sull68
We live in the Northeast and after 2 recent multi-day power outages (Hurricane Irene & October Snow storm) have decided to invest in a home standby generator. We have spoken to a couple of contractors and received a few options.

Our home has a 400 Amp meter that feeds 2 200 Amp panel boxes that are located next to each other in our basement. Each panel has its own 200 amp disconnect breaker in it there are no disconnects at the meter. We are interested in a whole house solution and both vendors felt this was possible with a 20KW Air-Cooled unit by using the load management feature.


Vendor A
Suggested a Generac 5887 20KW unit and a 400 Amp SE rated Nexus Smart switch.

Vendor B
Suggested a Generac 5875 20KW that included a 200 Amp SE rated Nexus Smart switch and then purchasing an additional a 200 Amp SE rated Nexus Smart switch. He suggested using one transfer switch for each panel. As opposed to the first vendor that would feed both panels from the one 400 Amp TS.

I am looking for feedback as to the advantages and disadvantages of each option. The installation costs are about the same but the equipment costs of Vendor B (2 200 Amp TS) is about $1,500 less. He had mentioned the cost of the 400 Amp TS is one of the reasons they do it this way. I believe that this is a long term decision and althought the idea of saving $$ is desirable I would like to know if I would be better with the single TS in the long run.
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ceb58
I would go with the 2 - 200amp ATS's. With the set up you have it is easier to feed the panels from 2 than 1. One advantage of 2 over 1 is if something happened to one of the ATS's you still have the other to get you by in a outage. If you have only one then you are screwed unless you can get it to transfer by hand.
I will say
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both vendors felt this was possible with a 20KW Air-Cooled unit
dont let them feel any thing make them produce the load calculation on the home per Art.220 of the NEC
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mdsulli
I did not want to bore people with too much information but I have very good usage data from the last 18 months that I had provided to both companies. I use a device to monitor energy usage and record the readings. Not sure if it has been discussed on this board or not but it is called the Energy Detective [url]www.theenergydetective.com[/url]. I have found it to be a great product and has helped our family be more aware of or usage in real-time.

I appreciate your feedback and obviously that is the direction I am leaning as it saves a significant amount of money. The redundancy advantage was mentioned by the company representative as well. He indicated that depending on which of the two switches failed it may require someone to manually start the generator.

Also still interested in others thoughts especially if someone has seen a similar configuration. Are there any advantages to a single 400 amp TS just trying to figure out why one company would propose that rather than the 2 switch option.
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Skip Douglas SkipD
mdsulli;11873 wrote:
I did not want to bore people with too much information but I have very good usage data from the last 18 months that I had provided to both companies. I use a device to monitor energy usage and record the readings. Not sure if it has been discussed on this board or not but it is called the Energy Detective [URL="http://www.theenergydetective.com"]www.theenergydetective.com[/URL]. I have found it to be a great product and has helped our family be more aware of or usage in real-time.
I do not believe that your measured usage readings meet the NEC's requirement for calculation. It is required that you calculate the TOTAL load possible if everything that is automatically connected to the generator is turned on. The only way around this is to use automatic load shedding equipment.
Skip Douglas
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murphy
How do the NEC calculations handle a central air conditioner and an oil furnace that are on the same thermostat and can't run at the same time?
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bucdup
I think the 2 x 200 amp option is just fine - especially considering the lower cost. With the new Nexus ATS and the DLM modules your load calculation is less of a concern than it used to be. The load shedding meets the NEC requirements for whole house installation. You can easily shed loads that are non-essential. Makes life pretty easy.

I have also found you usually get the best results using the Optional Method of NEC 220. To answer murphy's question, you do not have to figure both heat and A/C if they cannot run at the same time - you just figure the larger of the two loads. You must use the a proper NEC load calculation for the purposes of generator sizing. The TED readings are not allowed, even though that will give you a very good "real life" idea of generator load.

I seriously doubt the two ATS setup would result in a situation where you would have to manually start the generator due to a ATS failure. ATS failures are relatively rare anyway, your biggest issue will be you are purchasing a Generac.
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Enginerd
SkipD;11882 wrote:
I do not believe that your measured usage readings meet the NEC's requirement for calculation. It is required that you calculate the TOTAL load possible if everything that is automatically connected to the generator is turned on. The only way around this is to use automatic load shedding equipment.


I am not an expert on this but just about everyone I talked to is of the opinion that you don't need to calculate the "total load possible", just that you need to do a standard demand load calculation (I think it is Art.220 in the NEC) in the same way you would size service for the main panel. I also believe that data logging information can be substituted for the calculation but I am not 100% sure about that either.
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phabib
I looked into this a few months ago and what I understood was that you could log your usage at 15 minute increments for 30 days and then you'd have to size to meet the peak.
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Ginaelectric
sull68;11869 wrote:
We live in the Northeast and after 2 recent multi-day power outages (Hurricane Irene & October Snow storm) have decided to invest in a home standby generator. We have spoken to a couple of contractors and received a few options.

Our home has a 400 Amp meter that feeds 2 200 Amp panel boxes that are located next to each other in our basement. Each panel has its own 200 amp disconnect breaker in it there are no disconnects at the meter. We are interested in a whole house solution and both vendors felt this was possible with a 20KW Air-Cooled unit by using the load management feature.


Vendor A
Suggested a Generac 5887 20KW unit and a 400 Amp SE rated Nexus Smart switch.

Vendor B
Suggested a Generac 5875 20KW that included a 200 Amp SE rated Nexus Smart switch and then purchasing an additional a 200 Amp SE rated Nexus Smart switch. He suggested using one transfer switch for each panel. As opposed to the first vendor that would feed both panels from the one 400 Amp TS.

I am looking for feedback as to the advantages and disadvantages of each option. The installation costs are about the same but the equipment costs of Vendor B (2 200 Amp TS) is about $1,500 less. He had mentioned the cost of the 400 Amp TS is one of the reasons they do it this way. I believe that this is a long term decision and althought the idea of saving $$ is desirable I would like to know if I would be better with the single TS in the long run.


I have done the same install both ways and the money saved in the 400A transfer switch installation is equal to the additional labor of installing 2 transfer switches and wiring in between the both, Bonding both and grounding. Its just basically comes down to what will fit and where.
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Ginaelectric
Ginaelectric;11908 wrote:
I have done the same install both ways and the money saved in the 400A transfer switch installation is equal to the additional labor of installing 2 transfer switches and wiring in between the both, Bonding both and grounding. Its just basically comes down to what will fit and where.


I really would be much more concerned with feeding a 400 ampere service with a 80 amp Generator. You will need to shed and manage all the major loads to prevent an overload.
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ceb58
Enginerd;11902 wrote:
I am not an expert on this but just about everyone I talked to is of the opinion that you don't need to calculate the "total load possible", just that you need to do a standard demand load calculation (I think it is Art.220 in the NEC) in the same way you would size service for the main panel. I also believe that data logging information can be substituted for the calculation but I am not 100% sure about that either
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.



220.87 Determining Existing Loads. The calculation of a
feeder or service load for existing installations shall be
permitted to use actual maximum demand to determine the
existing load under all of the following conditions:
(1) The maximum demand data is available for a 1-year
period.
Exception: If the maximum demand data for a 1-year period
is not available, the calculated load shall be permitted
to be based on the maximum demand (measure of average
power demand over a 15-minute period) continuously recorded
over a minimum 30-day period using a recording
ammeter or power meter connected to the highest loaded
phase of the feeder or service, based on the initial loading
at the start of the recording. The recording shall reflect the
maximum demand of the feeder or service by being taken
when the building or space is occupied and shall include by measurement or calculation the larger of the heating or
cooling equipment load, and other loads that may be periodic
in nature due to seasonal or similar conditions.


This is the exception per the NEC
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