NEC Codes/ Generator Hookup

RonL · Oct 13, 2004

Do the new NEC codes come out in 2005? I used to keep up with NEC but its been a while. I need to do design and spec through this Winter and would like to have the latest code. Also, I always got the McGraw Hill guide to NEC as well as The American Electricians Handbook. Are these still valid guides?
I intend to install a whole house double throw switch for use with a generator. They are usually referred to as double pole. Is there some reason that neutral is not swiched out? It seems to me that when a generator is running neutral has the potential to carry current back into the grid. Do I need to go back and review my electrical theory? Along the same line when I build my new house it is going to be a vacation house until I retire. As such it will be unnocupied much of the time. I am thinking of installing an automatic propane powered generator on a branch circuit that would power heat, sump pump, refridgerator, freezer, security system, and some lights for when I am not there. I thinking that the propane system would be sized just large enough to run those emergency circuits. I also intend to get a diesel powered welder/generator that puts out 12 KW. I figure that If I'm home and the power goes out I can plug the welder/generator into one input of the whole house switch, turn on the welder/generator, and throw the double throw switch. Since power would be restored to the whole system the emergency subsystem would shut down. However. I'm thinking that there may be times when I may want to run both generators by shutting off the circuit breaker that feeds the emergency subsystem. The emergency subsystem would kick in and switch the subsystem on via the double pole, double throw switch. This switches the two hots over to the second generator. The hots are isolated but it seems to me that all the neutrals are bonded. Is this a problem? Do I need to go back and study electrical theory?

RonL

surfran · Oct 14, 2004

You are correct that the NEC's newest edition is 2005 and it is available now through the NFPA.

v8dave · Oct 15, 2004

Check with your local code enforcement agency, i.e., city or county building department. Different jurisdictions require different ways of handling the neutral line with a transfer switch.

Both my county building dept and the local utility company want the all neutral lines connected all the time. It makes sense as the neutral is really supposed to be a common ground (that is, it is a ground line run to all locations and not just an earth picked up at the plug location). But, some jurisdictions do require a tripple pole, double throw for transfer switches. Just ask, they'll tell you.

plaindave · Oct 18, 2004

Make sure that the switch you buy is designed for switching under load. When the power comes back on you will probably forget to turn off the main breaker and a standard switch will arc like you won't believe.

I built my own switch using circuit breakers and a kirk key interlock system. I have to open one breaker before I can close the other. 100% safe, it provides overload and short circuit protection, and breakers can be switched under load.

About 5 years ago I worked on emergency generator systems. I don't recall the reason but neutrals were almost never switched. I can find out if you really need to know, but I'll bet 95% or more of the jobs didn't switch the neutral.

RonL · Oct 21, 2004

Dave and Dave

The house will be powered by a two hundred amp service. The service entrance will be on a pole at the beginning of the driveway. There will be a meter box and a two hundred amp breaker. Neutral and ground will be bonded at this box. Since this is technically the service entrance and the box at the house is technically a sub panel I will be running four wires from the pole to the house and not bonding the neutal to the box. Since the two hundred amp breaker at the pole is the overload protection the breaker box at the house will be main lugs and will not need a main breaker. The double throw switch will be mounted next to the breaker box and will be the main shutoff. The double throw switch will a rated 200 amp full load make switch. From the breaker box I'll run a line to an automatic transfer switch and from there to a sub panel that runs the absolute necessities ( furnace, freezer, refridgerator, sump pump, lights , and alarm system). The infeed from the manual transfer switch will be the main power, of course, and a 50 amp plug to connect to a 12 KW welder/generator. The automatic transfer switch will be connected to a propane powered generator. The propane generator will maintain the house while I am not there. If I'm there I figure that I can use either or both generators. If I run both generators I figure that I can run the necessities and some "luxuries". However, there is a place in my mind that wonders if there is a problem with having the neutrals from multiple power sources bonded. It may be irrational. I'm going to break out my electrical texts and study the matter over the winter to set my mind at ease.
In looking at propane emergency standby generators I find that all generators up to about 20 KW use basically the same 100 amp automatic transfer switch. As far as the switch is concerned there is no cost premium to going to a larger generator. The smaller, less expensive generators are powered by air cooled engines that run at 3600 RPM. These seem to have less exact voltage and frequency control and less motor starting reserve. The larger, more expensive generators are powered by water cooled engines that run at 1800 RPM. These seem to have more precise voltage and frequency control and more motor starting reserve. They also appear to be more efficient in the use of fuel. In fact, several of the manufacturers use the same 150 cubic inch Ford engine to power their 20 KW generators. At full load this generator uses less fuel than some of the smaller generators at full load. At partial loads their does not appear to be a premium to using this generator compared to using a smaller generator at full load. In the long run it may be a better proposition to go with a larger water cooled generator. The larger generator would easily power the necessities when the house is unoccupied and power a number of luxuries when the house is occupied. I don't know what the future holds as far as the power grid is concerned, but I think that a water cooled generator would last for the rest of my life. What is the life span of an air cooled 3600 RPM engine?
When I build the house I plan on having an oil fired FHW heating system. Since I prefer to cook with gas, the stove will have gas burners with electric ovens. Because I like the recovery rate of gas water heaters I'm going to have a propane water heater. There will be a gas log fireplace in the dining room and one in the master bedroom. There will a wood stove in the " great" room connected to the kitchen via twin french doors. I plan on having a 1000 gallon fuel tank and a 1000 gallon propane tank. Weather can be severe in the mountains of New Hampshire but I figure with this setup I will be able to ride out even an extended power outage.
Well, enough rambling.

RonL

v8dave · Oct 22, 2004

Ron, I started a reply here but it got rather long winded and repeated some stuff I'd posted previously. So, I sent it by a PM.

varmint · Oct 22, 2004

I've been following this thread with interest, since I am going to be hooking up my little 6KW generator I just bought. In reply to your question "What is the life span of an air cooled 3600 RPM engine?" <font color="green"> </font> I noticed in the lit. that came with my rig that it meets the '06 Calif. emission requirements "for a service life of 250 hours" which doesn't really answer how long it will last, but you can reasonably guess that it's a lot less than a liquid cooled diesel, which I have heard will run for 12,000 in a generator application before a rebuild. That's for a Cat. engine, but others should be the same. Gas/LPG would be somewhat less.

v8dave · Oct 23, 2004

Around 1,000 hours would be a good life expectancy for a well maintained simple air cooled gas engine. Engines with pressure lubrication systems and or water cooling seem to last longer. Generators with the higher end Honda engines have pressure lubrication and last longer too. I'd guess that a small engine running on LP gas might last a bit longer because of the cleaner fuel.

The LP unit Ron is looking at has a small Ford auto engine. Some run at 3,600 rpm some at 1,800 rpm. Probably the 1,800 rpm version can be expected to last quite a while too.

Some diesel 3,600 rpm units can last quite a long time. I expect the Yanmar 5.5 KW (air cooled) diesel and Kubota (and Yamaha re-labeled) 6.5 KW (water cooled) diesel gensets will last multiple thousands of hours.

RonL · Oct 23, 2004

Dave

The way I intend to set up the system I don't believe that I will have to synchronize the generators if I run them both at the same time. The generators will be running seperate circuits and the generator's "hots" will be isolated. That is the question with the bonded neutrals. The welder/ generators that I am looking at are diesel fueled. If I am not there and the power goes out the automatic switch powering the sub panel will start up the propane generator and feed the sub panel with the essentials. If the power goes out while I'm there the sub panel will power up, of course, but I will have the option of throwing the manual whole house switch. If the welder/generator is powering the whole house and the breaker to the emergency sub panel is on, the propane generator will shut down. If the breaker to the emergency sub panel is off the propane generator will power the sub panel and the welder/generator will power the main panel. I figure with this setup I will be able to use either generator or both. With judicious use of the breakers I will be able to marshall my fuel supplies. I figure that for the most part during an extended power outage I will be using only one generator. There may be times when I may want to run both generators, such as when I want to run the washer and dryer to do laundry, when I want to run tools to do repairs, or when I want to fire up the dishwasher when the dishes pile up too deep.

Varmint,

I have been researching emergency generators for a while. My initial thoughts were that I would install a small air cooled automatic generator. There are some nice units in the 8 to 10 KW range powered by Honda GX series engines. These engines have earned a good reputation. The larger water cooled generators in the 12 to 20 KW range are roughly twice the price of the smaller air cooled units. The constant is the automatic switch and wiring. It pretty much costs the same to install an automatic unit up to about 20KW. The difference then is the initial cost of the unit itself. There is also the efficiency of the units. The water cooled units that run at 1800 RPM use fuel more efficiently. I am thinking now that over the long run in my circumstances that the sweet point is a water cooled 20KW unit. I am thinking that I will run a 100 amp feed from the main to the 100 amp automatic switch and from there to a 100 amp sub panel. The sub panel would feed the essentials ( heat, freezer, refridgerator, sump pump, lights, and security system). It would also feed the well pump, kitchen appliance outlets and some other " luxuries" ( home entertainment system). The well pump would be shut off when I am not there. The other "luxuries" would not be used when I am not there. The generator would have no trouble powering the essentials when I am not there. At 20 KW ( 83 amp at 240) the generator should provide enough power to run a number of " luxuries" and allow me to live through an extended power outage quite comfortably. The 20 KW generator uses 2.4 gallons of propane per hour at full load. Running the generator at full load 24 hours a day a 1000 gallon supply would last over seventeen days.
I was trying different projections for the future. One of my calculations figured on running the generator for a total of 2 weeks a year. Over a thirty year period that comes out to about ten thousand hours.
Again, enough of my ramblings.

RonL

MikeIA · Oct 23, 2004

If you don't want to shell out for a seperate service entrance and a double-throw transfer switch, you can go with a "Combination Service Entrance Device". This is one box that houses your meter, a main breaker, and a bunch of branch breakers. It has feed-thru lugs to power your house at full amperage, and an interlock can be installed to turn one of the branch breakers into a generator feed.
You won't be able to transfer the full 200 amps....I think it maxes out at 100 amps for branch breakers...but it didn't sound like you were going to, anyway.

The CSED I used was a Square D RC816f200c , which is Homeline, but you can get a Generator Kit for the QO series of CSED's , also.