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Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
Norcal sent in this pic of a receptacle he encountered:


[Linked Image]


This is the NEMA pattern used here:


[Linked Image]


Looks like 120v between neutral and ground .... gee, I wonder how that could have happened?

Remember this, the next time some fool insists that the neutral can't kill you. Or, wants to use the conduit as his 'neutral.' Or thinks bonding the neutral and ground (away from the main panel) is no big thing....


Joined: Jun 2004
Posts: 202
3
Member
Just trying to work out the wiring of the socket:

Is it a 3 pole 3 wire NEMA 14-50 configuration that can supply either 240 or 120 volts at maximum 50 amps? (assuming the neutral is counted as a pole?)

I'm assuming that when wired correctly:
X-G=240V
Y-G=240V
X-Y=240V
X-W=120V
Y-W=120V
W-G=0V

Is it normal practice to run the 2 "hot" wires, a neutral & an earth from the board to the socket?

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
32, in the USA we create 240 by 'center tapping' a single transformer, and grounding that center tap.
What that means is that there is 240 between the 'ends' of the transformer, and only 120 between either end, and the center. This grounded center tap becomes the neutral.
Since the transformer is center tapped, the "120" from either half is exactly out of time with the 120 from the other half.

So, for this plug ... the X and Y are for your 'hot' wires. There should always be 240 between then.
The "W" is for 'white' ... the color of the neutral wire. Either 'hot' wire should read 120 to this point.

The "G" is the 'ground' wire, a separate conductor that is kept separate from all other conductors until you reach the main disconnect at the service entrance. Only at that point are the neutral and ground wire connected.

The neutral is used for carrying current all the time; the ground is used only if something breaks- and then only so that the breaker can trip.

There ought to be very, very little voltage measured between the neutral and ground. More than one or two volts, and you have an issue that you need to look into. Most often, these voltage reading are 'ghost' readings, caused by a 'dead' wire running next to a 'hot' wire.

The pic shows 120 between the neutral and ground. There are several things that might cause this ... all of them bad.
The neutral may be open. If that is the case, every neutral 'downstream' of the open connection is now a 'hot,' just waiting for a chance to zap you.
Someone may have mis-wired the receptacle, and one of the 'hot' sections is really the neutral. This means that any 120 load on this circuit will be treated to 240.
The receptacle might be used on the wrong type of system. For example, if used on a three phase system, with three 'hots' and no neutral. If so, again, any 120 load will be exposed to 208 volts.


Finally ... and this is the possibility that is most dangerous ... it is possible some clever sort downstream decided to use the pipe as a 'neutral' ... and the pipe is not bonded all the way back to the panel. If that is the case, the pipe is now 'hot,' just waiting to kill someone.

Joined: Jun 2004
Posts: 202
3
Member
Is this pattern socket used for large items like stoves, ovens & clothes driers?


Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
Good point, 32 .... I think this is the new pattern for stoves. (Water heaters are different).

In that case, the ground and the neutral are almost sure to be bonded at the appliance. Not correct, but still likely. If the ground path were intact, this would result in a dead short as soon as the thing was plugged in. If the ground path were not intact, the stove case would become "hot".

Joined: Sep 2002
Posts: 917
Likes: 1
N
Member
If you notice the "3 phase" symbol on the corner of the cover its being used as a 240 Volt welder receptacle.

Joined: Apr 2004
Posts: 811
Member
Hey '32, in case you're interested, I have a list of (some) standard NEMA plug/receptacles on my Photobucket page:

NEMA Guide

The one in this photo is the 14-50R, 3-wire w/ Ground configuration, as you already pointed out. If you want to see the other two pages, let me know.

Ian A.

Last edited by Theelectrikid; 05/20/07 07:51 PM. Reason: Can't type!

Is there anyone on board who knows how to fly a plane?
Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
Thanks, Norcal. I had missed that marking ... A range recep used for three phase ... now I'm really getting worked up!

I wonder if the folks at DarwinAwards let you make reservations?

Joined: Oct 2006
Posts: 745
E
Member
32V:

Here in the US, residential electric hot water heaters are hard-wired using a 30 amp, 240 volt circuit. There is no plug/cord connection required. It's just two wires (one from each end of the transformer to derive 240 volts) and a ground (or earth) wire. In some juridictions, a disconnect switch might be required within sight of the water heater. Commercial installations can vary greatly, but are usually three-phase services.

Electric furnaces may require multiple circuits or simply use just one large circuit. These too are wired directly, though there may be a requirement for a disconnect switch to be located within sight if the circuit breaker(s) feeding the unit are not within sight. We do have some exceptions where padlock attachments for these breakers are considered an alternative to a separate disconnect at the furnace. In my home, the furnace is fed by a single 80 amp, 240 volt breaker with a padlock attachment in the main panel.

Outdoor air condintioning compressor units or heat pumps always require a disconnect switch within sight, but like water heaters, usually only require a 2-wire 240 volt circuit that is hard-wired. These can range from 20 to 60 amp loads, depending upon the size of the units.

Electric stoves that incorporate the cook top and oven in one appliance, (referred to as ranges in the US) utilize a NEMA 14-50 receptacle, or "socket".

When a separate electric cook top and oven are installed, they are generally hard-wired with no plug/socket connection. Some local codes may require these installations to be done differently.

Electric clothes dryers, the current requirement is for a NEMA 14-30 receptacle (socket) and matching cord. Dryers are always required to be plug (socket) and cord connected since they are portable appliances.

There was a time when the "earth" or "ground" wire and the "neutral" were permitted to use the same wire, resulting in just a three-wire connection for these appliances. This lasted for at least 50 years and I really don't know why it was deemed to be dangerous all of a sudden.

Electric ranges and dryers have certain components, like clocks, lights and motors that operate on 120 volts. For this reason, the new standards require that an insulated neutral wire be provided to these appliances.

Sharing the ground (earth) and neutral for minor 120 volt requirements is no longer permitted.


---Ed---

"But the guy at Home Depot said it would work."
Joined: Feb 2007
Posts: 65
B
Member
This is a 120/208 Y at 50A? I'm looking at the NEMA patterns, and I don't see any for this. The 15-50R is probably the closest, but that is for 250V, not 120/208.

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