Connecting A Generator

Let me start by saying that the safest way of connecting a portable generator (genset) or any other power source to a house wiring is via a transfer switch. But what if you don’t have one and a power outage caught you unprepared? Well, there are still some ways of energizing your house without a transfer switch. However, these methods may not be quite safe, may not comply with National Electrical Code (NEC®) and should not be used unless it is really necessary in an emergency. I personally don’t recommend doing this. However, I realize that people will do so anyway, so I have compiled here some information to help you do it right. This information is provided for general reference only without liability of any kind. Remember, you can always use extension cords to feed stand-alone appliances. If nevertheless you decided to power up your home without a transfer switch, I advise you to review the safety issues before going over the available options listed below.

HAZARDS ASSOCIATED WITH PLUGGING A GENSET DIRECTLY TO HOUSE WIRING:

• Feeding electricity back into the grid (“backfeeding“) presents a hazard for linemen or neighbours. Using main breaker instead of a transfer switch still leaves room for human error;
• Using male-to-male extension cords presents electrocution risk because a plug on the loose end can be electrically “hot”;
• Miswiring of cables can create a hazardous condition.

If you decided to go for it,  at least be sure to observe the following basic rules and remember- you do it at your own risk.

IMPORTANT SAFETY RULES TO FOLLOW:

• All work should be done by a qualified electician;
• Always turn OFF the main electric service breaker before you begin any wiring and wear rubber gloves to be on a safe side;
• In order to prevent backfeeding into service lines, keep the breaker in OFF position until you disconnect your portable generator;
• Keep the frame of the genset securely grounded;
• Run it only outdoors with exhaust away from doors and windows;
• Turn OFF the genset first before unplugging the power cable.

If you don’t have a transfer system, in my view, the simplest way of connecting a portable source to your house is by using the main service panel. First of all, flip the main circuit breaker to OFF (OPEN) position. Then the panel’s cover door can be removed.  Most portable generators rated 5000 watt or higher have 4-prong 125/250V twist-lock outlet NEMA L14-30. It provides two “hot” wires, the neutral and ground (see diagram above). Take a cord with 4-wire generator plug on one end and loose leads on the other. Connect these leads after the main breaker to the corresponding downstream lines of your building and ground. Make sure the connection is indeed downstream, not upstream, i.e. not to the lines that go to your meter. Turn off all individual breakers. After you start up the genset, to prevent its overload activate only those lines you need and do it one at a time. Alternatively, if you want a “pluggable” method, an electric drier outlet can be used as described below.

CONNECTING A 4-PRONG CABLE TO 3-PRONG OUTLET.

The homes built prior to 1996 may have a 3-hole 125/250V drier outlet NEMA 10-30R. It provides two “hot” wires and neutral (see diagram). The appliances used with such an outlet had a jumper between neutral terminal and grounding lug. People are often looking for a 3-prong to 4-prong adapter. Well, there is no such thing. In order to connect a generator output to the home wiring via an existing 3-hole drier outlet you can do the following. Take a standard 4-wire generator cord and remove its socket. This will expose four leads. Then there are two options. If you happen to have a loose mating 3-prong plug NEMA 10-30P,  attach it to two line wires and the neutral (refer to L14-30 pinout above). Alternatively, if you already have a 3-prong drier cord, you can tie its ends to L1, L2 and N of the generator cord by using wire nuts. In both cases the remaining green lead can be used to ground the genset (read below).

IMPORTANT SAFETY WARNING. A generator frame must be grounded per NEC®. If your model has a built-in GFCI, then its frame is supposed to be already bonded to the neutral (you should verify this with an ohmmeter). Since neutral buss in your home is grounded inside the main panel, the frame of your genset will likewise be tied to earth via this connection. However, if your model does not have a GFCI, you need to connect its frame to earth. There are several ways of doing this. A safer way is to install a metal rod and connect it to the genset frame. See Article 250 of the NEC® for the requirements. Alternatively, one can attach unconnected green lead in the generator cord to the cover plate of the drier outlet. To do it reliably, I would crimp a round lug to this wire and put this lug under any screw of the plate. Note that if you rely on the cord for the grounding, obviously it will work only when this cord is plugged at both ends. Be sure to do it before turning the genset on. In all cases check continuity between the frame and earth.

The maximum power you can provide to your house with such a scheme will be determined by the ratings of two 120/240V circuit breakers: the one that is in your generator and the one that is on the drier line, whichever is lesser. For example, if your genset has 30A breaker and your electrical panel has a 20 amp breaker for the drier, then maximum power you can draw will be 20Ax240V=4800 volt-amp. In reality you may get less because loads on L1 and l2 are often imbalanced.

To reduce risk, it is important to follow proper power-up and power-down sequences as described below.

POWER-UP SEQUENCE

• Turn OFF the main electrical service disconnect and attach a visible label “Do not turn on”;
• Turn OFF all branch breakers in your distribution panel including the double-pole breaker for the drier;
• Plug your modified cord first into the drier outlet, then into your genset (never connect it to genset first because someone can touch the plug on the other end which may be electrically hot!!! );
• Assure that frame of your generator is grounded (see the safety warning above);
• Start up the genset and let it stabilize for 5 minutes;
• Turn on the double-pole breaker that feeds the 120/240V drier line;
• Start turning on individual breakers on the critical branches that you want to backup one at a time.

A proper reverse procedure is equally important to reduce risk.

POWER-OFF SEQUENCE

• First of all, shut down the genset;
• Unplug the extension cord first from the genset, then from the outlet (never unplug the load side of the cable first since it may still be energized!!!);
• Turn ON the main service breaker (assuming the power is restored).

WIRING A GENERATOR 4-PRONG CABLE TO 4-PRONG OUTLET

Newer homes may have 4-prong 125/250V drier outlet NEMA 14-30R. It provides a separate ground hole besides L1, L2 and N (see diagram). For our task it has the right number of leads, but a wrong geometry . To connect a genset to such a receptacle one can replace the socket in the generator cord by a 4-prong plug NEMA 14-30P. This is a pretty much straightforward task. Then you need to follow the proper power-up and power-down procedures outlined above.

MORE OPTIONS

If you don’t have either the right NEMA plug 10-30P or drier cord handy, the things get more involved. If I had to do it, I would turn off the main circuit breaker, pull the drier receptacle off the wall, and disconnect all its wires. Then I would attach them to the generator cable one by one by using standard wire nuts with steel springs. I would of course double check that all circuits correctly match: “hot” wires are usually black or red and the neutral is white. The remaining green ground lead can be bolted to the metal wall box. I would also secure somehow the cable so it would not be hanging on its leads.

Finally, if your genset is rated at 4000 watt or less, then most likely it provides only 120V. If your building has a standard 120/240V system, it means it has two separate “out of phase” 120V lines.  In this case, unfortunately it’s impossible to power the whole house with a single 120V source. In theory, if you don’t have any 240V appliances (such as central a/c), you could supply electricity only to half of the electrical branches on one of the two 120V busses. If needed, you can additionally move some key branches to that backed up buss by swapping the respective circuit breakers.

NOTE. The information in this site is provided AS IS for technical reference only without guarantee and liability of any type, neither explicit or implicit. It expresses only a personal opinion of the author, and does not constitute a professional advice- see complete disclaimer linked below. If you use this information you do it AT YOUR OWN RISK. Always refer to NEC®, local codes and your product manual for wiring and safety requirements. It is strongly advised that all the work be done by a licensed professional (if you want to find one in your area and estimate the cost of the job- click here).