Connecting Portable Generator To Home Wiring: 4-Prong and 3-Prong (2023)

Let me start by saying that thesafest way of connecting a portable generator set or any other power source toahouse wiring is via atransfer system.The question of course is what to do if you don’t haveone andapower outagecaught you off guard? Fortunately, there are still some ways of energizing your housewithout a transfer switch, especially if you have a drier line. However,these methodsmay benot quite safe,would notcomply withNational Electrical Code (NEC®) and should not be used unless it is absolutely necessary inanemergency.

Ipersonally would not recommenddoing this. However, I realize that some people will do so anyway, so I have compiled here someinformation to help you do it technically right.Of course, thisinformationis providedfor general referenceonly without liability of any kind- this is not a professional advice!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, let’sfirst review the safety issues before going overthe available options listed below.

HAZARDS ASSOCIATED WITHPLUGGING A GENSET DIRECTLY TO HOUSE WIRING:

• Feedingelectricity back intothe grid (“backfeeding“)presents ahazard forlinemen and neighbors. Using main breaker instead of a transfer switch still leaves room for human error;
• Usingmale-to-male extension cordspresentselectrocution risk becausea plug onthe loose end can be electrically “live”;
• Miswiring can create a hazardous condition.

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

IMPORTANT SAFETY RULES TO FOLLOW:

• Always turn OFF the main disconnect before you begin any work and keep it in OFF position until you unplug andturned off your portable generator;
• Wear lineman rubber gloves;
• Run your power source only outdoors with exhaust away from doors and windows

OK, now let’s get down to business. Most portable generators rated 5000 watt orhigher have 4-prong 125/250V twist-lock outlet NEMA L14-30. It provides two “hot” wires, the neutral and ground as shown on the above diagram (the colors are for the mating cord; hot lead can also be red). Also see the configuration of other sockets.

If you have an electric drier outlet, it can beemployed as described below (which as I’ve said, may be dangerous if you are not careful).

CONNECTING A4-PRONG CABLE TO 3-PRONG OUTLET

Many homes in North America have a 3-hole 120/240V outlet. Some homes built prior to 1996 may have NEMA 10-30R. It provides two “hot” wires and neutral (see diagram). The old appliances that used such an outlet had a jumper between neutral terminal and grounding lug. People are often looking to buy a 3-prong to 4-prong adapter. Well, there is no such thing: nobody sells it because having plugs on both ends is not safe. In order to connect a generator output to the home wiring via an existing 3-hole outlet you’ll have to make your own cable. Here is how. Take a standard 4-wire generator cord and remove its socket. This will expose four leads. Then there are two options. If you have a loose mating 3-prong plug NEMA 10-30P, attach its X and Y terminals to two line wires, and attach W to the neutral (refer to L14-30 pinout above). Alternatively, if you have a 3-prong drier cord, you can tie its loose 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).

If yourmodel has a built-in GFCI, then its frameis supposed tobealready bonded internally tothe neutral (youcan verify this with an ohmmeter). Since neutral bus in your home is grounded inside the main panel, the frame of your genset will likewise be tied to earth via this connection. In this case you can just isolate the green lead in the cable and leave it unconnected. However, if your model does not have a GFCI, you may need to connect its frame to earth. There are several ways of doing this. A safer way is to install a copper or brass rod driven at least 2 feet deep into the earth and connect it to the frame via #6 AWG wire. See Article 250 of the NEC® 2020 for the requirements. Alternatively, if you are in a hurry, you can attach the green lead of the generator cord to the cover plate of the wall outlet. To tie it reliably, I would crimp a round lug to this wire and put this lug under any mounting 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 double-check continuity between the frame and earth by an ohmmeter.
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 20A*240V=4800 volt-amp. In reality, you likely will get less because loads on L1 and l2 are often imbalanced (and by the way, it is recommended to load the breakers only up to 80%).

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

POWER-UP SEQUENCE

• Turn OFF the main electrical service disconnect and attach a visiblenote “Do not turn on”;
• Turn OFF all branch breakers in your distribution panel including the double-pole breaker for the drier or another outlet that you plan to use;
• Plugyour modified cord first into the outlet and place a visible note “Do not unplug until genset is turned off”;
• Run the cord through an open door or a window to outside and connect itto your genset
• Assure that frame of your generator is grounded (see the safety warning above);
• Start it up and let it stabilize for 5 minutes;
• Turn on the double-pole breaker that feeds the 120/240V;
• Begin turning on individual switches on the critical branches which you want to backup one at a time.

A properreverse procedure isequally important to reducerisk.

POWER-OFF SEQUENCE

• Unplug the extension cord first from the genset, then from theoutlet (never unplug the load side of the cable first since itisenergized!!!);
• Shut down the genset;
• Turn ON the main service breaker (assuming the utility power is restored).

WIRINGA 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 awrong geometry. To connecta genset to such areceptacleone canreplace thesocket in the generator cord by a 4-prong plug NEMA 14-30P. This is a pretty much straightforward task.Then you needto follow the proper power-up and power-down procedures outlined above.

MORE OPTIONS

If you don’t haveeither the right NEMA plug 10-30Por driercord handy, the things get more involved. If I had to do it, I would turn off the main circuit breaker, pull thedrier receptacle off the wall, and disconnectallits wires. Then I would attach them tothe generator cable one by one by usingstandard wire nuts with steel springs. I wouldof coursedouble check thatall circuitscorrectly match: “hot” wires are usually black and red, and the neutral is white. The remaininggreen groundlead can be bolted to the metal wall box.I would also secure somehow the cable, so itwould notbe hanging onits leads.

USING MAIN PANEL

If you don’t have a high-current receptacle, you can connect a portablesource to your house at the main service box.Firstof all, of course, flip the maincircuit breakers toOFF (OPEN) position. Then you can removethe cover door.Remember that the panel may become energized from utility, so wear rubber gloves! Take a sufficiently long generator cord with 4-wire plug and bolt red and black leads to the “hot” bus-bars (the thick vertical metal plates) after the main double-breaker. Be sure your connection is downstream from main disconnect and not upstream, i.e.not to the linesthat go to your meter. Bond white and greenleads to the neural bar and metal case respectively. Place a sign not to touch anything until genset is off. Turn offallindividual breakers before starting upthe genset to prevent its overload. Finally, turn on your unit and after it warmed up, activate those lines that you need, and do it one at a time.

A final note. Ifyour model is rated at 4000 watt or less, then most likely itprovides only 120V. Ifyour 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, i.e. on one of the two 120V buses. If needed, you can additionally move some key branches to that backed up bus 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 or legal advice– seecomplete 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 allthe workbe done by a licensed professional (if you want tofind one in your area or estimate the cost of the job- check out this site).