be part of my client's contractor's scope.

 

[tinfoil]

We use a 5kW peak-coincident per customer value when we are sizing our individual distribution transformers, so long as there are six or more customers. But nearly all of our homes are vanilla 200A services. We are also aided by the fact that we are winter-peaking, and allow extra capacity (125%) out of our transformers when it is below freezing.

Your primary voltage was unstated. Assuming that it is reasonably large, you can get a LOT of homes on a piece of primary cable, without approaching thermal or voltage drop limits, even at single phase.

For example, our 1/0 Alum (our SMALLEST wire) underground cable is good for ~175 amps in duct, and our voltage is 7200V (L-G). This is 1250 kW of capacity, or 250 standard homes (assuming short distances so voltage drop is a non-issue. Three-phase would be triple that.

Of course, we don't come anywhere close to that in practice.
My point is that the primary infrastructure costs are almost independant of the individual home's loads, until you get to the transformers, and secondary services to the homes.

The secondary services themselves cannot take advantage of coincident factors, as you have to size them for each individual home's worst-case scenarios.

So I think your quandary boils down to sizing your transformers. (Unless you are looking at 500+ homes, in which case your local utility may insist on some 'upstream' improvemments at your client's expense).

 

[Mstrvb19]

tinfoil,
My task is to size the equipment for the 'upstream utilitiy', or what I call off-site power. This is for a resort where there will other large loads besides homes, of which we'll have around 430 units or various sizes. There will also be a resort hotel and some other amenities. I agree with your assessment of the secondary sizing. Basically, we have to determine the cost to the utility of new infrastructure and infrastructure upgrades.

 

[tinfoil]

As was mentioned in the thread linked above, you will have to think about what happens after a prolonged power outage. Your coincident factors tend to go towards 1.00 for Air-con loads in the summer, and for electric heat in the winter (plus domestic hot water if the outage is long enough). BYW, these are the largest loads in the house.

If you size the feeder for enough capacity to deal with the voltage drop issues under those conditions, you nearly always fix ampacity issues as well.

This will give you lots of margin under 'normal' conditions, and you will be quite close to the 'economic loading point' after accounting for I^2 * R losses.

 

[tommom]

Look in Westinghouse Distribution Engineering Handbook, vol.3. It's old, but there is an outstanding chart of coincident residential load info there.

 

[viktorel]

If it's true you'll better to consist "load table" with all consumptions, & show it to utilities service. It table definite necessarily in such load.
Viktor, Ukrane

 

[Jebb]

You had said before "Basically, we have to determine the cost to the utility of new infrastructure and infrastructure upgrades." I wouldn't try to estimate what is needed to upgrade the infrastructure on the utility side because you don't know any of the factors that the utility has to deal with (ie. feeder loading, substation loading, feeder layout, etc....) and gathering this info would be close to impossible to get without talking to the utility. Without knowing many of the facts, it sounds that your project has some substantial load to be served so I would contact the utility before going much futher in your estimating. Most utilities will sign a confidentiality agreement if you project is "top secret" right now. This will give them time to give you a study grade estimate (10 - 20%) so you can give a good estimate to your end customer and don't have to deal with the end results when the project goes over budget due to cost over runs or the utility says they can't complete the upgrades needed to meet your schedule.