Generator load shedding question

[LarryFine]

I'm looking to install a 17Kw Honeywell/Generac and ATS. The house is one story, approx. 1K sq ft, and is all gas except for the HVAC (heat pump and 10Kw of back-up heat, in attic and must have been added later on.)

There is an electric clothes dryer, but they don't restart by themselves. The question is can I skip interrupting the HVAC control since the generator has a greater output than is needed to support the A/C and back-heat?

 

[jumper]

First thought, as long as the qenny can handle any transferring load no problem.

Manual shedding later seems reasonable.

 

[kwired]

You have 10 kW of back up heat, do you have less than 7 kW of other load?

You mentioned the dryer and not automatically starting, but should they decide to run the dryer while on generator you could be getting close to generator capacity or even over if heat kicks on. Shedding would suspend the heat and keep from overloading generator. Heat pump load is likely much less than 10 kW, so you maybe just shed the W lead of the HVAC control system so it can't call for back up heat if necessary.

 

[LarryFine]

You have 10 kW of back up heat, do you have less than 7 kW of other load?

It is a relatively small house with gas appliances, so I'd say yes, less than 7Kw likely to be left on.

You mentioned the dryer and not automatically starting, but should they decide to run the dryer while on generator you could be getting close to generator capacity or even over if heat kicks on.

That could happen in any situation, and customer knows how to regulate (works for power company.)

Shedding would suspend the heat and keep from overloading generator. Heat pump load is likely much less than 10 kW, so you maybe just shed the W lead of the HVAC control system so it can't call for back up heat if necessary.

His concern is that heat works if he's not home during an outage, he has pets, but leaves nothing on.

 

[kwired]

It is a relatively small house with gas appliances, so I'd say yes, less than 7Kw likely to be left on.


That could happen in any situation, and customer knows how to regulate (works for power company.)


His concern is that heat works if he's not home during an outage, he has pets, but leaves nothing on.

You have two possibilities then, first is what NEC sees as possible connected load that may automatically transfer. Second is the reality that very little is running when nobody is home, NEC doesn't recognize the second one as a value you can use, kind of encroaches NEC stating it isn't a design standard IMO. Legally required standby system I can understand being more particular about what automatically transferred load is.

 

[gar]

181006-0947 EDT

LarryFine:

An internal combustion engine can run at full power rating continuously, but slightly overload it, and stall occurs. An electric generator can run at full power rating continuously, be greatly overloaded, possibly many times rating, still produce electric power, and if for only a short time, then not be damaged.

Starting current for an electric motor lasts for many cycles. I have shown plots in some prior post. This starting current, not inrush current, can be moderately larger compared to full power running current. Thus, my concern is that, if there is a large load on the generator when the heat pump turns on, then this might stall the generator.

.

 

[kwired]

181006-0947 EDT

LarryFine:

An internal combustion engine can run at full power rating continuously, but slightly overload it, and stall occurs. An electric generator can run at full power rating continuously, be greatly overloaded, possibly many times rating, still produce electric power, and if for only a short time, then not be damaged.

Starting current for an electric motor lasts for many cycles. I have shown plots in some prior post. This starting current, not inrush current, can be moderately larger compared to full power running current. Thus, my concern is that, if there is a large load on the generator when the heat pump turns on, then this might stall the generator.

.

Which means size of prime mover in relation to size of alternator is a determining factor to how the whole unit responds to high loading. Alternator may be able to take a high load, but prime mover marginally sized will at least slow down when first hit with the load.

Once saw the result of bolted fault on a tractor PTO driven generator - generator maybe 20-25 kW, tractor capable of delivering 100 HP maybe more - broke universal joint on the PTO coupling shaft when hit with that fault.