DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Electrical and Wiring > How are 30amp & 50 amp plugs connected?

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Old 07-11-2009, 09:08 PM  
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Originally Posted by bkplasma View Post
Nestor: Thanks for that great explanation. I think you did a great job clearing it up.
Thanks for the fan mail, Bkplasma. Think how much better it woulda been if I was sober when I wrote it.


I just re-read the original question and Scott is saying he has to wire a 30 amp plug and a 50 amp plug. A 220 volt 30 amp space heater will use exactly the same receptacle and plug as a 220 volt 30 amp electric clothes dryer, which you can buy in any hardware store. Similarily, a 220 volt 50 amp arc welder will use exactly the same plug and receptacle as a 220 volt 50 amp stove, which you can also buy at any hardware store.

Now, I'm not a licenced electrician, so if I say anything not quite right in this post, would the electricians in here please correct me. I'd rather be corrected than have all the newbies in here getting the wrong information.

What you need to do is first add up the amperage on all the fuses in your electrical panel and find out if the panel has enough capacity to add another 80 amps of load. If having both the space heater and arc welder running at the same time as all the other lights and receptacles and stove and dryer currently on the panel will exceed the panel's rated amperage, then you'll have to install a new panel, and that's way over my head.

If adding the space heater and welder doesn't overload your panel, then you need to run a three conductor cable (what gauge wire I don't know but the electricians in here should) to the main panel of your house, and connect the red, black and white wires. Now, don't assume that by connecting to breakers on opposite sides of the panel that you're connecting to opposite voltage sources. That is, one side of the panel isn't always powered by the red wire and the other side the black wire. It's the bus bars inside the panel that determine which circuit breakers get connected to the black wire and which get connected to the red wire. Typically, a panel will have several ADJACENT breaker locations where adjacent breakers get connected to opposite voltage sources. Electrical panels are designed this way so that two breakers servicing the same appliance can be "ganged together" so that you can't trip off one breaker without tripping off both of them. If you only have two locations like that on your panel, they are probably already being used for your electric stove and electric clothes dryer. If you have 4 such locations, you can move the 15 amp breakers that may be in those locations to other slots on the panel and use the "gangable" locations for your space heater and arc welder as it's better to have both breakers to a 220 volt circuit adjacent to one another.

You will be putting TWO 30 amp circuit breakers in the panel for the space heater, and preferably those two breakers will be on the same side of the panel and in adjacent locations so they can be ganged together. (Your electrical inspector might hassle you if they're not together and even ganged together somehow.) The red wire of your cable will connect to one 30 amp breaker and the black wire to the other. The white wire of your cable will connect to the bus bar in the panel where all the other white wires connect.

At the other end of the cable, you will install a receptacle intended for electric dryers. Inside that receptacle, you will see three connection points in a row on the back of the plastic female receptacle, as well as a fourth connection point for the ground wire. Connect the white wire to the middle point on the row of three connection points and the black and red wires to the connection points on either side. If the space heater doesn't need the white wire, then having the white wire connected to the middle connection point in the receptacle won't do any harm. To find out if it does use that white wire, maybe open the space heater and see if the white wire from it's cord is connected to anything or just has a big wire nut on it. If the space heater doesn't have a cord, use a cord from an electric clothes dryer.

Now, just plug your space heater into that dryer receptacle, flip your two 30 amp breakers on at the panel, and the space heater should work when you turn it on.

Wiring your welder is exactly the same process except that you would use a female receptacle for a stove, (and a cord for a stove if the welder doesn't have a cord) and you would use 50 amp breakers in your panel instead of 30 amp breakers. You'll have to find out from someone more knowledgeable about what size of three conductor cable to use for the welder. Also, when I say "three conductor cable", it means three wires BESIDES the ground wire. That's cuz normally, there shouldn't be any current in the ground wire, so it's pessimistic to call it a "conductor". Calling the ground wire a conductor is predicting there will be a short in the space heater circuit.

In the hardware store, the plastic bag that stove or dryer cords come in will say "range cord" or "dryer cord". Similarily, the cardboard boxes that range receptacles or dryer receptacles come in will say "receptacle for range" or "receptacle for dryer". This is only to keep things simple for homeowners, who, as a general rule, don't know squat about electrical wiring.

The way it really works is that the people that oversee the electrical standards in the United States have designed mating plug and receptacle configurations for all combinations of voltages and amperages and you simply use the designated plug and receptacle configuration for the appliances voltage and amperage. So, a 110 volt 15 amp receptacle and plug will have the standard polarized plug/receptacle configuration we see all the time, and a 110 volt 20 amp receptacle and plug will be different, with one horizontal prong instead of both being vertical. And, there are other prong and slot configurations for every combination of voltage (110, 220 and 440) and amperage (15, 20, 25, 30 and 50).

So, it's not like there's a special "dryer" plug and receptacle configuration that dryers use or a "range" plug and receptacle configuration that stoves use. There is a special 220 volt 30 amp plug and receptacle configuration that all 220 volt 30 amp appliances use, whether they're an electric clothes dryer or a 220 volt 30 amp space heater or a 220 volt 30 amp air compressor in an auto body shop. Ditto for stoves. There is a 220 volt 50 amp plug and receptacle configuration that all 220 volt 50 amp appliances use whether they're a stove or an arc welder or a milling machine in a machine shop.

Obviously, those plug and receptacle configurations only apply if the appliance has a cord that plugs into a receptacle. Often appliances will be "hard wired" to the appliance with no receptacle and plug to "unplug" the appliance. If you see a cable going directly into a 220 volt appliance, then that cable will be wired to a "terminal block" inside the appliance and the "white in the middle, red and black on either side" rule applies to the three wiring connections on the terminal block.

Now you know enough to start the legwork, which is finding out the rated capacity of your existing panel to see if it has sufficient capacity to also accomodate the space heater and welder. And, at the same time, find out how many locations in your panel will allow you to connect to opposite voltage sources with adjacent breakers so they can be ganged together for safety.

Hope this helps.

Last edited by Nestor_Kelebay; 07-12-2009 at 11:52 AM.
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