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generator sizing

k1c | Posted in General Discussion on April 9, 2010 06:53am

How should I size a generator?  That is, running watt and how much to add for start (surge) watt?  I’ve come across adding 20%, 100% and even running watt x 3. 

As for making the connection, I’ve come across the neutral/ground bonding issue.  I plan to use Square D manual transfer switch with breakers at the service panel and in the transfer switch.  The service panel is bonded but I plan to separate neutral and ground at all other connections.  This case seems to call for neutral/ground bonded generator.

However, I also came across this article:

“Note that many models do have their neutral bonded to the metal frame.  If such a model also has a GFCI (GFP), it may cause a problem:  when a neutral is bonded to ground in two places its current will split, generator’s GFCI breaker will trip, and no outlets will function.  Possible solutions to this problem are either to not ground the neutral a second time at the genset, or use a so-called neutral switching device (3-pole or 4-pole transfer switch).”

What is this about?  I don’t necessarily need a science lesson, although appreciated, but this is beginning to sound like it is not a diy job.  Does this article apply to portable generator?

If my generator happens to be floating neutral, can I still wire the transfer switch the same way as I described above? 

I have service panel which is bonded and a sub panel which is not.  I need to put transfer switch in each panel.

Thank you in advance.

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Replies

  1. Scott | Apr 09, 2010 07:05pm | #1

    I'm no expert, but I'll tell you this... I've got a 5KW generator rated at 6250W surge, and it doesn't do a good job running my 1hp, 240 volt well pump. It will start it, but after a while the pump starts to sound funny, and I'm worried that it's being hurt. If motor loads are part of your plan, buy the biggest one you can afford.

    There are often good deals on used ones.

  2. DanH | Apr 09, 2010 08:51pm | #2

    Normally you size a generator based primarily on the HP starting load, but as Scott points out there are other considerations.

    You need a generator that is large enough to start your largest motor load (the generator specs will often tell you this, or there are formulae to be found to figure it out).  But you also need to consider the largest combined running load.  With something like a well pump the motor starts fairly easily but then has to work harder as the head of water builds up, so just because it starts doesn't mean it will run.  So also size the generator based on the "full load amps" of the motor (and again there's probably a formula around that translates HP into that, if it isn't in the motor specs).

    Re the GFCI, I wouldn't think a generator intended for powering a house would have a GFCI, due to this issue -- the GFCI-equipped units are intended for situations where you plug directly into them.

    1. Clewless1 | Apr 10, 2010 09:41am | #5

      I know little about this topic. But I'd think a 5 KW gen set running a 1 hp motor ... which is what 0.745 kw (not sure if I got the conversion exactly right) ... that is at its full load capacity ... just after starting and before acquiring the full head, it's going to run at much less than that. It seems to me if he has 5 kw generator, it should anytime easily accomodate the well pump in addition to some other loads.

      Seems like he has a problem if his well pump is struggling. Maybe just because you have 5 kw generator doesn't mean you have the amps?? That's where my electricical falls apart a bit. But volts and watts IS amps, right?

      1. Scott | Apr 10, 2010 11:27am | #6

        Maybe I'm imagining something that isn't a problem, but the sound bugs me.

        The pump seems to start OK, and the generator labors under the load as one would expect. After the pump runs for a number of minutes and the pressure tank is about half full, the pump motor starts to make a sort of squirming sound that can be heard 200 feet up in the pumphouse. The tank now has enough water for us to use, but the pressure switch hasn't yet opened. At that point I always shut if off.

        I  should say that this is a fairly rare occurance... our power doesn't go off that often.

        1. gfretwell | Apr 10, 2010 12:07pm | #7

          The squishing sound is somewhat normal. That is the pump bypassing as it approaches iti's pressure limit.

          Maybe you are just paying more attention when it is running off the generator.

          Have you measured the voltage? Do you have a way to look at frequency? Better digital meters will do that.

          1. Scott | Apr 10, 2010 12:26pm | #8

            >>>Maybe you are just paying

            >>>Maybe you are just paying more attention when it is running off the generator.

            It's certainly more pronounced when running on the gen. In fact, it's completely absent when running off the service.

            >>>Do you have a way to look at frequency? Better digital meters will do that

            You mean the AC frequency (60hz)? What would that tell me?

            I'll have a look to see if my meter measures this.

          2. gfretwell | Apr 10, 2010 03:21pm | #9

            The frequency of any non-inverter generator is based on the rotor speed so if the frequency is falling off it indicates the generator is slowing down, either from load or from a shaky governor.

            The speed of the pump motor is again depending on the frequency so they are all related.

            As a sanity check you can plug in an old style motor driven clock if you have one. They also have a synchronous motor. If it is losing time, the generator is slow but it might take a while to see the difference.

          3. Scott | Apr 10, 2010 08:41pm | #10

            Thanks, that all makes sense. Yes, this is an older non-inverter type, so RPM is crucial. I'll check it out.

          4. DanH | Apr 10, 2010 09:34pm | #11

            But all common single-phase AC motors "slip" to a degree, so a minor variation in AC frequency isn't a big deal, and not as big a problem as a drop in voltage.

  3. DanH | Apr 09, 2010 10:34pm | #3

    The whole ground thing is a touchy issue with generators.  If you read the code a certain way most standby generator setups in the country would be illegal, since they have neutral-ground bonding in the breaker panel, while the code (if read a certain way) appears to require it at the generator, and many generators come with neutral & ground bonded.  But having bonding in two places is a code violation.

    The same with grounding of the generator.  The code seems to require it on the one hand and forbid it on the other hand.  (I personally favor grounding the generator to a nearby ground electrode, in addition to tying it to the house ground.)

    The thing is, the code doesn't really fully acknowledge the existence of temporarily connected, standby generators -- it primarily assumes fixed installations where the genset is bolted down and hardwired.

    1. gfretwell | Apr 10, 2010 01:10am | #4

      There is a good write up of the grounding in the Soares grounding book but basically, in any system, you only bond the neutral once. If you are not switching the neutral in your transfer equipment you will NOT bond the neutral in the generator. If the transfer equipment does switch the neutral you do bond the neutral in the generator.

      The instructions with the generator should tell you where that bond is and how to lift it if necessary.

  4. junkhound | Apr 11, 2010 09:37am | #12

    That is, running watt and how much to add for start (surge) watt?  I've come across adding 20%, 100% and even running watt x 3. 

    The 20% is the right number for the GENERATOR!. 

    Now, the other side is what is driving the generator!.  Now you are talking 3X in some cases for a 2 cycle prime mover or old B&S one lunger.

    My old Datsun 1200 cc 55 HP (at 4000 RPM) running at 1800 RPM drives a 12 kVA Harbor Freight.Northern hyd type generator.  It will start and run a  15HP motor with no problem. 

    The ONLY reason it will run that big of load load is the engine does not 'poop out' when hit with the start current from the motor.  A 'little' 15 HP one lung gas banger can stall out and drop below the power curve (i.e. cannot develop enough torque to drive the generator back up to speed when hit with motor inrush current) 

    BTW, you really need to talk KVA and not just watts or HP when discussing generators.

    1. DanH | Apr 11, 2010 02:09pm | #13

      BTW, you really need to talk KVA and not just watts or HP when discussing generators.

      True, since a motor may have a high inductive reactance at certain points in its load curve, with the voltage and current seriously out of phase, so that watts doesn't come close to KVA.  But unfortunately consistent KVA figures for small motors and generators are apt to be hard to come by.

      1. Scott | Apr 12, 2010 01:20am | #14

        >>>But unfortunately

        >>>But unfortunately consistent KVA figures for small motors and generators are apt to be hard to come by.

        Exactly.

        Art's point is most excellent, but it's the exact reason why most manufacturers avoid stating KVA ratings.... it's easier to entice buyers with half-truths than with the full truth.

        Honda, Yamaha, (and others).... take note.

        Scott.

  5. k1c | May 08, 2010 01:45pm | #15

    I think I finally got the informations I need for using a portable generator.  I got the final information I need from a site called Generatorsdirect.com.  I decided the information from this site fairly matches what I was thinking from the research I've done. 

    There are many brands of generators and some people recommend the most expensive (mainly Honda) or some say that cheaper is fine for use in blackout times.  I decided that Honeywell as recommended by above seller has all the features I want:  enough watts, low noise and electric start.

    Deciding how much watts I needed was more tricky but finally I decided to put a blindfold over my eyes and follow the recommends from above seller, as well.  Finetuning the watts and amps I need was going overboard.  There are not that many generators with range of that many watts, anyway.  This seller recommends that for average house with power needs at blackout times to feed heating, refrigerator and some lights will do fine with 5000 watt range generator.  Smaller was 3000 range and biggest was 10,000 watt range.  I just needed the middle.

    The generator I selected comes with floating neutral electric connections.  That means the neutral wires and ground are not connected on the generator body itself.  Its manual says I must ground the generator during use.  I plan to take the ground from outdoor power connections to the manual transfer switch to the ground point for the main service panel.

    I hope this information helps out and thank you to all who helped out.

    1. Scott | May 08, 2010 09:39pm | #16

      I followed everything you said up until:

      >>>I plan to take the ground from outdoor power connections to the manual transfer switch to the ground point for the main service panel.

      This doesn't constitute grounding the generator, but I suppose it may bond the branch circuits to your service ground. In my case I buried a grounding plate and ran a bare piece of copper to a lug that I fastened to the genset chassis.

      1. gfretwell | May 08, 2010 11:58pm | #17

        If his transfer switch does not switch the neutral he is exactly right. You do not reground the neutral and the grounding of the generator case will be via the green wire connection to the service.

      2. k1c | May 09, 2010 01:41am | #18

        generator sizing

        This is what I plan to do about the grounding.  Since the generator comes with floating neutral, the manual said to ground the generator.  I think this means I have to sink 8' copper rod into the ground, right?  I wasn't sure if I need to go to this trouble and the ground around the house is rocky and cement-like gray dirt.  I would not be mixing the neutral and ground anywhere along the way to the transfer switch and transfer switch will also have separate neutral and ground buses.   I planned to take the ground wire from the transfer switch to the point where the main service panel is grounded.   This ground wire will not enter the service panel.   The main panel is grounded to copper water line from the street.  I know many people don't like this arrangement, but the house has worked for about 60 years this way.  I also know that this water line has firm contact with earth due to leak repair that was done.

        Anyway, thank you to all for the advices and the help.  I enjoyed reading all the discussions.  I found a interesting article that sort of drew the picture for me about the grounding issue.  I lost the site address while converting it to the pdf, but here is the title:  Understanding the Neutral to Ground Connection - It's Meaning and Consequences.  It is by Capt. David Rifkin.

        1. gfretwell | May 09, 2010 11:01am | #19

          If you are connecting this to the house, it will be grounded via the equipment grounding conductor (the green or bare wire). The neutral will get grounded in the panel by the main bonding jumper if you select a transfer switch that only transfers the hot conductors. You accomplish nothing by driving another rod but you certainly can if it makes you feel better. It still needs to be bonded to the service ground.

          1. k1c | May 26, 2010 04:58pm | #20

            generator

            Sorry about the late reply.  I didn't think to check back, and I don't get notified by email.  I plan to use Square D manual transfer switch where I have to remove the circuit completely from the main box.  I saw it in a how to video.  I think the automatic transfer switches get wired where only the hot wire need to be rerouted, but I thought the manual switch is better for the simple technology (least chance of malfunction).

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