120/240v 3 Wire System vs 120/240v 4 Wire System

[MarkEE]

We have an application where 120v 3 wire equipment must operate in older buildings where single phase 120/240v 3 wire system is the only available power. The manufacturers of the equipment state that the three prong plug must be utilized for proper operation. The only solution I have come up with is to use a portable transformer 240v - 120/240v 4 wire to establish a true ground system. Any comments or ideas?

 

[alehman]

It appears by "120V 3 wire" you mean 2 circuit conductors and a ground? (by convention 3-wire means 3 circuit conductors, with or without a ground). The transformer idea will work provided you ground the secondary neutral per local code (and provide overcurrent protection required by code).

 

[jbartos]

Suggestion: Single phase, three - wire, 120V/240V system is supposed to consist of two hot wires, neutral and ground. If you do not have ground, it is better to rewire the old installation and wire the new installation with ground, which will enable having 3 prong receptacles, one hot wire, neutral and ground. The neutral is connected to the ground at the point of utility service entrance; usually at utility service disconnect switch/breaker.

 

[Joest]

One possible solution is to install a 3-prong outlet and run the ground wire to the plumbing system if it is metal and nearby.

 

[alehman]

Not to pick on Joest too much, but connection of a equipment ground to a metal water pipe as the only current path is in violation of the NEC and can be very dangerous for several reasons.
1. Implicit assumption by future users that the ground is good because there is a 3-prong receptacle.
2. Water piping may not be electrically continuous and of sufficiently low resistance for fault clearing current.
3. The water pipe may not be connected to the building service entrance ground. If it is not, this can be a very significant fire and electrocution hazard for users of plumbing fixtures should there be a ground fault in the utilization equipment.

 

[rbulsara]

I am a bit confused.
3 wire means 2 hot and a Neutral. Ground is always extra.
(Similarly 4 wire is 3 hots and 1 N, plus ground)

What are the three prongs? 2 hot and a ground? or 2 hot and a Neutral?.

1 hot, 1 N and 1 G is not a 3-wire system.

The Neutral of 120/240 must be grounded at the service entrance equipment. I am not sure where the problem is and why u need a transformer to establish a ground?

Is it that this plug does not have a ground? Is it required by Code? I would think so.

 

[alehman]

I suspect this is an old system with two-prong 120V receptacles, and there is no grounding conductor run with the circuits. Many old houses (like mine) are wired this way.

 

[jbartos]

Suggestions to rbulsara (Electrical) Nov 25, 2003 marked ///\\I am a bit confused.
///Nothing wrong with that.\\3 wire means 2 hot and a Neutral.
///True.\\ Ground is always extra.
///Yes.\\(Similarly 4 wire is 3 hots and 1 N, plus ground)
///Yes, in majority of cases.\\What are the three prongs? 2 hot and a ground? or 2 hot and a Neutral?.
///1 hot, 1 neutral and 1 ground if related to 120V receptacle which is normally installed in residences, commercial and industrial spaces.\\\

1 hot, 1 N and 1 G is not a 3-wire system.
///Not quite. It is two wire system plus ground.\\The Neutral of 120/240 must be grounded at the service entrance equipment.
///Yes.\\ I am not sure where the problem is and why u need a transformer to establish a ground?
///The transformer is not needed. The correction of wiring deficiencies is needed, which is often very expensive and troublesome.\\Is it that this plug does not have a ground?
///Yes.\\ Is it required by Code?
///The ground and three prong 120V receptacles are required by NFPA 70-2002 National Electrical Code.\\

 

[Joest]

MarkEE,

Any hopes of still getting the Bill Mitchell demo software we talked about in another post? My email address is listed at the top of the following post faq800-480. Thanks in advance.

-Joest