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Joined: Oct 2000
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Fact: The 2002 NEC includes mandatory rules that cover voltage drop.
"647.4(D) Voltage Drop. The voltage drop on any branch circuit shall not exceed 1.5 percent.
The combined voltage drop of feeder and branch-circuit conductors shall not exceed 2.5 percent.
(1) Fixed Equipment. The voltage drop on branch circuits supplying equipment connected using wiring methods in Chapter 3 shall not exceed 1.5 percent.
The combined voltage drop of feeder and branch-circuit conductors shall not exceed 2.5 percent.
(2) Cord-Connected Equipment. The voltage drop on branch circuits supplying receptacles shall not exceed 1 percent.
For the purposes of making this calculation, the load connected to the receptacle outlet shall be considered to be 50 percent of the branch-circuit rating.
The combined voltage drop of feeder and branch-circuit conductors shall not exceed 2.0 percent.
FPN:The purpose of this provision is to limit voltage drop to 1.5 percent where portable cords may be used as a means of connecting equipment.
695.7 Voltage Drop.
The voltage at the controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor starting conditions.
The voltage at the motor terminals shall not drop more than 5 percent below the voltage rating of the motor when the motor is operating at 115 percent of the full-load current rating of the motor.
Exception: This limitation shall not apply for emergency run mechanical starting. [NFPA 20, 6.4]"
Joe Tedesco, NEC Consultant
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interesting that an article that deals in 60v should stipulate 1/2 the FPN suggested drop elsewhere in the code......
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Joined: Nov 2000
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The rule in 695 will not only require larger service/feeder conductors, but may very well require a larger transformer. Look at what the starting current is on a large motor. I have seen electric fire pumps up to 250 hp and know that some are even larger.
Don
[This message has been edited by resqcapt19 (edited 07-28-2002).]
Don(resqcapt19)
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Welcome back from your vacation! I posted this information because I wanted to clear the air.
I was sent an email message last week as a subscriber to a major newsletter where there were words that were not totally true as they relate to VD.
See you over on the other threads??
Joe Tedesco, NEC Consultant
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For you cheap guys like me that haven't yet reviewed NEC Article 647 {new in 2002?} it looks like it's online in toto at: http://www.equitech.com/support/647.html
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Joined: Oct 2000
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647 appears to address a very specific application via SDS, and as the article would deal with 'sensitive' electronics the nature of VD being mandatory supplements/modifies ( 90.3) chap's 1-4.
note how the evil nuetral* is done away with... 647.3 General.
Use of a separately derived 120-volt single-phase 3-wire system with 60 volts on each of two ungrounded conductors to a grounded neutral conductor shall be permitted for the purpose of reducing objectionable noise in senstive electronic equipment locations provided that the following conditions apply:
(1) The system is installed only in commercial or industrial occupancies.
(2) The system’s use is restricted to areas under close supervision by qualified personnel.
(3) All of the requirements in 647.4 through 647.8 are met. my question here is, does this system supply normal 120V equipment , or is said 'equipment' rated/listed to run off the 60v/60v SDS ??? anyone?..... * (trade slang.....)
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Article 647 is not really new in the 2002 NEC, except for the Article number.
Came from the 1999 NEC Article 530, Motion Picture and Television Studios and Similar Locations, Part G. Separately Derived Systems with 60 Volts to Ground
Joe Tedesco, NEC Consultant
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yes Joe, i can imagine that, in addition to VD, any form of a 'ground loop' would also be a concern,the issue of overall power quality at hand.... as such we are directed in 647 to 250.30 ..... (B) Ungrounded Systems.
The equipment of an ungrounded separately derived system shall be grounded as specified in 250.30(B)(1) and (2).
(1) Grounding Electrode Conductor. A grounding electrode conductor, sized in accordance with 250.66 for the derived phase conductors, shall be used to connect the metal enclosures of the derived system to the grounding electrode as specified in 250.30(B)(2). This connection shall be made at any point on the separately derived system from the source to the first system disconnecting means.
In a separately derived system that is ungrounded, a bonding jumper must be installed to connect the disconnect enclosure and equipment grounding conductors to the grounding electrode system. which now brings up this nifty little ex a tad back in A.... Of course this does not apply, being in A, and having rid the nuetral from the system, yet it is of interest in that 647 may in fact provide for the 'fix' for it...... Exception No. 1: A bonding jumper at both the source and the first disconnecting means shall be permitted where doing so does not establish a parallel path for the grounded circuit conductor. Where a grounded conductor is used in this manner, it shall not be smaller than the size specified for the bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path. To add to this little 'code loop' here, pursuant to the 'PQ' angle, we now delve into 'listings'.... The practice of grounding the secondary of an isolating transformer to a ground rod or running the grounding electrode conductor back to the service ground (usually to reduce electrical noise on data processing systems) is prohibited if an item in (1) or (2) of 250.30(A)(4) is available. However, an isolation transformer that is part of a listed power supply for a data processing room is not required to be grounded in accordance with 250.30(A)(4), but it must be grounded in accordance with the manufacturer's instructions. somebody here's had to have been there..... ( speak up, don't be shy...) ![[Linked Image]](https://www.electrical-contractor.net/ubb/biggrin.gif) [This message has been edited by sparky (edited 07-28-2002).]
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Thanks for the 530>>647 update, Joe. Another clump of hair fell out, but this new jointly financed RowGain-Ultra/Krazee-Glue should fix that. You can surely pick me out, eh? ![[Linked Image from drbukk.com]](http://www.drbukk.com/images6/Likter.JPG)
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Joined: Apr 2002
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A bit off-topic, but the center-grounded 60/120V configuration had another interesting application in the disco days of the late 1970s. It became a cost-effective solution to 4.8kW of medium-based sign lamps strung up through many feet of wiremold and what seemed to be miles of 14THHN. The idea behind the tapped grounding allowed sets of 16 SCRs on an 8x8 matrix to control three groups of 64 lamps in various patterns, giving control of individual rows and columns of colored lamps. We used a 10kVA, 480-120/240V dry-type transformer, powered from a dedicated 240V circuit, providing 60/120V for the SCR/lamp matrix. This arrangement allowed for switching of both conductors to each lamp, and limited the lampholder voltage to below 150V. [A microprocessor and keypad controlled the SCR-gating patterns.]
It looked great when it was completed, but we didn’t want to think about a sign lamp for a long time. Disco still sucks.
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