Load Calculation for a Generator

[Little Bill]

If you're installing a stand-by generator and a load calculation is required, how do you do that?

If it's mostly general use receptacles and lights, do you use the sq ft, or if you know what loads are going to be used on the circuits, do you use that?
Or is it a combination of the two?

This is just a 17Kw generator with an ATS that also has a built in load center. It is prewired with a whip to go into the panel to pick up the desired circuits. I think it is for 14 circuits.

 

[augie47]

That's a real good question and one I bet you will end up running by your AHJ.
As for me, I think some adaptation of 220.83 would be a good start.

 

[jcassity]

look up the gen's kw/kva rating. you said 17kw already,,,, just checking thats not kva.

pay attention to the gens efficiency rating as well. if you find out its efficency is 80% then its going to give you two numbers,, kva and kw. just design around the kw value.

if you dont know the loads then build out a senerio in your minds eye of what it would power,
*think emergency circuits and save all the nice to have circuits for last especially the 2pole devices.
All it takes is one takeless water heater to throw all your math into outter space. Mine is an 18kw uint Stieble Eltron made in germany... been very very pleased with it. This one change reduced my montly power consumption by 20% but its instantanious command for power is very astronomical. the thing draws 44A on each leg at 55deg input water temp when i have 125deg dialed in.

for the most part its a simple as adding up the watts of devices then your design not exceeding the efficiency rating of the genset.

its a decent size gen and an exceptional size gen for just basic outlets and lights.

if the equipment doesnt have watts listed on its name plate, then your just going to do volts x the manuf list amps to get your watts.

for your information, in the telecommuications world, we target 80% of the genset kw rating as our max loading value.

if your customer knows the devices and you know the outlets and lights max wattage design, then your able to add it all up.

 

[chris kennedy]

220.87

 

[Smart $]

...
If it's mostly general use receptacles and lights, do you use the sq ft, or if you know what loads are going to be used on the circuits, do you use that?
Or is it a combination of the two?

...

Combination of the two... ft? for general lighting and receptacles plus specific loads including demand factoring that applies. No different than doing a load calculation for a subpanel.

 

[templdl]

Thought should also be given to the generator's ability to start a motor load given the other loads that it may be supplying at the same time. Would the genset be able to support the starting current of a motor when it already has a load?

 

[Little Bill]

With the cheaper prices and availability of Gensets today, it is hard to explain to a customer just what they can and can't back up depending on the size of the genny.
I ask what that absolutely need, then what they would like to have. What they have a hard time grasping is we (the electrician) have to move complete circuits, not just a light or receptacle here and there. I will say "so you want the master bedroom backed up?" and they will say "no just this or that in there." I try to explain that I have to move the whole circuit and they can choose what they operate in that room. It doesn't sink in!:happyno:

I try to explain that I have to present a load calculation based on the entire circuit(s). If these were manual transfer switches that would be a different story.

Anyway, thanks for the info on the load calc.!
Luckily with the install I'm about to do, all the major appliances are gas, so there shouldn't be any problems with those.

 

[goldstar]

A Generac "Essential Circuits" system (if that's what you're using) will give you a 71 amp output (probably a 90 amp breaker on the unit) at 240 VAC according to their spec sheet. Do a Google search for "generator sizing guides" and several will pop up. Use one of them to do your calculation. Plan what "essential circuits" you intend to connect to the load center like the furnace, refrigerator, well pump, TV, some lights, etc. Everything after that is a bonus. Bear in mind that if you're planning on connecting and starting a 3-ton AC condenser unit the 17 KW unit will probably handle it (check the starting amperage) but it's going to take a large portion of your running amperage. I generally like to discourage homeowners from connecting their AC units to their generator panels. Doing so eats up 4 of the spaces in a 16 circuit panel. If they want to cool off during a power outage tell them to take a ride in their car.

 

[Smart $]

... Do a Google search for "generator sizing guides" and several will pop up. Use one of them to do your calculation. ...

If it's the AHJ requiring an NEC compliant load calculation, the referenced guides' calculation method may not suffice.

 

[Little Bill]

A Generac "Essential Circuits" system (if that's what you're using) will give you a 71 amp output (probably a 90 amp breaker on the unit) at 240 VAC according to their spec sheet. Do a Google search for "generator sizing guides" and several will pop up. Use one of them to do your calculation. Plan what "essential circuits" you intend to connect to the load center like the furnace, refrigerator, well pump, TV, some lights, etc. Everything after that is a bonus. Bear in mind that if you're planning on connecting and starting a 3-ton AC condenser unit the 17 KW unit will probably handle it (check the starting amperage) but it's going to take a large portion of your running amperage. I generally like to discourage homeowners from connecting their AC units to their generator panels. Doing so eats up 4 of the spaces in a 16 circuit panel. If they want to cool off during a power outage tell them to take a ride in their car.

This is a RTS (residential transfer switch) and requires a 70A breaker.

 

[kwired]

With the cheaper prices and availability of Gensets today, it is hard to explain to a customer just what they can and can't back up depending on the size of the genny.
I ask what that absolutely need, then what they would like to have. What they have a hard time grasping is we (the electrician) have to move complete circuits, not just a light or receptacle here and there. I will say "so you want the master bedroom backed up?" and they will say "no just this or that in there." I try to explain that I have to move the whole circuit and they can choose what they operate in that room. It doesn't sink in!:happyno:

I try to explain that I have to present a load calculation based on the entire circuit(s). If these were manual transfer switches that would be a different story.

Anyway, thanks for the info on the load calc.!
Luckily with the install I'm about to do, all the major appliances are gas, so there shouldn't be any problems with those.

I guess it depends on if you have an AHJ that is going to be really picky on the load calc.

Say you have a 12 x 12 bedroom. With no other information that room is going to be 3VA per square foot - 432 VA of load.

Now maybe they want to be able to use a 1300 watt heater in there - 432 is not enough. Forget the 3VA figure and get some real data for the calculations is the way I would generally look at it. I usually would not have an AHJ looking all that closely to my load calculation though.

 

[RICK NAPIER]

A couple of things worth noting is that a lot of these units have a lower kw rating on natural gas then on propane. Also a number of these units have load shedding available so you can put your A/C on it you just can't be sure it will run.

 

[jcassity]

A couple of things worth noting is that a lot of these units have a lower kw rating on natural gas then on propane.

good point,, and to that point *all of them* are a lower kw rating ,,and the EPA influences have become more strict in the past two years than you can imagine.

 

[renosteinke]

Generator sizing is a real problem. IMO, the NEC is simply WRONG on this topic.

Why? Because there are different requirements for different generator scenarios - with one of the most severe conditions being an optional residential genny with an automatic transfer. That is, they 'over engineer' the one circumstance where an undersize genny poses no real problem; so what if the genny stalls?

Now, I'd just write this off to code zealotry, except that 110 also requires us to pay attention to the manufacturers' instructions. Genny makers typically want the genny to operate at 80% (or more) of its' rated capacity. Running the genny lightly loaded is simply damaging to the genny.

Enter your typical house with its' 200-amp service. The NEC would require you to have a 200-amp genny, or add things like 'load shedding' or a separate genny-fed panel. Yet, go to this same house on the hottest summer day, fire up the kitchen and the air conditioning, and you'll probably measure an actual load of 120 amps. That is, every possible need of the house would be served by 25KW genny; manufacturers would advise against anything over 30KW, yet code would mandate a 48KW unit.

Oddly enough, this is not an issue with 'required' gennys, as they are sized to their specific loads- and not the overall service size. Go figure.

Oddly enough, Hillbilly Hank doesn't have this problem with his suicide cord - heck, if the portable genny stalls, he just yells at the missus to shut some things off. Maybe he'll remember to shut off his main breaker, and maybe he won't. Oops. Meanwhile, he can drink cold beer and watch TV, while his code-compliant neighbor tries to figure out why his $$$ unit won't start.

 

[ceb58]

Generator sizing is a real problem. IMO, the NEC is simply WRONG on this topic.

Why? Because there are different requirements for different generator scenarios - with one of the most severe conditions being an optional residential genny with an automatic transfer. That is, they 'over engineer' the one circumstance where an undersize genny poses no real problem; so what if the genny stalls?



Enter your typical house with its' 200-amp service. The NEC would require you to have a 200-amp genny, or add things like 'load shedding' or a separate genny-fed panel. Yet, go to this same house on the hottest summer day, fire up the kitchen and the air conditioning, and you'll probably measure an actual load of 120 amps. That is, every possible need of the house would be served by 25KW genny; manufacturers would advise against anything over 30KW, yet code would mandate a 48KW unit.

The first statement is exactly why the code was changed for 2011. Dealers were selling and installing undersized units that would stall out and leave people in the dark. Some with medical issues.
Cannot figure where you come up with the second statement. No where in 702.4 (B)(2) dose it say that. I am currently doing a 4200 SqFt home that will have a back up gen. for the bottom floor. Home will have a 300 amp service but by the load calculations for the first floor it will need a 14 Kw gen. but for starting of the A/C unit and room for addition we are installing a 20 Kw.

 

[templdl]

Genny makers typically want the genny to operate at 80% (or more) of its' rated capacity. Running the genny lightly loaded is simply damaging to the genny.

Yes, overloading a genset is senseless. But to oversize a genset is costly by paying for unused capacity unless yu are taking into consideration overloads that may occur or growth.
But running a genset underloaded where did you find the documentation from a manufactured that states that it will damage the generator? I know that oversizing a genset is a poor design and a waist of money.

 

[renosteinke]

"Dealers were selling and installing undersized units"

I don't agree with that assertion at all. EVERY genny job I've had has started with the customer thinking he needed a unit far larger than he really needed. Moreover, the genny dealers have always been instrumental in persuading the customer that he didn't need, or even want, the big unit.

To be fair, the genny itself is probably the least important part of the job. Far more attention needs to be given to the transfer switch, the exercising regime, and -perhaps most important- the NOISE issue.

Medical issues? I can't help it if someone applies the wrong part of the NEC to a job. "OptionaL" and "life critical" are quite different applications.

The homeowner with the "small" house will necessarily wind up with an oversize genny, due to the starting current required for motors (HVAC compressors). Or, they'll have to do without air conditioning.

Last Christmas I received a 10-hr power outage as a 'present.' All I NEEDED was enough power for the furnace blower and a few lights. Apply all the math you want, and you'll not come close to matching the 125-A service my house has. Do you really think I'd be best served by a 30KW unit?

During that power failure I visited the local home store- where I noticed several folks looking at the portable gennys. Each stated that they had a small genny, one that had failed to start. None were aware of the need to 'exercise' the machines (UNDER LOAD), or to protect the fuel in storage. Those gennys had been sitting in the garage, collecting dust for three years (since the last outage). Small surprise they failed.

Me? There's a reason my firm is called "Amish Electric." I did just fine with non-electric heat and light for that little hiccup.

I don't believe in selling folks dinosaurs that will fail when needed.

 

[goldstar]

Now maybe they want to be able to use a 1300 watt heater in there - 432 is not enough. Forget the 3VA figure and get some real data for the calculations is the way I would generally look at it. I usually would not have an AHJ looking all that closely to my load calculation though.

I see your point but if we had to size services on "maybe" or "if" a homeowner might want to plug in 1300 watt heaters in every room then a mere 200 amp service would never be adequate for a house.

 

[goldstar]

A couple of things worth noting is that a lot of these units have a lower kw rating on natural gas then on propane. Also a number of these units have load shedding available so you can put your A/C on it you just can't be sure it will run.

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 controlling breaker will allow. You have to base your calculations on the connected load to this panel.

 

[kwired]

I see your point but if we had to size services on "maybe" or "if" a homeowner might want to plug in 1300 watt heaters in every room then a mere 200 amp service would never be adequate for a house.

I have wired and been in houses with all electric heat with either baseboards or other units in each room with the average being 1000 -1500 watt units in each room and still was able to supply the house with 200 amps, and maybe even 125 or 150 if the house wasn't too big.

Have also seen 100 amp services with pretty fair amount of electric heat connected and they never had an issue with tripping main breaker. They likely should have been more than 100 amp according to art 220, but nobody ever changed the supply when adding the heat. More than once I have also seen similar with 25-30 kW of fixed heat connected to a 200 amp service - electric range, water heat, and dryer plus remainder of house - 220 says this needs to be more than 200 amp but it never trips the main.

I don't go around making these installs intentionally, I have added some load at times and told owner they are pushing the limit on what they have and may need to upgrade the service. The demands from all families is not the same either, something 220 can not really account for.