neutral vs ground

[Todd Dorer]

I have a project designed by a Fl. EE. The system, as designed, are 120/240 circuits from the main service disconnects, with no neutral, only a ground, feeding 120/240 xfmr?s varying distances away, at which points we will serve traffic cabinets containing electronic equipment.

The intent of the EE is to derive a neutral at the xfmr. His reasoning is that in order to use the 240v in the drop calc one cannot install a neutral with the circuit conductors.

1st, I cannot find information regarding the calculation "rule". Please help.

2nd, without the neutral, are we not creating a potential hazard in the absence of a low-impedance path back to the source?

It is not my intent to berate the EE. I only wish to ensure a safe and code compliant installation.

Regards

 

[wirenut1980]

Sounds like the EE is going to run just 240 V plus ground out to an isolation transformer with 240 V primary and 120/240 V secondary, at which point the neutral point is derived for the 120 V loads. The "calculation" the EE is referring to is voltage drop calculation. I see no problem with the proposal from where I am sitting :grin: .

 

[Todd Dorer]

Could one not simply install the neutral and not use the xfmr? Or is it necessary to use th xfmr in order to use the "240" in the voltage drop calc?

Thanks

 

[charlie b]

His reasoning is that in order to use the 240v in the drop calc one cannot install a neutral with the circuit conductors.

True, but not important. If the voltage drop were, for example, 4.8 volts on a 240 volt system, that would equate to a 2% VD. But if there were a neutral, and if we were calculating the VD on the basis of a 120 volt supply, you would get a calculation result of 2.4 volts, and that would still be a 2% VD.

What is also true is that the neutral would not be needed, if all you are serving is the primary of a transformer.

. . . without the neutral, are we not creating a potential hazard in the absence of a low-impedance path back to the source?

The neutral is not intended to perform that function. The Equipment Grounding Conductor is what establishes a low-impedance path back to the source, in the event of a short circuit to the external metal case of any connected equipment.

It is not my intent to berate the EE. I only wish to ensure a safe and code compliant installation.

I can?t speak for other EEs, but I for one sincerely appreciate any help I can get. If you don?t understand what I have issued in my design, or if you think I did something that would violate the code, I would want to hear about it.

 

[mivey]

No safety problems.

As for Volt drop. Forget the 240 volt loads as this is not important (other than transformation losses). What is important is the 120 volt load, in particular, the unbalanced 120 volt load.

Take the worst case scenario where there is only 120 volt loads on one leg of the second transformer. You have decreased the volt drop because you have doubled the supply voltage for a major portion of the circuit.

Instead of having the volt drop for 120 volts all the way back to the source, you have the reduced volt drop for 240 volts for most of the way, and the volt drop for 120 volts for the portion after the transformer.

 

[gar]

080624-1826 EST

Todd:

If I understand what you are describing the following is the circuit:

There is a 240 V center tapped source at the disconnect. The center tap is tied to an EGC and to a grounding electrode.

From the disconnect three wires are run to a remote location where there is a stepdown transformer from 240 to 120 (for this discussion disregard the possibility of a 240 center tapped secondary at the remote transformer). The two primary wires are hot relative to ground because the source center tap is grounded. The third wire is the EGC wire. This wire has the same potential as the source center tap and earth at the source, and if at the source a neutral had been run somewhere else the potential of that hypothetical neutral at that center tap would also be the same as the EGC.

At the remote transformer there is electrical isolation of the primary from the secondary and the EGC. The EGC would be tied to the transformer frame and any other conductive parts at the remote transformer.

In my judgement at the remote transformer the secondary neutral of that remote transformer, the incoming EGC, the outgoing EGC, all conductive parts, and a grounding electrode would be connected together. The code people will have to verify this.

Suppose the 120 V load on this remote transformer is 2400 VA. That means the 120 V load current is 20 A. This current reflected to the primary of the remote transformer is 10 A (240*10 = 2400). If the loop resistance (2 times one wire) of the primary wire is 2.4 ohms, then the voltage drop on the primary side is 24 V or 10%.

Now consider no remote transformer. But split the load in two and run a neutral from the source to the remote location. The current in the neutral is zero, on either side of neutral the current is 10 A. The resistance on each hot wire is 1.2 ohms. With a balanced load there is no current in the neutral and no voltage drop along the neutral. The voltage drop on a 120 V side is 12 V or 10% so long as the balancing load exists on the other side. If that load burns out, then the voltage drop for the working side goes to 20 %. 10% from the hot wire and 10% from the neutral.

In one case you have the cost of a remote transformer, and the other the cost of a fourth wire and a slight drop in light output from the working load when when the balancing load fails.

.