branch circuit issue

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madmike

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Experts,

I was called to a new Govt installation still in construction, experiencing stray voltages on de-energized duplex receptacles.

I found 1/2" conduit fed from a branch panel, served by (3) 20 amp breakers, with a five wire (12 AWG solid) 208V service feeding a group of receptacles. Phase "A" fed two duplexes, phase "B" fed one duplex, and phase "C" fed one duplex outlet. A total of four duplex outlets.
With various combinations of phases shut off, the stray voltages read between 5 and 35 volts.

Does this appear to be a compliant installation? It supplies electronic display consoles at a tracking facility being built.

r,
MM
 
Re: branch circuit issue

The installation may be OK. Was it a digital multimeter you were using to measure the 5 to 35 volts? They have a very high input impedence. When you turn off the breaker to the circuit you are measuring voltage on, you basically have an antenna whose lenght is the run back to the panel plus the meter leads. You are bound to measure some voltage from inductive and capacitive coupling between the wire you are measuring and other circuits.

Steve
 
Re: branch circuit issue

Put a load on the circuit, and see what you get. Sounds like ghost voltages. I would bet you are using a digital meter.
 
Re: branch circuit issue

Steve66 and Dereckbc,

Many thanks for the reply. Yes, ghosts, fer sure, caused by the E-field in the conduit, not having dedicated neutrals for each circuit. Although I measured voltage, no current would have been available.

My problem is with running three phases, protected at 20 amps each in the panel board, on 12 AWG solid wire, using a single shared 12 AWG neutral. If the three loads only carry 50 % of the breaker rating, the neutral will be overloaded. This is to be an electronic display console load. The run is approximately 50 feet from the panel board to the J-box. The four duplex outlets I mentioned earlier are connected at the J-box.

MM
 
Re: branch circuit issue

It doesn't sound like you are very familiar with three phase power systems. If the three branch circuits are from different phases (if the building has 3 phase service, and the 3 breakers are in a vertical column right next to each other, they would be on different phases) the currents will cancel each other out on the neutral.

This does not take into account harmonics, which are caused by things like switching power supplies. Harmonics can cause neutrals to be overloaded, but that would depend on the exact details of the equipment connected.

Don't worry about the ghost voltages. That is unavoidable, and separate neutrals would make no difference.

Steve

[ June 22, 2004, 09:23 AM: Message edited by: steve66 ]
 
Re: branch circuit issue

Remember, this is a three-phase panelboard feeding four duplex outlets. Duplex outlets are for single phase loads. And, these are electronic loads, meaning they will be non-linear.

"For three-phase circuits supplying single-phase loads that have non-linear current characteristics and share a common neutral, current in the neutral can exceed current in the phase conductor" IEEE Recommended Prctice for Powering and Grounding Electronic Equipment, IEEE Std 1100-1999, paragraph 6.4.1.1.2.

If this was feeding a balanced three-phase load the neutral current would not be an issue.

So my question is; does the NEC permit this or is the netural required to be oversized? I am not that familiar with the NEC and have not been able to find anything on this. Would appreciate some guidance here please.

MM
 
Re: branch circuit issue

Originally posted by madmike:
So my question is; does the NEC permit this or is the netural required to be oversized?
Click to expand...
Yes it is permitted, no an oversized neutral is not required but permitted. You have a design issue. The code will allow you to either put in a single neutral, oversized neutral, or seperate neutrals.
 
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