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.
Load Calculation for a Generator
- Thread starter Little Bill
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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
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
This is an excerpt from the Q & A section of the specs shown on the EG Direct website regarding a Generac 17 KW unit :
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.
This is an excerpt from the "Reviews" on the same website :
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."
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
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
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
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.
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
This particular transfer switch doesn't have the load shedding capabilities.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
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...
You do not have to have specific load sharing in the ATS.
Tapatalk...
goldstar
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
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
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
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
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
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
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
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.
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
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
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
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
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
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
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
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
Senior Member
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
Senior Member
- Location
- New Jersey
- Occupation
- Electrical Contractor
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.
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