Service disconnect and generator transfer switch setup

[howardrichman]

I had posted recently about 2 services w/ 2 meters on detached garage. The 1 meter is for the house needing 300 amp conductors. When I did a load calculation, the house uses no more than approx. 135A or 32KW. The house was wired with many circuits, and wants to keep the breaker quantity instead of rewiring to reduce the excess of existing breakers. The homeowner wants a generator for the house. I was originally to use a 300 amp transfer switch at the house. Bring in 350MCM alum. service conductors in, and run 2 and 4 SER to a 200 amp main, and a 100 amp sub panel; tapping the load side of the 300 amp breaker in the transfer switch. I doubt he'd want to buy a generator of that size or load shed the hell out of a smaller unit. The other option is to install a trough there w/ two whips to the panels, and use a smaller transfer switch inside or out; to a smaller panel for essential circuits. I'd rather put the trough in the basement, but the trough size would come past the panels for code.

What to do?
HR...

 

[Smart $]

... I doubt he'd want to buy a generator of that size or load shed the hell out of a smaller unit. ...
What to do?

What does the owner want?

Assuming you are talking ATS, your generator has to be rated not less than the calculated (135A) or unshed load... not the combined rating of supplied equipment.

 

[howardrichman]

What does the owner want?

Assuming you are talking ATS, your generator has to be rated not less than the calculated (135A) or unshed load... not the combined rating of supplied equipment.

I used a conventional load calculation method. 1st 10K@ 100%, remaining loads@ 40% etc. 3250sq. ft. 2 of all appliances; mother/daughter setup. I'd like to give the practical, then the alternative plan for power generation. For the whole house gen, I'd need a 36KW unit. An alternate plan would be a small 10-14KW unit from an existing sub panel for outages. big cost differences.


HR...

 

[Smart $]

I used a conventional load calculation method. 1st 10K@ 100%, remaining loads@ 40% etc....

Known by Code geeks as the Optional Method.

 

[GoldDigger]

I used a conventional load calculation method. 1st 10K@ 100%, remaining loads@ 40% etc. 3250sq. ft. 2 of all appliances; mother/daughter setup. I'd like to give the practical, then the alternative plan for power generation. For the whole house gen, I'd need a 36KW unit. An alternate plan would be a small 10-14KW unit from an existing sub panel for outages. big cost differences.


HR...

There are really four different ways to go:

1. Whole house ATS and a generator rated for the calculated house load.
2. Whole house ATS and a generator combined with automatic load shedding to bring the load down to the generator rating.
3. Critical loads subpanel with an ATS feeding it alone and a generator rated for the calculated load of that subpanel.
4. Manual transfer switch, possibly just a breaker interlock, for either whole house or subpanel. If whole house the owner is expected to shed loads manually before tripping the transfer switch.

The fourth one is by far the lowest cost and simplest, but is not a great idea if the house is going to be unattended for days at a time and the owner is worried about losing frozen and refrigerated food.

 

[Iron_Ben]

I used a conventional load calculation method. 1st 10K@ 100%, remaining loads@ 40% etc. 3250sq. ft. 2 of all appliances; mother/daughter setup. I'd like to give the practical, then the alternative plan for power generation. For the whole house gen, I'd need a 36KW unit. An alternate plan would be a small 10-14KW unit from an existing sub panel for outages. big cost differences.


HR...

A 32 kilowatt demand for a home is unusual. I've seen a few that high, but not many. We had a 10,000+ SF all electric house in our service territory that peaked around 70 kw. If the home has a smart meter, perhaps the utility can provide a year's worth of actual rather than "calculated" usage and demand data. I retired from the poco almost three years ago, and when I left we could provide that for any customer. All they had to do was ask, and by now it may be readily available on line (not certain as I moved and left the service area far behind).

I don't know if that would be compliant with 220.87 and otherwise kosher, but it seems like it should be.

 

[electrofelon]

Of course I dont know what the loads are, but I find you can usually run almost the whole house with a not so large generator if you just pick off a few of the large things like dryer, electric cook top, etc. Granted some people may want everything to work and that is up to them and their checkbook, but getting rid of those few large things will reduce costs dramatically. Try to stay with a 200 amp or smaller transfer switch. I would probably just bring 2 sets of service entrance conductors, a 100 and a 200, and hit a main breaker panel with one (probably the 200 A set), and the service rated transfer switch with the other.

Optional standby systems and NEC load calcs are a big annoyance to me. I would like to see the ability to use less conservative calcs for something like a resi generator. Its a shame to have to spend all that money on the equipment only to have it often 25% or less loaded.

 

[kwired]

Of course I dont know what the loads are, but I find you can usually run almost the whole house with a not so large generator if you just pick off a few of the large things like dryer, electric cook top, etc. Granted some people may want everything to work and that is up to them and their checkbook, but getting rid of those few large things will reduce costs dramatically. Try to stay with a 200 amp or smaller transfer switch. I would probably just bring 2 sets of service entrance conductors, a 100 and a 200, and hit a main breaker panel with one (probably the 200 A set), and the service rated transfer switch with the other.

Optional standby systems and NEC load calcs are a big annoyance to me. I would like to see the ability to use less conservative calcs for something like a resi generator. Its a shame to have to spend all that money on the equipment only to have it often 25% or less loaded.

You may have to read between the lines a little, but most electric clothes dryers would be naturally shed when power was lost as you would have to press the start button to make them start again, so in a way they are not automatically transferred, and you wouldn't have to worry about that particular appliance adding too much load and throwing the main generator breaker if you were not home when the transfer occurred. I'm sure others will not think this way about it though, but is something to think about.

 

[GoldDigger]

You may have to read between the lines a little, but most electric clothes dryers would be naturally shed when power was lost as you would have to press the start button to make them start again, so in a way they are not automatically transferred, and you wouldn't have to worry about that particular appliance adding too much load and throwing the main generator breaker if you were not home when the transfer occurred. I'm sure others will not think this way about it though, but is something to think about.

A lot depends on the mechanism in the dryer. Before the electronic controls the "timing" element was essentially the centrifugal starting switch in the motor. If power came back on before the drum had slowed enough to close that switch it would resume with no additional action when the power came back on.
Kwired correctly said "most", including AFAIK all with electronic controls, but if you are going to rely on this feature for load shedding you should try it out first to confirm that your dryer does what you want.

 

[kwired]

A lot depends on the mechanism in the dryer. Before the electronic controls the "timing" element was essentially the centrifugal starting switch in the motor. If power came back on before the drum had slowed enough to close that switch it would resume with no additional action when the power came back on.
Kwired correctly said "most", including AFAIK all with electronic controls, but if you are going to rely on this feature for load shedding you should try it out first to confirm that your dryer does what you want.

How many residential grade gensets/ATS will start the prime mover and transfer to standby power before the dryer drum has stopped?

The dryer we have now does start and stop on it's own during the "wrinkle guard" period of the cycle, but this part of cycle doesn't apply any heat, just tumbles and sounds the end of cycle buzzer every few minutes.

 

[iwire]

How many residential grade gensets/ATS will start the prime mover and transfer to standby power before the dryer drum has stopped?

The dryer we have now does start and stop on it's own during the "wrinkle guard" period of the cycle, but this part of cycle doesn't apply any heat, just tumbles and sounds the end of cycle buzzer every few minutes.

How many inspectors are going to accept this?

IMO none that actually ask you about load shedding.

 

[kwired]

How many inspectors are going to accept this?

IMO none that actually ask you about load shedding.

I did say you kind of had to read between the lines. If you don't have an inspector or need to file a permit, I probably don't worry too much about the load from a dryer for this reason. But if you do, I agree it may be hard to sell them on this being a method of load shedding.

JMO, but NEC crossed the line of being a design manual/specification here. If it were a legally required emergency standby system then they have more business with such requirements, but an optional standby system should not have such requirements - that is the designer or installers problem to deal with.