Generator sizing

[laketime]

Having some trouble figuring out what to supply for a customer who wants and emergency back up generator. They want to back up these items:

(2) water heaters; supplied by (2) 2pole 30amp breakers
(2) refrigerators
(1) well pump; supplied by a 2pole 20amp breaker
(1) master bedroom; supplied by (2) 1pole 20amp breakers
(1) family room; supplied by a 1pole 20amp breaker
(1) washing machine
(1) master bath; supplied by a 1pole 20amp breaker
(1) boiler; supplied by a 1pole 20amp
(2) kitchen circuits; 1pole 20amps
(1) dishwasher; 1pole 20amp

I am coming up with something in the range of 25kw. Does that sound about right.

This is a single family dwelling with a single phase 120/220v 200 amp service

 

[augie47]

Possibly a bit high, but without specifics it;s just estimating...considering motor start up..
When it come down to $$, homeowners often decide it's not that important to wash dishes and clothes....

 

[robbietan]

Possibly a bit high, but without specifics it;s just estimating...considering motor start up..
When it come down to $$, homeowners often decide it's not that important to wash dishes and clothes....

I agree. Clothes washing and dish washing are hardly fare for sizing generators.

 

[kingpb]

The connected load is one thing, but the running load is what matters for minimum size anyway, after that you need to know the order in which the loads will start.

If all the loads you have listed could come on simultaneously then that will be the determiing factor. You need to create a duty cycle.

 

[necnotevenclose]

I'd suggest sending that list to a few gen reps. They can help size and discuss if the gen should be sized a bit bigger for any future needs of the stand-by gen.

 

[Robert Hutcherson]

If the customer is capable of managing loads, I would suggest installing a manual transfer after the service main and ahead of the main panel. This would allow for greater flexibility for using loads. Additionally, you could likely use a 10 to 15 kw generator and provide more economical proposal for customer. Just one persons opinion, and certainly not the only opinion or idea.

Good Luck.

 

[kingpb]

Based on many assumptions, if all loads came on simultaneously, with some diversity, i.e. ckts not all loaded to max, the caterpillar program indicates you may need a 40kW machine.

 

[gar]

2100825-1018 EST

How much money does this family have?
How many and what type of people live in this house?
How long are power outages expected?

When my great-great-grandfather was the first permanent white settler in Delta County Michigan there were no roads, no electricity, no phones, there was running water in the Whitefish River, and friendly Indians. In those days people could survive with their own abilities.

When there is a power outage today what do you really need? Somewhat depends upon the likely length of the outage. But mostly you need refrigeration, heat, a little light, and some communication capability. An all electric house probably requires a fairly large generator. If a house was mostly gas, meaning gas heat, cooking, and dryer, then you might scrape by with 1 KW, but marginally. Personally I have a 5 KW portable and gas for most of the high energy requirements. With this I can run my two freezers, one refrigerator, some miscellaneous items, and my two neighbors refrigerators.

The type of people in the house has a great influence on what is needed. If these are typical persons of today that are mechanically and electrically clueless, have never been boy scouts, then they have to foot a large bill for a backup system that will see little use.

Instead of a very large generator one might put some of that money into a passive solar water heater. By the way you can dry clothes outside in 0 deg F weather. You can cook on a wood fire in the backyard, and heat water the same way. You can wash dishes and clothes by hand, that is the old way.

The whole point is that there are a lot of ways to solve the problem. There has to be a balance between cost and what is purchased.

Today I believe everyone should have a generator, but not too large. I think the power system is going to become less reliable, and there may be unexpected problems.

Relative to the original question you really need to analyze the family, and their real needs. Then you do an intelligent analysis of how to design the overall system.

.

 

[sinnadurai sripadmanaban]

generator sizing

generator sizing

For accurate results you should give actual load like-bulbs&wattage,home appliances & wattage ,hp of pump etc and time of use like from...to.....for each item.Then add in a MS excel worksheet(Time in X-axis,Load in Y-axis).Finally add for future loads considering starting method used for the pump-S.Sri

 

[thofa4u]

back up generator ratings

back up generator ratings

considering load a 15 kw generator is ideal, but 4 future reasons a 20 kw is suitable.

 

[kbsparky]

They could do with a 10 kW unit, but would have to manage those 2 water heaters for non-simultaneous operation.