Load Calculation for a Generator

[templdl]

sw

Anyway, the point I'd like to make here is with a 16 circuit breaker panel controlled by a 70 amp main breaker, isn't the load calculation basically already done for you ? Obviously you can't load the panel with more than what the controlling breaker will allow. You have to base your calculations on the connected load to this panel.

Interesting questions. Given that breakers do have a TCC how do they coordinate with the generator's ability to product enough power to trip them, specifically the 70at main? It appears as though that unless the TCC of the main breaker is know and the genset's capacity to product power are compared there are only assumptions being made. There seems to be a conflict between the mainbreaker size as well as the size of and number of the breakers is the panel preventing the overloading of the genset. Regardless of the ratings and number of breakers you can't rely of the breakers to prevent overloading the genset. The installers must use their where with all to take the actual load and its diversity into consideration to reduce the possibility of overloading the genset.

 

[templdl]

Point well taken Rick but I believe this is an "Essential Circuits" system we're talking about. It comes with a 16 circuit main lug breaker panel that requires a 70 amp controlling breaker in the main panel as Little Bill pointed out. This panel comes with the following compliment of breakers :

5 - 15A/120V
5 - 20A/120V
1 - 20A/240V
1 - 50A/240V
1 - 40A/240V

There is no load shedding module with this type of transfer switch. IMHO, I don't believe Generac's intent when fabricating this panel was to field load it exactly to the breakers it's shipped with. These panels are subject to field modification (with respect to the breakers used). What can you possibly want to use during a power outage that would require a 2-P 40 or a 2-P 50 breaker ? In essence you can replace the 50 and 40 amp breakers with s/p 15's and 20's if you wish. Again, IMHO I would not use this type of transfer switch/breaker panel if I were connecting AC units seeing as how it has no load shedding module. If the AC unit was calling for cooling at the time of a power outage, chances are it might trip the breaker on the generator at start-up if other appliances were also calling for power at the same time.

Anyway, the point I'd like to make here is with a 16 circuit breaker panel controlled by a 70 amp main breaker, isn't the load calculation basically already done for you ? Obviously you can't load the panel with more than what the controllingw breaker will allow. You have to base your calculations on the connected load to this panel.

The best to do in order to understand what it would take to overload the genset is to understand the trip curves for the breakers. If you do so you probably be somewhat surprised that breaks will not protect the genset. The question is if the genset can generate enough current for a long enough period of time to trip that 70at breaker.
This topic would be put to bed quickly should a TC for the 70at breaker be provided as well as the damage curve for the genset.

 

[goldstar]

This is an excerpt from the Q & A section of the specs shown on the EG Direct website regarding a Generac 17 KW unit :

Q: Can the circuit breaker complement on the 16 circuit 100 amp switch for the Model 5873 generator be changed? What I am looking at is 1 50 amp 240v, 1 20 amp 240 v, 6 20 amp 120v, and 5 15 amp 120v for my house. The 50 amp is a 4 ton 14.5 SEER AC, the 20 amp 240v is a well water pump, and the rest are regular house circuits. Also, can ArcFault and GroundFault breakers be used with the switch? Thanks for your help. Like your website.
by Leon R Henry from Pennsylvania on August 28, 2013

A: You can accommodate the 50A 240V and 20A 240V no problem but you are only able to do a total of 10 120V circuits (5-20A and 5 - 15A). The only changes that can be made is going down in amperage on the circuits and you cannot change a 240V circuit into 2 120V circuits as this will void the UL listing and the warranty.

Apparently I've been voiding the UL listing on these for some time now. I don't think the violation has so much to do with changing the breakers out as it is about changing the wires inside the pre-manufactured whip to the main breaker panel. Apparently this is the way Generac submitted this xfer switch/breaker panel unit to UL. And, they've designed it in such a way that there is only enough neutral bar spaces to accommodate the s/p breakers already installed. If you want to add s/p circuits you have to either extend the floating neutral bar or wire nut several neutrals together. But, what if you have no need for a 2-P-20,a 2-P-40 or a 2-P-50 amp breaker ? You've now purchased a $500 piece of equipment of which you can only use 10 out of the 16 breaker positions. Make no sense to me. AND, BTW, this reply is from a sales company and not from Generac directly.

The transfer switch will accept AFCI breaker and the following models are approved by Generac.

Siemens Part No. Q115AF - 15A or Q120AF - 20A (These can be obtained from a local electrical wholesaler and will simply replace any of the single pole circuit breakers supplied in the pre-wired transfer switch distribution panel).

by Jim, Product Expert

This is an excerpt from the "Reviews" on the same website :

Generac Power Systems offers two versions of a 17 kW home standby generators.

The 17 kW generators are extremely popular because they pack enough power to run a 5-ton air conditioner.

This model sells the best out of the four versions for a few reasons:

1. Automatic Transfer Switch
Guardian's 100-Amp Distributed Load Center ensures the easiest, trouble free installation. You simply pick 16-circuits that you want to power during an outage. Most people choose this option.

If you want to power more than 16-circuits or run two air conditioners simultaneously, you should consider the Guardian 20kW Smart Circuit standby generator system instead.

Again, this is just a personal preference, I would not have picked the Essential Circuits system if I were including a 5 ton AC unit as part of the total load. IMHO without the load shedding module you run the risk of tripping the breaker on the generator when the AC unit starts, especially if other loads call for power at the same time (i.e.well pump, refrigerator(s), septic pump, etc.) And, as templdl pointed out "This topic would be put to bed quickly should a TC for the 70a breaker be provided as well as the damage curve for the genset."

 

[Little Bill]

The HO wants to put his heating system on the generator/transfer switch. He has a packaged unit that is gas. How can I just put the power for the gas portion of the unit on the genny without putting on the A/C part?

 

[kwired]

The HO wants to put his heating system on the generator/transfer switch. He has a packaged unit that is gas. How can I just put the power for the gas portion of the unit on the genny without putting on the A/C part?

I take it you mean the heating and cooling are both powered by the same circuit. You need to interrupt portion of control circuit for cooling whenever power is supplied by the generator with aux load shedding relays or something similar anyway.

 

[Little Bill]

I take it you mean the heating and cooling are both powered by the same circuit. You need to interrupt portion of control circuit for cooling whenever power is supplied by the generator with aux load shedding relays or something similar anyway.

This particular transfer switch doesn't have the load shedding capabilities.

 

[GoldDigger]

If there are any circuits which are not powered when on generator, you can just drive a relay from that to interrupt the cooling call circuit. Or you can just drive it from the service side of the ATS.

You do not have to have specific load sharing in the ATS.

Tapatalk...

 

[goldstar]

If you just connect the 120 volt AHU to the xfer switch/sub-panel the heating will run in the winter months. If you don't connect the condenser to the sub-panel the AHU will still come on (if the outage is in the summer months and the T-stat is calling for cooling) but the condenser won't. The selector switch on the T-stat will determine heating or cooling.

 

[kwired]

If you just connect the 120 volt AHU to the xfer switch/sub-panel the heating will run in the winter months. If you don't connect the condenser to the sub-panel the AHU will still come on (if the outage is in the summer months and the T-stat is calling for cooling) but the condenser won't. The selector switch on the T-stat will determine heating or cooling.

I believe his heating and cooling are in the same unit and powered by same branch circuit so he needs to interrupt the proper controls if he wants to be able to use heating but not cooling when running on standby power.

 

[kwired]

If there are any circuits which are not powered when on generator, you can just drive a relay from that to interrupt the cooling call circuit. Or you can just drive it from the service side of the ATS.

You do not have to have specific load sharing in the ATS.

Tapatalk...

That would require the relay coil to be energized all the time - except of course when there is no utility power, I kind of like the idea of powering a relay directly from the generator input to the transfer switch and using normally closed contacts to control whatever is being shed. This of course would mean shedding would happen anytime the generator is "exercised" with no load transfer also, but may not be a big deal for things like air conditioning or water heating. Any way to be able to mechanically detect the position of the transfer switch would be the best method though, and I think most auto transfer switches do have aux contacts that can be purchased if they are not preinstalled.

 

[kingpb]

You cannot simply look at the loads being supplied by the generator to determine if they fall within the nameplate rating of the generator. The sequence of starting those loads is very critical. If the generator is already carrying load, then adding a large load could cause the generator to go into load rejection even if the added load is still within its overall rating.

 

[jcassity]

i saw this conversation coming...
customer says "i just want" to eventually "i want the whole house on gen"

post one speaks to general lighting and recpts ,, motor loads now are in the mix.

gen is simply too small to cover all major loads running at once , not to mention some loads trying to start up.

my earlier response was based soley on what you asked, now i agree with everyone else

 

[kwired]

i saw this conversation coming...
customer says "i just want" to eventually "i want the whole house on gen"

post one speaks to general lighting and recpts ,, motor loads now are in the mix.

gen is simply too small to cover all major loads running at once , not to mention some loads trying to start up.

my earlier response was based soley on what you asked, now i agree with everyone else

And a reminder of the cost difference usually sends them back to "i just want".

Remember a manual transfer switch can back up the entire service even if the generator is not able to handle all the load, but with an ATS the system must be able to handle the load it transfers because it does so with no supervision. I don't know that I believe this should be a code requirement or not, but it is a good design decision anyway. What is the point of automatic transfer if the main trips? You have lost automatic backup that was intended if the main trips and you are not there to notice it tripped.

 

[goldstar]

I believe his heating and cooling are in the same unit and powered by same branch circuit so he needs to interrupt the proper controls if he wants to be able to use heating but not cooling when running on standby power.

Not sure what I'm missing here. From my experience I know there is a control unit inside the AHU. A T-stat tells that control unit whether to provide heating or cooling. In the case of cooling (if the T-stat is switched to the cooling mode) when that T-stat calls for cooling it will send 24 vac to the condenser, energizes a relay within the condenser and starts the condenser. It's not going to do that in the winter during heating season. Am I missing something or not interpreting this properly ?

 

[Smart $]

Not sure what I'm missing here. From my experience I know there is a control unit inside the AHU. A T-stat tells that control unit whether to provide heating or cooling. In the case of cooling (if the T-stat is switched to the cooling mode) when that T-stat calls for cooling it will send 24 vac to the condenser, energizes a relay within the condenser and starts the condenser. It's not going to do that in the winter during heating season. Am I missing something or not interpreting this properly ?

Heating season doesn't present the problem. Cooling season does. If the condenser load isn't shed, it must be added into the load calculation.

 

[goldstar]

Heating season doesn't present the problem. Cooling season does. If the condenser load isn't shed, it must be added into the load calculation.

I Understand that. Remember, this isn ESSENTIAL CIRCUITS system. If you're not connecting the condenser to the generator sub-panel what difference does it make ? It can call for cooling for the entire time the generator is running and it will never come on.

 

[Smart $]

I Understand that. Remember, this isn ESSENTIAL CIRCUITS system. If you're not connecting the condenser to the generator sub-panel what difference does it make ? It can call for cooling for the entire time the generator is running and it will never come on.

Great... if the heating and cooling (condenser) are not on the same circuit. Some HVAC systems are powered by one circuit.

 

[brother]

I have not read all these posts, but would the calculation for a generator by the OP be under NEC 2011 ARTICLE 702 Optional Standby Systems. 702.4 Capacity and Rating to be specific.?

 

[Smart $]

I have not read all these posts, but would the calculation for a generator by the OP be under NEC 2011 ARTICLE 702 Optional Standby Systems. 702.4 Capacity and Rating to be specific.?

704.2(B)(2) to be more specific.

 

[goldstar]

Great... if the heating and cooling (condenser) are not on the same circuit. Some HVAC systems are powered by one circuit.

I'm sorry but apparently I've never had the experience of working on a residential heating/cooling system such as you describe. Would you please post a link to the type of system your speaking about so I can chat intelligently with you ? Thanks.