Connecting a portable generator to home main panel

Discussion in 'Electrical Forum discussion & Blog' started by dabiz7, May 19, 2010.

  1. dabiz7

    dabiz7 New Member

    Messages:
    42
    Have started looking into the possibility of having a portable generator available to run a few circuits in the house in case of power outage. I understand the need for the manual transfer switch, but I am a little unclear about how it all operates. A few questions:
    1) Doesn't the transfer switch operate by completely isolating each side of the switch from each other? When switched to main service, then the generato side of the switch is completely isolated and vice versa. Is that right?

    2) If yes, I don't understand why I can't just install the transfer switch between the service entrance and the main panel. When power is available, transfer switch is over to 'main' and all works normally.
    When power goies out, move transfer switch to 'back-up' , cut off all breakers in the main panel, switch back on the few that I want to operate, fire up the generator and I'm in business.

    If I am missing something, please advise. Thanks!
  2. drick

    drick In the Trades

    Messages:
    392
    That is correct - almost. The hot conductors from the generator and your power co. must remain isolated from each other. The neutrals and grounds are actually all connected together.

    You can.

    You can also back feed a 2 pole circuit breaker in your existing panel (assuming you have space for one) and as long as you remember to turn off the main before connecting the generator. Of course if you forget or do it in the wrong order you could kill yourself, a line worker, burn down your house, etc. To solve that problem you may be able to use one of these: www.interlockkit.com. They prevent the main and the generator breaker from being turned on at the same time.

    -rick
    Last edited: May 19, 2010
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,015
    Location:
    New England
    A transfer switch is a break before make kind of switch, which ensures that you never have two sources connected to the same output protecting both your generator and the power company and it's employees from the possibility of power being fed back into the distribution network. Unless you buy a generator that is large enough to handle all of the load of your house, you should not consider placing it before the main breaker, as you could end up with it overloading your generator the first time you flip the manual transfer switch. This is especially important if you ever decide to move to an autostart generator and an auto transfer switch to minimize outages problems. You may not be home when the power goes out...an automatic system is nice. Normally, people isolate a few critical circuits: heat, refrigerator, a well pump (if you have one), and maybe a few lights and outlets...maybe the stove if the generator can handle it.
  4. Smith333

    Smith333 Member

    Messages:
    108
    I put one of these in a few months ago: http://www.reliancecontrols.com/ProductDetail.aspx?30408B

    It mounts right next to the main panel. Rewiring is a breeze if you have enough space in your main panel. It has double-throw switches so it's impossible to backfeed.

    I've only had to use it once so far, but it's able to let me run my well pump, fridge/freezers, and lights. Paired it up with a Generac 6500W generator. The well pump has to be run by itself, but its something I can live with
    Last edited by a moderator: May 19, 2010
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,015
    Location:
    New England
    If you decide to do this, you should only consider doing it per the local codes with a permit and inspection. This almost certainly will require an approved transfer switch, which could be manual or automatic. While many methods could get you power into the panel, most of them could kill you or someone else if you mess up. the risk of property damage or injury is just too great. In the dark, you might just do something out of order or make a mistake...do it right, or don't do it.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,641
    Location:
    Cave Creek, Arizona
    The transfer switch is to ENSURE that you can NEVER have the generator and the power company circuits interconnected at the same time. Regardless of how careful YOU are to operate a manual circuit breaker(s) when needed, there is no other way to be sure OTHERS do it correctly.
  7. mattbee24

    mattbee24 In the Trades

    Messages:
    100
    Location:
    Fremont, OH
    What I like to do for little portable generators is this:

    Get a little 8 space sub panel and mount it right beside your main panel. Put in (2) double pole 30a (or whatever size breaker you need for the generator) breakers in the sub panel side by side. Use a mechanical interlock for the 2 breakers so only one can be on at a time. Fill the rest of the sub panel with breakers for the items that will be used when power is out. Then feed one of the breakers in the Sub panel from the main panel with an appropriate sized breaker. Feed the other breaker in the sub panel with the generator.

    This way is usually cheaper than an actual transfer switch, and you don't have to worry about turning breakers off or overloading the generator.
  8. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    Thanks guys for posting a lot of recommendations on procedures on the attachment of a generator to a dwelling unit but I am sorry to say not one of you have come close in the proper installations and what the laws will allow. Poor old jar came pretty close but that installation could still have some hidden dangers.

    Unless you have an in-depth understanding of Article 702 and 250 of the NEC then it would be a good idea to simply run a cord to the receptacle that is already on the generator. Be sure to use a good quality cord and ensure the conductor size is sufficient for the load being served.

    Why put someone in danger just because we want to boost our self esteem in proving our extensive knowledge in something we know nothing about?

    What you don’t want to do is back feed a breaker in your panel and then just remember to turn the main off. The voltage going back to the transformer will be stepped up to the primary level and energize the utility conductors which may be intact all the way back to a substation where the same step-up in voltage will take place.

    This is a common find by utility linemen during inclement weather and slows the agenda of restoring power to the many who have no power and those who desperately need power.
  9. drick

    drick In the Trades

    Messages:
    392
    >>Poor old jar came pretty close but that installation could still have some hidden dangers.

    Hmm,

    Ok, as an example xyz electric installs a generac transfer switch with external inlet for a portable generator setup used by a private residence. We'll assume the circuits connected to the transfer switch have a maximum load that can be safely handled by the generator. We'll also assume the employees of xyz electric aren't idiots and know a thing or two about electrical wiring.

    JW, in my simple example please elaborate on the 'hidden dangers' you elude to.

    I'd also be interested in hearing about what you would install in the above example as a transfer mechanism for a portable electric generator of less then 10KW. Keep in mind this is for Joe Homeowner and not a fortune 500 company so try not to price yourself out of the market (Otherwise Jo Homeowner will resort to plugging his generator into the dryer outlet and manually switching off the main).

    -rick
  10. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina

    Well drick if XYZ was an electrical contractor then he would have knowledge of Sections 250.34 and 35 of the NEC and would not install a “portable generator” to a dwelling unit in any manner.

    Then again we have some idiots that have the mind set that if it works then it must be alright. This is like saying my Honda has 160 MPH on the speedometer so it must be okay for me to drive at this speed.

    The simple answer to your question is, I would not wire a portable generator to a dwelling unit for any amount of money. I would recommend that the home owner use cords and plug in their appliances directly to the generator.

    WE must assume that as an electrical contractor I have the knowledge of the laws set forth by my state’s Department of Insurance and Licensing Board and would make the installation of any generator on any type of building according to the adopted codes in my area.

    We must also assume that as an electrical contractor I have no control over the self proclaimed experts on electrical installations therefore have no control on how they would handle their installation. Yes I am talking about every poster in this thread.
  11. Jim Port

    Jim Port Electrical Contractor

    Messages:
    156
    Location:
    Maryland
    Isn't 250.34 & 35 just a modifier of 250.30 when the conditions of being cord connected etc are met? Otherwise 250.30 would be required.

    Using 250.34 & 35 to disallow any portable generator installations seems counter to the way I have heard optional systems are commonly installed and approved.
  12. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,534
    Location:
    North Carolina
    The neutral of the generator must be bonded and where this takes place is very important.
    A portable generator has the neutral bonded to the frame. This means that the neutral must be isolated in the service of a dwelling should this generator be connected to the wiring of the house.
    Of course the neutral could be lifted from the frame of the generator and left bonded in the service equipment but at this point the generator is no longer portable, it is electrically connected to the dwelling.
    Using the grounding electrode and bonding of the building would then mandate that the generator is no longer a separately derived system with the neutral being isolated from the frame of the generator and the generator is not longer portable.
    Leaving the neutral bonded to the frame and isolating the neutral at the building service the generator would be required to be connected to a grounding electrode. 250.32(A)(3) requires that this electrode to the bonded to the neutral at the same place where the system bonding jumper is installed. 250.32(A)(1) requires that this system bonding jumper be installed somewhere before the first disconnect. This would be somewhere before the receptacle on the generator.

    It would be so much easier and cheaper to just buy a couple of 50 or 100 feet #12 cords and use the receptacles furnished with the generator than all this bonding and earthling of a portable generator.

    Jim Port
    First let me say that the way optional standby systems are commonly being installed and approved are incorrect as outlined by the code.

    250.32 addresses separately derived systems which would also include transformers. 250.34 and 35 addresses generators even if the generator is being as a separately derived systems as covered in 250.35(A).
    When the optional standby system is a portable generator then section 250.34 applies and there the requirement to connect to earth is relieved as long as the system is used as outlined in 250.34 (A), (B), and (C).
    Take note to the Fine Print Note found under 250.34(C) and we can see that if the portable generator is connected to the premises wiring then it must comply with some pretty strong rules and the common method of installation and approval just went out the window.
  13. andrew79

    andrew79 Licensed Electrician

    Messages:
    18
    Location:
    Ontario Canada
    siemens makes a nice panel for this too....it isolates the neutral as well as both hots by using a three pole breaker with a mechanical interlock and you can size your breakers as you want. Sells for around $180 in canada.
  14. billpowers

    billpowers New Member

    Messages:
    1
    Location:
    richmond va
    generator connections

    I read the comment about the Siemens panel. I looked into the Siemens panel at an electrical supply house and a guy at the supply house told me about a kit that you can buy from interlockkit.com. It was a breeze and makes your panel work like the Siemens panel and only takes an hour or two to put on. I agree that people MUST keep there generators outside even if it is raining. Having it in a garage is dangerous.
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