Generator feeder questions

[LarryFine]

I guess there aren't that many.

Since five of the main disco's six neutral terminals are in use (neutral in, neutral out, EGC out, and two GECs), and the ATS/generator's neutral uses the sixth hole, can the EGC (#8cu based on the 70a generator's feeder) either:

A. Land on a lug bolted to the main disco's enclosure?

B. Be split-bolted to one of the existing bare GEC's?


Added: So the capacity questions have to do with which loads may be left on when the ATS switches over, right, and not everything in the house a careless customer might attempt to use?

 

[jap]

The customer on this job lives alone, and is handicapped. He basically lives in front of his computer. His power usage is that of a shed.

It almost seems my questions are being ignored, and I started this thread. :? Please go over my last few posts to put together the facts.

I don't mind the sizing questions and answers, but I would like my questions addressed, too, like whether the disco/ATS nipple requires bonding.

The rule for that the auto from switch requiring the generator to handle the full load is in the 2011 code.

Although you mentioned the panel only had only 10 or so breakers in it, I don't recall you ever mentioning what the actual full load of the service was. When I heard 4/0 I assumed it was a 200 amp service.

I also recall you mentioned there was no load shed in place in your installation


Jap>

 

[jap]

Everything in the house will be left on in a power outage when the generator comes on if there is no load shed in place.

That's the whole point of the rule for the generator to be sized for the full load when using an auto xfr switch.

All the other other opinions of how all those loads won't be on at the same time or there's no way it coul ever get that high may be true.

But the code says it has to be sized for the full load not what you assume it may or may not be.

Jap>

 

[ActionDave]

Everything in the house will be left on in a power outage when the generator comes on if there is no load shed in place.

That's the whole point of the rule for the generator to be sized for the full load when using an auto xfr switch.

All the other other opinions of how all those loads won't be on at the same time or there's no way it coul ever get that high may be true.

But the code says it has to be sized for the full load not what you assume it may or may not be.

Jap>

There is no load to shed.

If there were an air conditioner to hook up to a load shedding relay would you be alright with the same size generator?

 

[LarryFine]

The rule for that the auto from switch requiring the generator to handle the full load is in the 2011 code.

What defines "full load"? Does that mean a typical 200a service must have a 50KW generator if there is no load shedding?

Although you mentioned the panel only had only 10 or so breakers in it, I don't recall you ever mentioning what the actual full load of the service was. When I heard 4/0 I assumed it was a 200 amp service.

The service is 200a, although it's way overkill in this house, should have been 100a. Maybe 1000 sq. ft., one floor, no central AC.

I also recall you mentioned there was no load shed in place in your installation.

The ATS has a four-circuit module, but I haven't connected anything to it. To shed anything besides HVAC, wouldn't I have to employ a 24v transformer and normally-energized contactors?

 

[LarryFine]

Everything in the house will be left on in a power outage when the generator comes on if there is no load shed in place.

Are you saying that it's required to make that presumption? Is that based on the same rules as load calculations, or actual loads, like lights, computers, TVs, cooking appliances, etc?

That's the whole point of the rule for the generator to be sized for the full load when using an auto xfr switch.

All the other other opinions of how all those loads won't be on at the same time or there's no way it coul ever get that high may be true.

But the code says it has to be sized for the full load not what you assume it may or may not be.

Again, how is that number derived? What defines "full load"?

 

[JFletcher]

A 16 kilowatt generator is good for 67 amps per leg. Do a load calculation, if it comes out under 67 amps, you are good to go. No further action needed, no load shedding required.

10 circuits? Let me guess, you have a water heater, a dryer, a range, an oven, two small appliance branch circuits, one circuit serving half the house, a bathroom circuit, dishwasher circuit, and the 10th serving the carport, laundry, and outside receptacle. Amiclose? Even all of that on at one time you are not going to shut that generator down on overload.

As you stated, the dryer will not come on by itself after a power failure.

 

[LarryFine]

Agreed, a dryer won't self-restart, which brings me to repeat: Do the capacity questions have to do with which loads may be left on when the ATS switches over, or everything in the house a careless customer might attempt to use?

I will study the panel in more detail Monday.

 

[JFletcher]

We've always sized based on a load calculation, not a worst case scenario. If the customer is cooking Easter or Thanksgiving dinner for 15 people, has all for electric range burners on high, has the oven on high, someone's taking a shower so the water heater is on, and someone is doing a load of laundry, then yes, there is a real chance that *IF* the power were to go out right then and there, when the generator picks up the load it will choke and cut out. That real chance has about a .001% probability of happening over the course of a year.

Eta: this is not a hospital, an airport, or a nuclear facility... It's for a house. It's not legally required nor an emergency standby generator.

 

[kwired]

Again, this house has no AC, and dryers use centrifugal switches, so they won't restart after a power failure.

Also, we're still on the 2011 NEC. Is that sizing rule in effect here?

All that definitely make sense! And makes me understand why you can't just go by the average usage!

I have more questions, but don't want to hijack this thread.
I'll either use search options or start a new thread.
Thanks!

If one used just average usage you should be able to only have 30 amp service conductors, right?

Everything in the house will be left on in a power outage when the generator comes on if there is no load shed in place.

That's the whole point of the rule for the generator to be sized for the full load when using an auto xfr switch.

All the other other opinions of how all those loads won't be on at the same time or there's no way it coul ever get that high may be true.

But the code says it has to be sized for the full load not what you assume it may or may not be.

Jap>

Are you saying that it's required to make that presumption? Is that based on the same rules as load calculations, or actual loads, like lights, computers, TVs, cooking appliances, etc?


Again, how is that number derived? What defines "full load"?

IMO this is another place where NEC has crossed it's own line of not being a design manual. Legally required standby system - I understand needs to be able to transfer load and be capable of supplying that load. Optional standby system - IMO it is just a poor design issue if it can't run the load.

Outside of an AHJ being extra particular about how to handle this, I wouldn't count the dryer - it won't automatically start. I probably would also not count ranges, ovens, etc. Those are usually attended while in operation - my belief is the intent (or at least good design that NEC should keep it's nose out of) is to not automatically transfer too much load should the transfer occur when nobody is home.

Things like disposers, microwaves, garage door openers, are all in use while someone is there. The dishwasher or even clothes washer may have timed cycles or be left running when someone leaves, often will not automatically restart after a power failure though.

What does automatically transfer that is potentially significant loads in most cases whether anybody is home or not? HVAC, water heater, well pumps, pool pumps, hot tubs.

 

[jap]

this is not a hospital, an airport, or a nuclear facility... It's for a house .

You phrase that as so the rule shouldn't apply to just a stupid little old house.

But it does.

JAP>

 

[electrofelon]

You phrase that as so the rule shouldn't apply to just a stupid little old house.

But it does.

JAP>

Yes in does. In reality, it probably depends on how rigorous your inspectors are, or if you have inspections for this type of thing at all. Here, I just use my judgement. I'll put a 11KW generator on a 200A ATS with no load shedding all the time.

 

[mkgrady]

What defines "full load"? Does that mean a typical 200a service must have a 50KW generator if there is no load shedding?


The service is 200a, although it's way overkill in this house, should have been 100a. Maybe 1000 sq. ft., one floor, no central AC.


The ATS has a four-circuit module, but I haven't connected anything to it. To shed anything besides HVAC, wouldn't I have to employ a 24v transformer and normally-energized contactors?

Yes, you either have to supply a control transformer and relays or use the Generac relays that don’t need control wiring. They make relays that look at frequency. If the frequency dips they shutdown that load. You can use up to eight of these relays. You can even use a lockout feature that keeps certain loads off while the generator is running. Not sure how the relay knows it is being fed by the generator.

 

[LarryFine]

In reality, it probably depends on how rigorous your inspectors are, or if you have inspections for this type of thing at all. Here, I just use my judgement. I'll put a 11KW generator on a 200A ATS with no load shedding all the time.

I pulled a permit, so there will be an inspection. A few years ago, I installed a 20KW with two 200a ATSs in master/slave mode, and used one shedding module to shut down the three HVAC units, but nothing else. There was no mention of load shedding by the inspector (different county).

 

[LarryFine]

You can even use a lockout feature that keeps certain loads off while the generator is running. Not sure how the relay knows it is being fed by the generator.

In this ATS, the module is installed in, and controlled by the ATS. I am capable of designing any control circuitry required.* The question is whether any is required here. I'm hoping not, since I'm almost done and didn't figure shedding in the price.

Again, I am returning tomorrow to make final connections, which means routing the service conductors through the ATS; everything else is done. I will check the circuits. I'll take some pictures and post them, along with the major circuit breakers.


*I say that because I routinely install new commercial-kitchen fire-suppression wiring, both with and without pre-wired control boxes (I prefer the latter) and perform upgrades to existing systems, which can be more challenging, but more enjoyable.

 

[kwired]

Yes, you either have to supply a control transformer and relays or use the Generac relays that don’t need control wiring. They make relays that look at frequency. If the frequency dips they shutdown that load. You can use up to eight of these relays. You can even use a lockout feature that keeps certain loads off while the generator is running. Not sure how the relay knows it is being fed by the generator.

One simple method is to supply the load through a normally closed contactor, with the contactor coil supplied by the generator output. It will interrupt the load when generator runs a routine exercise cycle. If there is any kind of aux contact capability on the transfer switch you can run through that and then it would only shut down the load when transfer switch is in standby position.

 

[jap]

One simple method is to supply the load through a normally closed contactor, with the contactor coil supplied by the generator output. It will interrupt the load when generator runs a routine exercise cycle.

That's just a silly way of doing things.

JAP>

 

[kwired]

That's just a silly way of doing things.

JAP>

Why? This was for something you know you don't want to run when on standby power. About the only way it fails is if contactor coil is burned out, but the fact it hardly ever gets energized reduces that chance quite a bit.

Allowing the item to run if generator is lightly loaded is an entirely different ballgame from isolating the item any time you are running on standby power.

 

[JFletcher]

You phrase that as so the rule shouldn't apply to just a stupid little old house.

But it does.

JAP>

My comment wasn't aimed at saying that a house does not need to be calculated correctly, it does, it's just that as this is not a 700 or 701 application, the impact of overloading the generator is quite minimal.

As far as I know, there is no NEC approved way to calculate the actual loads that a house will see when flipped onto generator power. Some things that wind up in the calculation should not be, like the microwave, the dryer, and anything else that does not restart automatically.

For the record I have never seen a 50 KW generator on a house with a 200-amp service. The largest I've seen was at a million-dollar + house that had a 36 kilowatt liquid cooled generator with a ATS to a 320 / 400 amp service, no I did not go inside it to see if there were any loadshedding modules installed. We have installed load shed modules on a few houses with dual or triple hvac to prevent generator overload

 

[kwired]

My comment wasn't aimed at saying that a house does not need to be calculated correctly, it does, it's just that as this is not a 700 or 701 application, the impact of overloading the generator is quite minimal.

As far as I know, there is no NEC approved way to calculate the actual loads that a house will see when flipped onto generator power. Some things that wind up in the calculation should not be, like the microwave, the dryer, and anything else that does not restart automatically.

For the record I have never seen a 50 KW generator on a house with a 200-amp service. The largest I've seen was at a million-dollar + house that had a 36 kilowatt liquid cooled generator with a ATS to a 320 / 400 amp service, no I did not go inside it to see if there were any loadshedding modules installed. We have installed load shed modules on a few houses with dual or triple hvac to prevent generator overload

Couple larger houses I have wired, did not have a generator when built but they did mention interest in possibly having a generator someday. Those houses had multiple service panels - I placed most the loads that were likely to be desired to be on standby power all in the same panel so one could more easily supply that panel via a transfer switch when the time came to install a generator.