Generator sizing - Kohler Software

[designer82]

I am using the kohler software to size a generator. This is for a commercial kitchen (which is part of a high school)

All the kitchen loads need to go on the generator.

I have put all the equipment on 1 step on the generator but wondering if this is overkill, maybe I should apply a demand and put some loads on a 2nd step?

Appreciate your thoughts

 

[designer82]

Section 220.56 allows a 65% demand factor in my case, so maybe put 65% worth of loads on the generator 1st step then the remaining 35% on the 2nd step?

Or just do 65% on one step and that's it?

 

[tjkusa]

How do you plan to step the loads separately? If they are all on the same bus the generator will see them on transfer regardless of how many steps you put in the software without additional controls.

What is the sKVA on the single step load results? Is the sKVA large, driving the size of your generator up well above the running load or is the gen. sizing result the same same approximately as your load calculation and running kW result? If the latter, not much reason for stepping the loads. Generally only with large and/or high quantity of motors (6x FLC inrush software likely approximates) are you going to have a large sKVA requiring the need for multiple steps.

To step the loads separately, you could:

• have 2 panel boards each supplied from its own respective ATS and set transfer time on one ATS 10s longer than the other (or any variation of similar arrangement)
• Interlock pilot control circuit of any large motor loads you have with timer delay to not allow start until after 10s upon power up

Both of these options will add cost so probably best to avoid stepping unless you have a large starting kVA for single step. One freebie may be to look over any large HVAC unit user manuals that will be on site as sometimes you will find they have a programmable or preset startup delay of about 30s or other duration upon power up. If they have this, you could input this equipment as a separate step and reduce your sKVA.

 

[designer82]

Thanks for the response. That makes sense for sequencing the loads.

I think the better question is whether you size the generator for the demand load or for the connected load?

Enter all the connected loads in the Kohler Power Systems Software? Maybe the software already applies a demand factor on its own

 

[tjkusa]

You’re welcome.

My background is primarily wastewater. We would size for peak demand. Most process applications were n+1 redundancy so if we had 5 blowers, we would size generator to start and run a peak demand of 4 blowers with up to 6 steps as needed. 1st step is always general loads you have no control of which may turn on the instant the bus is energized (lighting, receptacles, electric unit heaters, etc.), next step should be largest skVA requirement you can control with a start delay, 3rd step is 2nd largest skVA requirement, etc.

You could explore applying the 220 demand loads based on your experience with the loads at your site.

Check out the Kohler application manual for their software: https://www.tawinc.com/Assets/tawinc/files/application-manual.pdf

I am more familiar with CAT, Cummins, and mtu sizing software. These do not apply a demand factor for you. You would have to input manually at load level. Some do however default to 75% running load for motors i.e. 75% FLC which is reasonable in some applications.