Generator transfer switch

[ceb58]

Moreover, the scenario you pose only reinforces my rationale. When, if ever, are you going to pull the max load on your house when no one is home? When have you ever seen a house pull the calculated load with everybody home? (Jim in Tampa don't answer this).

I think you are missing the whole point of 702.5. The gen. must be sized for the load it is to supply. Now if some one wants every thing to work in the home as if there were no outage,then it must be sized as such. If they want to only run essential items then it must be sized to that load.
A home could have a calculated load of 350amps by Art.220 but the HO could say they only want certain items to operate. If that load is say 30amp then you could install a 10kw with an ATS on a house with a 400amp service. How you accomplish this is a design issue. Ether load shedding or a separate panel with the back up protection.
Same scenario. If they want every thing to work as normal and the calculated load is 350amp but you use 220.87 (1) and the actual demand (peak) is 120amp then it could be sized to that.
If we,as EC are ask to provide an install then we are bound to follow what code tells us. If a HO wants a price on a gen. to run every thing then we are to follow the code and price as such. Now if the HO is the cheap,bottom feeding S.O.B. as you described or they just cannot afford it then we can give a price to run loads to get them through what ever. Ether way it is supposed to work as it should.

 

[ActionDave]

I think you are missing the whole point of 702.5. The gen. must be sized for the load it is to supply. Now if some one wants every thing to work in the home as if there were no outage,then it must be sized as such. If they want to only run essential items then it must be sized to that load. How you accomplish this is a design issue. Either load shedding or a separate panel with the back up protection.

This is what I am focused on. I am in favor of 702.5 as it existed prior to '08. I don't see why someone needs to give up the convenience of an ATS just because they don't have a separate panel.

 

[ceb58]

This is what I am focused on. I am in favor of 702.5 as it existed prior to '08. I don't see why someone needs to give up the convenience of an ATS just because they don't have a separate panel.

I wish some one would post the ROP and the CMP's comments on the change from 05 to 08. Then we can go from there.

 

[M4gery]

I was told that an easy way to lower the calculated load was to install a relay on the low voltage side of the AC condensing units to prevent them from turning on when the ATS switches over the the generator. Is this true, will it fly?


This is provided that the HO doesn't plan on using the central AC during generator power.

 

[LarryFine]

I was told that an easy way to lower the calculated load was to install a relay on the low voltage side of the AC condensing units to prevent them from turning on when the ATS switches over the the generator. Is this true, will it fly?

Yes. I've suggested just breaking the 24v supply at the transformer so no part of the system can run.

This is provided that the HO doesn't plan on using the central AC during generator power.

Or, they can manually shed other loads so the can use the HVAC if the genny can handle it using with a simple manual bypass.


The big concern here is the maximum load the genny might see when the home is unattended.

 

[iwire]

The big concern here is the maximum load the genny might see when the home is unattended.

Unattended homes are usually lightly loaded.

I think the big concern was the sale of larger generators, not surprisingly this rule was pushed by the manufactures.

 

[M4gery]

So in the OP's situation, he would be fine using the 200A transfer switch that they planned on purchasing, as long as it's installed after the service equipment. After the ATS he can install a sub panel and move all the essential circuits over to it. Or he can change it around so that 1 of the 2 current panels will now act as a sub panel after the ATS. Since the ATS is not switching the whole house, only a single panel, it only has to carry the load of that panel.

Is that all correct? Thanks.

 

[ActionDave]

Unattended homes are usually lightly loaded.

Exactomundo.

I think the big concern was the sale of larger generators, not surprisingly this rule was pushed by the manufactures.

Would not surprise me.

 

[iwire]

So in the OP's situation, he would be fine using the 200A transfer switch that they planned on purchasing, as long as it's installed after the service equipment. After the ATS he can install a sub panel and move all the essential circuits over to it. Or he can change it around so that 1 of the 2 current panels will now act as a sub panel after the ATS. Since the ATS is not switching the whole house, only a single panel, it only has to carry the load of that panel.

Is that all correct? Thanks.

Pretty much, all that maters is that the NEC calculated load the generator is expected to power is less than the generators rating.

How you get there is up to you, as Larry correctly points out selective load shedding is also allowed and might be very simple in a building with AC that does not have to run.

We installed a 400 KW fuel cell as a optional standby supply but it cannot pick up the 350-400 kw of load all at once, the manufacturer does not want the load to jump more than 50 KW at a time so controls had to be installed to bring the load on in steps.

 

[ActionDave]

So in the OP's situation, he would be fine using the 200A transfer switch that they planned on purchasing, as long as it's installed after the service equipment. After the ATS he can install a sub panel and move all the essential circuits over to it. Or he can change it around so that 1 of the 2 current panels will now act as a sub panel after the ATS. Since the ATS is not switching the whole house, only a single panel, it only has to carry the load of that panel.

Is that all correct? Thanks.

I would say yes. Headed out the door; if there is a detail I missed some one will add the correction whether it be code related or grammatical.

 

[LarryFine]

Unattended homes are usually lightly loaded.

I was thinking about during heating and cooling seasons.

 

[iwire]

I was thinking about during heating and cooling seasons.

I understand how HVAC systems cycle automatically Capt. O. but unless it is an odd home there will be no cooking, lighting, home entertainment systems, tools, clothes washing etc going on when no ones home.

 

[ceb58]

Yes. I've suggested just breaking the 24v supply at the transformer so no part of the system can run.

Or, they can manually shed other loads so the can use the HVAC if the genny can handle it using with a simple manual bypass.


The big concern here is the maximum load the genny might see when the home is unattended.

A transfer switch with load shedding can be installed. They have up to 4 relays that drops off loads and can be configured to bring those loads back on if there is sufficiently available supply.

I dont see the concern with a load on an unattended home. If it is sized to handle the load of a attended home then it should not be an issue with it unattended.

 

[HughesBros.Electric]

Say you mount a 200 amp ATS SE and the HO only wants like a 12kw gen. to run a few things when power is off. How can you protect the gen. from over loading when you lose power and the gen. kicks on and you only have a 12kw gen. trying to run your house? How do you shed circuits? Thanks

 

[ceb58]

Say you mount a 200 amp ATS SE and the HO only wants like a 12kw gen. to run a few things when power is off. How can you protect the gen. from over loading when you lose power and the gen. kicks on and you only have a 12kw gen. trying to run your house? How do you shed circuits? Thanks

Look at my post #33 or a sub panel can be installed to bring the necessary circutes to or you can install an ATS that has the branch circutes built in (same as a sub panel) or you can install a manual transfer switch and let the HO shed the loads.
This is another reason, IMPO other than 225.31 that a disconnect with OCP be installed between the gen. and transfer switch. As I posted before some of the gen. have what looks like a breaker on them but they do not provide over-current protection.

 

[hurk27]

Look at my post #33 or a sub panel can be installed to bring the necessary circutes to or you can install an ATS that has the branch circutes built in (same as a sub panel) or you can install a manual transfer switch and let the HO shed the loads.
This is another reason, IMPO other than 225.31 that a disconnect with OCP be installed between the gen. and transfer switch. As I posted before some of the gen. have what looks like a breaker on them but they do not provide over-current protection.

I did a kind of automatic load shedding system for one home owner with start stop relay control, because when the power drops out the start stop circuit will drop out, and I put a single start/reset button to reset when the power comes back on, I interlinked it from the utility side so the home owner couldn't reset it while on the generator, it was kind of goofy over kill but it worked.:roll: another I used current relays I had picked up at a yard sale, they were adjustable all the way down to 1 amp, and had 20 amp contacts, so if one appliance was on it wouldn't allow the controled appliance to run until it was done, I used them to keep one freezer off while another was running then once it was cooled down it allowed the other freezer to come on which another current relay on it would keep the first freezer off, this way the home owner didn't have to do a thing, worked well for other loads that cycled like the AC condenser and a electric water heater, just couldn't find a low cost supply of them to make it worth while.

 

[ceb58]

another I used current relays I had picked up at a yard sale

You have cool yard sales. All I have found was a two legged coffee table

 

[jzadroga]

As I understand it the code is concerned with safety not convienience.

90.1 Purpose.
(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.
(B) Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

I feel that a generator sized for the normal loads as opposed to entire load possible is not a safety hazard.

But to be clear, I would only install one supplied with a breaker that protects the the generator from overload.

 

[PaulWDent]

This is why:

This is why:

What is the reason a generator with an automatic transfer switch has to be sized to carry the whole load of the house? I assume that means everything turned on like all the lights and appliances. I could see if it was a criticle care generator, but an optional standby generator for a residence? Whats the worst thing that would happen if the load was to much for the generator? The only thing I could see is the breaker for the generator trips. Not a safety hazard just a nuisance like AFCI's. Most houses could run very well on a smaller generator as the usual load is rarely the entire connected load.

Answer: Because when the switch is in the ultility position, it IS carrying the whole load of the house! Just because the generator can't supply the full load doesn't reduce the amps through the switch when the thing is selecting utility. You would need a 350-amp transfer switch even with a 1 amp generator, unfortunately.

 

[laketime]

another I used current relays I had picked up at a yard sale

Nothing like installing yard sale material for a customer:roll: