Whole house generator

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fisherelectric

Senior Member
Location
Northern Va
Customer did his own research and bought a Generac 5875-0 to run his house (20K, NG). Called me to come wire it up for him. He has about 3000sq ft (1980s) with all gas furn, HWH, and cooking, elec dryer plus standard kitchen appliances. The AC, which is his main concern, has LRA 130 and RLA 26. This generator is rated at 83 amps. Transfer switch has built-in load management capability. I could not find out if this generator would handle the LRA needed to start this AC properly so I called Generac and spoke to a tech there who told me that they do not publish the "surge ratings" of their generators but that this unit had a "surge rating" of 143 amps. The specs say it will handle up to 5 tons AC, which is what the customers AC is. Does anyone with more knowledge and experience with these units than me know of any reason why this 20K generator wouldn't work. A Generac "dealer" told this customer he needed at least a 35K unit. Thanks

Steve
 

S'mise

Senior Member
Location
Michigan
.... a "surge rating" of 143 amps. The specs say it will handle up to 5 tons AC, which is what the customers AC is. Does anyone with more knowledge and experience with these units than me know of any reason why this 20K generator wouldn't work. A Generac "dealer" told this customer he needed at least a 35K unit. Thanks

Steve

It will start the 5ton AC. Remember, the generator has to be able to handle the load served(the entire house!) If the dryer and AC are load shed you are probably fine, but you should do a load calc on the house to be sure.
 

satcom

Senior Member
Just a note, we ran into a few of these homeowner purchased gen sets where they miss checking the NG supply volume and after they are installed they have to up grade the NG supply if the gas utility will approve it always check with your gas supplier before you order
 
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goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
Just a note, we ran into a few of these homeowner purchased gen sets where they miss checking the NG supply volume and after they are installed they have to up grade the NG supply if the gas utility will approve it always check with your gas supplier before you order
I just want to add a comment to this. Several years ago I had a plumber install a gas line for one of these generators. Unbeknownst to me at the time he had tapped off the 1/2" gas line after it had hit the furnace, water heater and dryer. Long story-short, it fired up instantly in the summer when I installed it but when the winter came there wasn't enough pressure in the gas line at the generator inlet to fire the unit. It kept trying to start until it finally blew a hole in the side of the engine. Generac stood behind the generator and replaced the engine but the tech who came out to service the unit told me I needed a 1" gas line at the generator gas inlet to insure the proper pressure (even though they break it down to 1/2" or 3/4" with the flexible hose). The plumber came back and re-installed a 1" gas line directly after the meter and there hasn't been a problem since.
 

kwired

Electron manager
Location
NE Nebraska
I just want to add a comment to this. Several years ago I had a plumber install a gas line for one of these generators. Unbeknownst to me at the time he had tapped off the 1/2" gas line after it had hit the furnace, water heater and dryer. Long story-short, it fired up instantly in the summer when I installed it but when the winter came there wasn't enough pressure in the gas line at the generator inlet to fire the unit. It kept trying to start until it finally blew a hole in the side of the engine. Generac stood behind the generator and replaced the engine but the tech who came out to service the unit told me I needed a 1" gas line at the generator gas inlet to insure the proper pressure (even though they break it down to 1/2" or 3/4" with the flexible hose). The plumber came back and re-installed a 1" gas line directly after the meter and there hasn't been a problem since.

Why would gas pressure be any different in winter? Unless furnace, water heater or dryer do not require AC power to operate (water heater is most likely) there will be no gas usage at time of starting the generator if it is starting during a power outage. Unless this happened while starting for weekly or monthly exercising.
 

goldstar

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
Why would gas pressure be any different in winter? Unless furnace, water heater or dryer do not require AC power to operate (water heater is most likely) there will be no gas usage at time of starting the generator if it is starting during a power outage. Unless this happened while starting for weekly or monthly exercising.
That's exactly when it happened. FYI, summertime you don't use the furnace, dryer and water heater may have had a demand for gas at times other that those that the generator was exercising, etc.
 

satcom

Senior Member
Why would gas pressure be any different in winter? Unless furnace, water heater or dryer do not require AC power to operate (water heater is most likely) there will be no gas usage at time of starting the generator if it is starting during a power outage. Unless this happened while starting for weekly or monthly exercising.

In the winter months the utilities reduce the supply pressure to keep up with the demand, last winter a few days they were so low it cooled the house, I can just imagine a gen set trying to start.
 

kwired

Electron manager
Location
NE Nebraska
In the winter months the utilities reduce the supply pressure to keep up with the demand, last winter a few days they were so low it cooled the house, I can just imagine a gen set trying to start.

I would think they would increase pressure to keep up with demand. Just like POCO's raise voltage when demand increases to compensate for voltage drop. If they were to drop pressure the pressure at the users end would just get even worse.
 

Volta

Senior Member
Location
Columbus, Ohio
I don't know anything about intentional lowering of the pressure when it is cold, but I do know that furnaces will be running a bit more in the winter months. Stands to reason that the supply pressure will be lower.
 

mtfallsmikey

Senior Member
Why would gas pressure be any different in winter? Unless furnace, water heater or dryer do not require AC power to operate (water heater is most likely) there will be no gas usage at time of starting the generator if it is starting during a power outage. Unless this happened while starting for weekly or monthly exercising.

Not only is a load calc. needed for the electric, but one is also needed for the gas supply. That will depend on if there is a 1/2 psi or 2 psi system installed. might have to upsize the main from the meter. 1 c.f of LP gas has a capacity of producing 2,516 BTU, same amount of N.G. has 1,030 BTU.
 

satcom

Senior Member
I would think they would increase pressure to keep up with demand. Just like POCO's raise voltage when demand increases to compensate for voltage drop. If they were to drop pressure the pressure at the users end would just get even worse.

The gas Utilities are under contracts with the major gas suppliers if they exceed the volume contracted for they pay a premium price, if the pipe line can carry the extra volume, but that is not the big issue, some of the gas network is aged and was designed to supply lower volumes, many of the older low pressure systems have been replaced with high pressure at the customer site, but even with these improvements, cold weather still strains the systems
 

kwired

Electron manager
Location
NE Nebraska
The gas Utilities are under contracts with the major gas suppliers if they exceed the volume contracted for they pay a premium price, if the pipe line can carry the extra volume, but that is not the big issue, some of the gas network is aged and was designed to supply lower volumes, many of the older low pressure systems have been replaced with high pressure at the customer site, but even with these improvements, cold weather still strains the systems

What does delivery pressure have to do with volume purchased? It is either being used or it is not no matter what the delivery pressure is.

This is analogous to supply voltage for electrical distribution. The amount of energy supplied by the generation plant is same for a given connected load at the customer regardless of the voltage. Changes in voltage will alter the total usage somewhat but if final voltage is not within tolerated range there will be equipment problems. Same for gas supply, if proper pressure and volume is not maintained there will be equipment performance problems, otherwise it still takes same amount of gas to do same amount of work.

Cold weather for an undersized system will cause a drop in pressure because the demand is more than system can supply. Increasing pressure can compensate for this but may have other consequences. I am not a gas man but this is fairly basic physics here, similar situation exists with other pressurized systems like air, water, steam, etc.
 

renosteinke

Senior Member
Location
NE Arkansas
"Customer did his own research."

"Dealer recommended ....."

Just when did this become your concern? If the customer is of the opinion that electricians are but semi-retarded chimps, good only for pulling wire, let them stew in their own decisions.

Note that clearly in your contract. The scope of the job is "to install wiring connecting customer selected equipment." Note also that the customer "has been advised that his equipment selection may not be appropriate." Make clear that you do not make any claims regarding the performance of the system, that you are simply installing the wire.

If you're feeling really nice, you might accidentally use a larger pipe size, to facilitate installing larger wire should a larger genny proove necessary.

One final note: more HVAC stuff is coming equipped with 'soft start' or VFD circuitry. Such would greatly reduce the starting current needed.

In any event, if the customer chooses to accept the risk, it's his problem.
 

fisherelectric

Senior Member
Location
Northern Va
I don't think the customer regards electricians as chimps, I think he's trying to save a couple thousand bucks by buying direct rather than paying a dealer. I don't mind having some input into his choice of generator, I would rather hook-up the correct size generator the first time and I am not interested in selling him a generator, any service contract or warranting any generator (that is spelled out in the contract), I will be holding the permit for the electrical install so I do have some responsibility to make sure it's done right. I recommended a local plumber buddy of mine to run the gas and he'll be responsible for that.
 

kwired

Electron manager
Location
NE Nebraska
20 kw generator may start a 5 ton compressor without much trouble, if the compressor is the only load at the time. If other loads are running it may not be able to do it, and most likely you will see a pretty severe voltage drop while it is starting.
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
In the winter months the utilities reduce the supply pressure to keep up with the demand, last winter a few days they were so low it cooled the house, I can just imagine a gen set trying to start.

I always figured the demand was so high that they could not keep up the pressure. No different then adding loads to an already maxed feeder supplied by POCO transformers that will be adequate most of the time.
 
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