Wire run length for inside buildings

[gminnc]

Hello,
I am dealing with a situation where a medical Ultrasound system is displaying RFI noise in one room, but no noise when it is moved to a room 100 feet away. Measuring the Neutral to ground with the system on, drawing 14 amps on a 20 amp line, we get 2 VAC. Our manufacturer stated they would like to see that level be .25 VAC, and that the problem comes from using too small of a guage wire, for the distance between the sub-panel and the system. Is there a table containing showing the guage wire, current, and the max run length?

I will check this in the morning,
Thanks for your help.
P.S. I am an Ultrasound repair person, not an Electrical Engineer, so I am not familure with the electrical codes.

 

[dereckbc]

Re: Wire run length for inside buildings

There are engineering tables, but they are derived from this formula.

CM=(25.8 x I x L)/VD

Where
CM = Circular Mills of Cable
I = Load Current
L = One Way Cable Distance
VD = Voltage Drop

I will let you do the rest.

The problem is you are going to need a 1/0 to achieve less than .25 volt drop. I would like to see the look on your electricians face when you ask him to run it.

I think your best bet is an isolation transformer. You really need to consult with an electrical engineer, knowledgeable electrician, or both.

Good Luck

[ November 18, 2003, 05:38 PM: Message edited by: dereckbc ]

 

[don_resqcapt19]

Re: Wire run length for inside buildings

What is the neutral to ground voltage in the room 100' away? Have you checked either of the locations with a miligauss meter? Is this a problem with equipment that has just been installed, or a problem that just appeared on older equipment?
In my opinion, properly designed single unit equipment should not be effected by 2 volts between the neutral and the grounding conductor. Don

[ November 18, 2003, 06:26 PM: Message edited by: don_resqcapt19 ]

 

[dereckbc]

Re: Wire run length for inside buildings

gminnc, just to add another note, the .25 VAC between N-G translates to a .5 VAC circuit voltage drop. This would be a .41% voltage drop. The most stringent engineering requirements I have ever witnessed are 3.3% at ful load. At 3.3% your N-G voltage would be 2 VAC or a 4 volts circuit voltage drop.

To get the voltage drop you are wishing for is not reasonable or feasible. It can be done, but it would require extraordinary large oversized conductors, or an isolation transformer right next to the load.

I, like Don, have serious doubts your noise is created by a 2 volt N-G. I think you have another problem which has been overlooked.

I know there is a member named Roger who works a lot with hospital environments, and may bee can make some suggestions as to what voltage drop requirements are acceptable for the application

 

[roger]

Re: Wire run length for inside buildings

Hello all, I have to agree with Dereck and Don, most med equipment manufacturers will always blame voltage and or leakage current levels that are practically unachievable for the problem. They also like to say there is an intermittant loss of grounding that can't be found. (they think nobody will balk at their claims.)

I would be curious to know who the manufacture is but that probably is not overly important.

You are moving the unit into another room and I am assuming (actually pretty sure) another circuit, so I do understand looking for a control common to both areas, ie the 2vac, but not dwelling on it when it is only a problem in one room.

Which room is closest to the distribution point.


I would start with testing and researching the circuit in the offending room. Have an electrician determine if this is a multi-wire branch circuit and what is on the other circuit conductors if it is.


Back to Derecks point of the 1/0 conductor, in many GE, Phillips, and others xray installations, over sized neutrals and EGC's are normal, simply for noise problems.

Of course in some of these designs I have seen the manufacturers create their own problems, (ground loops) not as much recently though.

Please don't leave us hanging, keep us informed.

Roger

[ November 18, 2003, 10:47 PM: Message edited by: roger ]

 

[gminnc]

Re: Wire run length for inside buildings

WOW!!

Thank you all for such a speedy reply. I did not expect that many answers in just a few hours.

I would rather not name names, but it has been an issue, and we in the past have "fixed" our problems. It has been "fixed" by the hospital or clinic running a dedicated line that is a relatively short distance from the sub-panel using 10 or 8 gauge wire.

I was requested to search for "Official" documentation we could provide the customer. By the time I told them to wait for your replies, management reversed their request, and stated that they will recommend that the hospital hire an Electrician.

As for your replies, even though management is no longer concerned, I still would like to learn for my own sake.

Thank you all.