Load Calculation for a Generator

[Smart $]

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.

Well to be completely honest, I have not had the experience either. I was just repeating information provided by other participants, whose information is generally quite reliable.

 

[kwired]

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.

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.

Smart more less answered for me.

I haven't seen such a unit either - at least in a dwelling with gas source for the heating, and with units with electric heat there is usually multiple circuits, but the heating load is usually the larger load in those cases and we wouldn't be discussing wanting to run the heat but not the cooling anyway. Just because you or I haven't seen one doesn't mean it is impossible to ever find one either.

Dwelling or non dwelling, with an ATS the generator must be able to carry the transferred load, or provide ways of limiting the load to what the generator can carry.

 

[goldstar]

Just because you or I haven't seen one doesn't mean it is impossible to ever find one either.

I suppose you are correct unless we're talking about a heat pump or something similar. Little Bill did mention that the appliances were gas fired so I made the assumption that the heat was as well.

Dwelling or non dwelling, with an ATS the generator must be able to carry the transferred load, or provide ways of limiting the load to what the generator can carry.

A typical residential forced, hot air/cooling system consists of furnace coupled with an air handler and a cooling condenser. My point was that if you didn't connect the condenser to the generator sub-panel you wouldn't have to worry about the condenser load as part of your load calculation. You would have to connect the AHU because of the furnace and heating but that's only a few amps. Now, if the HO wanted to have the capability of AC during a power outage that's a different story. Again, I wouldn't use this type of xfer switch/sub-panel if that were the case.

I'm sorry to have gone off on a tangent and pirated this thread away from you Little Bill.

 

[kwired]

I suppose you are correct unless we're talking about a heat pump or something similar. Little Bill did mention that the appliances were gas fired so I made the assumption that the heat was as well. A typical residential forced, hot air/cooling system consists of furnace coupled with an air handler and a cooling condenser. My point was that if you didn't connect the condenser to the generator sub-panel you wouldn't have to worry about the condenser load as part of your load calculation. You would have to connect the AHU because of the furnace and heating but that's only a few amps. Now, if the HO wanted to have the capability of AC during a power outage that's a different story. Again, I wouldn't use this type of xfer switch/sub-panel if that were the case.

I'm sorry to have gone off on a tangent and pirated this thread away from you Little Bill.

I understand everything you said and didn't think you were that far off tangent, you just did not want to accept the fact that the heating and cooling are possibly all on the same circuit, but the load is much less when in heating mode. It is not all that likely to find that in a dwelling, but certainly not impossible, if that is the case you have to somehow disable the cooling if the generator can not handle that load. If it were not a dwelling unit the same still applies, and you are more likely to find such a unit in a non dwelling that is gas heat but also contains the cooling compressor and blower(s).

 

[goldstar]

I understand everything you said and didn't think you were that far off tangent, you just did not want to accept the fact that the heating and cooling are possibly all on the same circuit, but the load is much less when in heating mode. It is not all that likely to find that in a dwelling, but certainly not impossible, if that is the case you have to somehow disable the cooling if the generator can not handle that load. If it were not a dwelling unit the same still applies, and you are more likely to find such a unit in a non dwelling that is gas heat but also contains the cooling compressor and blower(s).

OK. Just wasn't thinking out of the box and like most others, never saw a set-up like that.

 

[kwired]

OK. Just wasn't thinking out of the box and like most others, never saw a set-up like that.

There are many roof top units that have gas heat and the AC all in one unit fed by a single electric circuit. They are often high enough capacity they will require three phase input and is part of the reason why you will not find them in a typical residential application. Most units with heat and cooling all in one I have seen in dwellings have had electric heat instead of gas though, and may or may not be heat pumps. OP wouldn't even be asking his question for one of those units though because the heat load is likely higher than the cooling load and his generator wouldn't handle either load.

 

[Little Bill]

I didn't take a picture of the unit, but it's as described by some here. It is a complete packaged unit that is not at all peculiar. Many homes that don't have the room, or the customer doesn't want any part inside, have these units.

It is on one 30A circuit. The power is divided internally to the various components. It is gas heat with of course, AC A/C.

It would be very difficult to run a control wire to the unit and even more difficult (for me) to know what wires could be interrupted being that they probably go through some logic board. Biggest thing would be the distance. The unit is on the other end of the house from the genny/transfer switch. No basement, and a swimming pool between the unit and switch.

 

[kwired]

I didn't take a picture of the unit, but it's as described by some here. It is a complete packaged unit that is not at all peculiar. Many homes that don't have the room, or the customer doesn't want any part inside, have these units.

It is on one 30A circuit. The power is divided internally to the various components. It is gas heat with of course, AC A/C.

It would be very difficult to run a control wire to the unit and even more difficult (for me) to know what wires could be interrupted being that they probably go through some logic board. Biggest thing would be the distance. The unit is on the other end of the house from the genny/transfer switch. No basement, and a swimming pool between the unit and switch.

Well you are making it harder to shed just part of the load when you mention what you need to go through to get wiring to it.

Where is the thermostat? You can just interrupt the cooling control lead there if it is easier to get to. The cooling lead should be connected to the "Y" terminal of the thermostat. There could be some wireless control options available.

 

[goldstar]

At this point Little Bill you have little or no choice but to rely on your customer's integrity and hope that they don't leave the T-stat for the AC set in the automatic mode while they're out of the house in the summer months. As it is they are being given the capability of selecting 16 out of X# of circuits that they absolutely want to have active during a power outage. If they choose larger loads like the pool motor(s), well pump, double wall oven, etc. they stand a greater chance of damaging or over-stressing the generator if more that one of these loads calls for power at the same time while on emergency power.

If you can get your hands on the manual for the AC/heat unit and take a look at the wiring diagram we might (collectively) be able to come up with a solution using X-10 or Insteon wireless controls as a fall back.

 

[goldstar]

There could be some wireless control options available.

You post a lot faster than I do.

 

[Little Bill]

At this point Little Bill you have little or no choice but to rely on your customer's integrity and hope that they don't leave the T-stat for the AC set in the automatic mode while they're out of the house in the summer months. As it is they are being given the capability of selecting 16 out of X# of circuits that they absolutely want to have active during a power outage. If they choose larger loads like the pool motor(s), well pump, double wall oven, etc. they stand a greater chance of damaging or over-stressing the generator if more that one of these loads calls for power at the same time while on emergency power.

If you can get your hands on the manual for the AC/heat unit and take a look at the wiring diagram we might (collectively) be able to come up with a solution using X-10 or Insteon wireless controls as a fall back.

My problem now is convincing the inspector that the HVAC can be on the switch with the exception of the A/C.

 

[kwired]

At this point Little Bill you have little or no choice but to rely on your customer's integrity and hope that they don't leave the T-stat for the AC set in the automatic mode while they're out of the house in the summer months. As it is they are being given the capability of selecting 16 out of X# of circuits that they absolutely want to have active during a power outage. If they choose larger loads like the pool motor(s), well pump, double wall oven, etc. they stand a greater chance of damaging or over-stressing the generator if more that one of these loads calls for power at the same time while on emergency power.

If you can get your hands on the manual for the AC/heat unit and take a look at the wiring diagram we might (collectively) be able to come up with a solution using X-10 or Insteon wireless controls as a fall back.

But with an ATS, the transferred load must be within the capacity of the generator, or it must have automatic control to keep the load within the generator rating. No one is necessarily there to supervise the load with an ATS. A manual transfer switch means someone is there to make the transfer and therefore someone is there to supervise the load.

 

[goldstar]

But with an ATS, the transferred load must be within the capacity of the generator, or it must have automatic control to keep the load within the generator rating. No one is necessarily there to supervise the load with an ATS. A manual transfer switch means someone is there to make the transfer and therefore someone is there to supervise the load.

I understand the fact that the NEC wants to make this as fool proof as possible. But, what's the worst that can happen in this scenario ? The generator starts and if all loads call for power at the same time either the generator will crap out or the breaker will trip. If the homeowners left for a week's vacation and they left their AC in the automatic mode - they're stupid. If they're just gone for the day, when they come home they turn off the AC and re-start the generator.

FWIW, If you call one of the Generac sales companies they will tell you, flat out, that this unit will start a 5 ton AC unit. Here's what it states on the Generac website :

Guardian Series 17kw Provides essential circuit power protection for your home, ensuring the necessities remain powered during an outage or up to whole house coverage when paired with one of Generac?s load shedding devices. Capable of backing up a five ton air conditioner.

As I've stated before, using a xfer switch without a load shedding module is not the correct xfer switch to use in this instance. But, being the situation as it it I think the best thing to do (collectively) is to try and help Little Bill out of this.

 

[kwired]

I understand the fact that the NEC wants to make this as fool proof as possible. But, what's the worst that can happen in this scenario ? The generator starts and if all loads call for power at the same time either the generator will crap out or the breaker will trip. If the homeowners left for a week's vacation and they left their AC in the automatic mode - they're stupid. If they're just gone for the day, when they come home they turn off the AC and re-start the generator.

I agree to the point that the NEC has become a design manual, which it says in 90.1 that it is not intended to be such.

It is good design practice if installing something that will automatically operate that it stays in operation or the intended purpose of making it automatic becomes a mute point, so shedding of non essential loads is still a good idea, and if the AC is deemed essential then a larger generator may be the better design practice.