VD ? Sensitive Electronic Equipment

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Sam.Ying

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According to NEC 647.4(D) the total voltage drop for sensitive electronic equipment shall not exceed 2.5 %
But if all equipment is supplied by a UPS, will a VD near 5 % than be in accordance with the NEC?
If not?. Than I am confused!!

Thanks,
Sam
 
Sam.Ying said:
According to NEC 647.4(D) the total voltage drop for sensitive electronic equipment shall not exceed 2.5 %
But if all equipment is supplied by a UPS, will a VD near 5 % than be in accordance with the NEC?
If not?. Than I am confused!!

Thanks,
Sam
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No it will not meet the requirement. I work with some 647 applications using a UPS. The reason for the tight VD tolerance with 647 qpplications is the lower voltage nature of the 120/60 systems and over current protection devices. By keeping the VD to 2.5% or less means the circuit cunductors are very low impedance, especially the EGC path under fault conditions. It really comes down to ohm's-law of E/R=I. Since E=60 as measured to ground, R has to alao be low to ensure I will be high during a fault to trip the OCPD
 
dereckbc said:
Since E=60 as measured to ground
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E=60? Did you say that wrong or am I missing something?

dereckbc said:
By keeping the VD to 2.5% or less means the circuit cunductors are very low impedance, especially the EGC path under fault conditions.
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I also have to ask why the EGC would be considered more important with sensitive equipment. "especially the EGC path under fault conditions"? If this were true there'd actually be a requirement for a larger EGC. I know that you know the EGC can be smaller and still work fine to clear a fault because it happens so fast.
 
Physis 3 said:
E=60? Did you say that wrong or am I missing something?
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No I did not say it wrong, you are missing something, 647 applications are 120/60 systems, there is no grounded circuit conductor (neutral), only two hot conductors plus ground. Each hot conductor is 60 VAC to ground and 120 L-L. Think of it like a single phase 240 circuit L1-L2=240, L-G=120.

647 systems are what is called balanced power systems.

Physis 3 said:
I also have to ask why the EGC would be considered more important with sensitive equipment. "especially the EGC path under fault conditions"? If this were true there'd actually be a requirement for a larger EGC. I know that you know the EGC can be smaller and still work fine to clear a fault because it happens so fast.
Click to expand...

Think about it for minute, consider it is 60 volts to ground and apply ohm's law. ;)
 
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All right, now I know.

This article 647 has no relevance to me at all, because I have a 3 phases/4wire system.

I have to learn to read the entire article, including the Scope in 647.1

Thanks again

/Sam
 
Sam.Ying said:
All right, now I know.

This article 647 has no relevance to me at all, because I have a 3 phases/4wire system.
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Okie Dokie! :grin: That doesn't mean a 3-p/4-wire sytem cannot be applied to 120/60 sytems because I have done been there and done that.
 
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Ok, sorry Dereck, I really didn't think you'd be off as much as I thought you were. I still don't have a code book and I'm not familiar with the section. I was presuming you meant 120v. 60 cycles. It makes a lot more sense after reading your response.
 
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