sizing generators to electrical service

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Cleveland Apprentice

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I couldn't find the section in art. 700, maybe I overlooked it. I am looking at installing a whole house back-up generator. The house currently has a 100 amp service. How do I size the gen? Does it need to be sized to 100% of the load/ocp a certain percentage less than 100%? I thought it was a certain percentage like 80%, but not sure. If anyone could cite the code section, I'd greatly appreciate it. Thanks
 
In the '08 Code it's 702.5.
It does not provide any percentage but allows that the generator will be capable of supplying the transferred load.
(As noted in that section, load management can be employed to regulate that load)
 
Do not load generator more than 50%

Do not load generator more than 50%

Add up the wattage of all the equipment that you wish to operate at one time ( for air conditioning ~ 1 kw per ton) and double. Should not operate elec. dryer or elec. water heater or range at same time. -- Best to contact the manufacturer ( Kohler, Generac, or other ) and get them to size the generator but have the list of your electric equipment and wattages on hand. Do not cheap out on transfer switch or connection to your home --- do not want to feed back power on utility lines and injure or kill one of their workers.
 
One word of warning though, Generac will lie to you about what they will run. I had several installs where the manufactures rep told them Oh yeah! It will run your A/C! I knew it wouldn't, but moved the circuit anyway to show the customer it wouldn't. Yep, generator bogged down. Startup killed it.
 
augie47 said:
In the '08 Code it's 702.5.
It does not provide any percentage but allows that the generator will be capable of supplying the transferred load.
(As noted in that section, load management can be employed to regulate that load)
Click to expand...

I believe that means everything. You can not
Add up the wattage of all the equipment that you wish to operate at one time ( for air conditioning ~ 1 kw per ton) and double. Should not operate elec. dryer or elec. water heater or range at same time.
Click to expand...

augie, am I correct?
 
I believe the intent of that Section is that the load must be controlled in such a manner that the generator can not be overloaded and thus cause a loss of power. The decision as to which loads can be connected can not be left up the the occupant if you are using a automatic transfer switch. Many TSs now have internal switching which can be used to select which appliances can operate at any one time.
 
121111-2146 EST

My following comment is from the perspective of an informed user, and experience.

I have a 200 A panel. Under normal typical daytime use my power consumption is usually below 3 kW. You can see plots of this at http://beta-a2.com/energy.html . Not all of this load is necessary under emergency conditions. I have a 5 kW Honda as a backup and it is adequate for my real necessary electrical needs. It is still a gas hog. Probably between 5 and 10 gallons per day.

What do I really need to run. One refrigerator, two freezers, furnace fan motor, TV, radios, some lights, and various computers. Besides these I have also run the washing machine motor, and the dryer motor.

If all motors do not come on at once my generator can handle the inrush currents. So on starting up the generator one sheds loads.

If someone were trying to sell me a backup generator they would probably be suggesting upwards of 15 kW. How often do I need this backup? Not often. Within the family we have four generators of about comparable size, three houses and a shop. Can not power much at the shop, but can run computers and lights.

.
 
gar said:
My following comment is from the perspective of an informed user, and experience.
Click to expand...

Which is interesting but entirely irrelevant to the requirements of the NEC regarding a whole house generator if the design includes a automatic transfer means.
 
121112-0747 EST

iwire:

Not irrelevant.

First, design for what your real emergency needs are.

Then, design the transfer switch circuits to service only those needs. Use the NEC specifications for those needs. For emergency needs you do not need a whole house powered.

Consider the great northeastern blackout. Where do you get gasoline for that amount of time. I had to drive 40 miles to get gas at that time because the local gas stations had no power to pump gas. The same problem exists on the east coast at the present time.

Suppose you use natural gas to power the generator. This is fine and desirable. But what if there is an attack on our infrastructure. It seems to me it is best to design around maximum efficiency under the maximum of the real minimum load requirement. With an automatic transfer switch system the load conditions before and after transfer are essentially the same. Therefore, the probability of all motors starting simultaneously is virtually zero. Even the probability of two motors starting at the same time is close to 0.

As I discussed in another thread if the engine-generator package has higher inertia, meaning a moderate flywheel, then motor inrush current can be handled with a smaller engine. An engine more nearly sized to the steady state load requirement, rather than the peak inrush requirement. The generator can still be larger if desired, this only adds a small amount of additional windage loss. An excessively large engine, one designed for peak inrush, is where the very high losses occur.

I believe every home should have a backup generator capability, also backup in other respects. The country has become too dependent on centralized sources for basic needs.

.
 
121112-0836 EST

I have to revise my statement on probabilities. It is not too likely that all motors are on at the time of transfer, but it is possible. So we could simultaneously see the inrush of all motors at transfer because these will essentially have to start after transfer.

So the transfer system, either in the transfer switch or separately, needs to sequence or randomize the connection of the inrush loads after transfer. Or the generator package has to have sufficient inertia to handle the inrush of all the loads.

.

.
 
Per 702.4 2011 NEC
Quote:
(2) Automatic Transfer Equipment. Where automatic
transfer equipment is used, an optional standby system
shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of
supplying the full load that is transferred by the automatic
transfer equipment.
(b) Load Management. Where a system is employed that
will automatically manage the connected load, the standby
source shall have a capacity sufficient to supply the maximum
load that will be connected by the load management system.

Quote:
System Capacity. The calculations of load on the
standby source shall be made in accordance with Article
220 or by another approved method.
 
gar said:
121112-0747 EST

iwire:

Not irrelevant.

First, design for what your real emergency needs are.

Then, design the transfer switch circuits to service only those needs. Use the NEC specifications for those needs. For emergency needs you do not need a whole house powered.
Click to expand...

That is not whole house backup, that is backing up selected loads. OP is asking about whole house back up.
 
hillbilly1 said:
One word of warning though, Generac will lie to you about what they will run. I had several installs where the manufactures rep told them Oh yeah! It will run your A/C! I knew it wouldn't, but moved the circuit anyway to show the customer it wouldn't. Yep, generator bogged down. Startup killed it.
Click to expand...

If the generator is 5 kVA and the A/C is 4 kVA it will run it, but in most cases will have a hard time delivering the surge necessary to start it. If you find a way to "soft start" the AC unit, it will work. They don't exactly lie to you, they just don't tell you all the truths.

I have a 8000 watt portable generator. It will start my 3 ton heat pump, but only if there is very little other load at the same time, and it really draws the voltage down and still has a hard time getting started. Once it is started I can run other loads within reason, with little problems. This heat pump only draws about 6 amps when in heating mode, but it's rated load is more like 22-24 amps.
 
kwired said:
If the generator is 5 kVA and the A/C is 4 kVA it will run it, but in most cases will have a hard time delivering the surge necessary to start it. If you find a way to "soft start" the AC unit, it will work. They don't exactly lie to you, they just don't tell you all the truths.

I have a 8000 watt portable generator. It will start my 3 ton heat pump, but only if there is very little other load at the same time, and it really draws the voltage down and still has a hard time getting started. Once it is started I can run other loads within reason, with little problems. This heat pump only draws about 6 amps when in heating mode, but it's rated load is more like 22-24 amps.
Click to expand...

Please see the other post about generator sizing, these are automatic units, so unless the homeowner just happens to be next to the panel to shut off all other loads, it's not going to work. Not many people want to just run their A/C only from the generator. The Generac rep makes no mention to the customer about having to pay extra for a soft start or load management to make it work. Typical salesman!:lol:
 
121113-1008 EST

For anyone that wants to answer:

The connected load via an automatic transfer switch to a gasoline engine generator consists of:
1 - 20 cu-ft refrigerator
2 - 20 cu-ft upright freezers
2 - 1/3 HP furnace blower motors
1000 W of tungsten filament incandescent bulbs
Miscellaneous small items.

Assume all loads will simultaneously start from a cold condition at transfer time. This means motors are stopped, and lights cooled to room temperature.

What size engine generator would you select?

I am going to the battery show, and won't be back until tonight. I will more accurately define the loads if necessary.

.
 
hillbilly1 said:
Please see the other post about generator sizing, these are automatic units, so unless the homeowner just happens to be next to the panel to shut off all other loads, it's not going to work. Not many people want to just run their A/C only from the generator. The Generac rep makes no mention to the customer about having to pay extra for a soft start or load management to make it work. Typical salesman!:lol:
Click to expand...

You did not mention anything I wasn't already aware of. Withholding some of the truth is not exactly the same thing as telling a lie, and is used to market almost any product.
 
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