bilwei
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This article refers to conductors operated above 2000 volts. Is this 2000 volts with respect to ground?
Article 310.6 Shielding
- Thread starter bilwei
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Yes. "Ground" is where the electrons would go, if the insulation were to breakdown. So insulation systems are rated in terms of voltage to ground.
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- Semi-Retired Electrical Engineer
Both situations are essentially the same. At all events, the phase-to-phase situation is less severe than the phase-to-ground situation.
The voltage between phases can never be higher than twice the voltage from phase to ground. An electron will have to go through one conductor's insulation system, in order to make it to ground. So the insulation system is based on making sure the applied voltage to ground is not enough to drive an electron all the way through.
It will have to go through two conductor's insulations system, in order to make it from one conductor to the other. It would take twice as much voltage (i.e., twice the insulation system's rated voltage) to be able to drive an electron all the way through.
The voltage between phases can never be higher than twice the voltage from phase to ground. An electron will have to go through one conductor's insulation system, in order to make it to ground. So the insulation system is based on making sure the applied voltage to ground is not enough to drive an electron all the way through.
It will have to go through two conductor's insulations system, in order to make it from one conductor to the other. It would take twice as much voltage (i.e., twice the insulation system's rated voltage) to be able to drive an electron all the way through.
bilwei
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The purpose of my original question was to determine if the NEC requires a shielded or nonshielded cable for my application. I will expound on the subject a bit more in detail.
Application: What Type of wire is required to feed a 800 hp 4160 volt rated refrigeration screw compressor motor? It will feed from the power company transformer thru an underground PVC conduit to a main service disconnect. Then from the disconnect underground to the building. We will stub up into the building and continue to the starter with IMC conduit and subsquently to the motor.
Application: What Type of wire is required to feed a 800 hp 4160 volt rated refrigeration screw compressor motor? It will feed from the power company transformer thru an underground PVC conduit to a main service disconnect. Then from the disconnect underground to the building. We will stub up into the building and continue to the starter with IMC conduit and subsquently to the motor.
Charlie:
I have to disagree that this is not voltage to ground, this could be phase to phase voltage. Other articles (like 215.10 - GFP protection for feeders) specifically say "voltage to ground". 310.6 only says voltage.
In addition, I don't think they mean RMS voltage. My take would be anything over 2000V peak. So basically if there is any way to say the voltage is over 2000V, sheilded conductors are required.
At 4160V, I don't think there is any question that sheilded conductors are required. Even the RMS voltage to ground (if grounded) would be more than 2000V.
(BTW, some exceptions for this were eliminated in the 2005, according to the handbook).
Steve
I have to disagree that this is not voltage to ground, this could be phase to phase voltage. Other articles (like 215.10 - GFP protection for feeders) specifically say "voltage to ground". 310.6 only says voltage.
In addition, I don't think they mean RMS voltage. My take would be anything over 2000V peak. So basically if there is any way to say the voltage is over 2000V, sheilded conductors are required.
At 4160V, I don't think there is any question that sheilded conductors are required. Even the RMS voltage to ground (if grounded) would be more than 2000V.
(BTW, some exceptions for this were eliminated in the 2005, according to the handbook).
Steve
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- Lockport, IL
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- Semi-Retired Electrical Engineer
You are certainly free to disagree. But you have not presented a strong argument in favor of the notion that "phase to phase" is intended, by default, unless "voltage to ground" is explicitly stated. The voltage rating of any component is based on the ability of its insulation system to prevent leakage from "where the current is" to "where the current should not be." For a cable, the place that "the current is" is the metal conductor, and the place that "the current should not be" is beyond the outer layer (be it insulation, shielding, jacketing, or armor). So what is there just beyond that outer layer? Another conductor? No, it is planet Earth, and things that are electrically connected to planet Earth.steve66 said:
Never in our industry does anybody express or speak about AC waveforms in terms of their peak values. What never? No never! What NEVER? Well, HARDLY EVER! (With apologies to Sir William Schwenk and Sir Arthur.)steve66 said:
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NEMA WC 70 (ICEA S-95-658) Section 1.3.1 describes voltage rating as ?? the operating voltage between phases or, in direct current, between conductors.?
WC 70 is the ?STANDARD FOR NONSHIELDED POWER CABLES RATED 2000 VOLTS or Less.?
I no longer have access to a legible copy of WC 71 which deals with cables over 2000V; however, I do know it essentially defines voltage rating of cables as the phase to phase voltage of a three phase system.
WC 70 is the ?STANDARD FOR NONSHIELDED POWER CABLES RATED 2000 VOLTS or Less.?
I no longer have access to a legible copy of WC 71 which deals with cables over 2000V; however, I do know it essentially defines voltage rating of cables as the phase to phase voltage of a three phase system.
Maybe I'm not reading this right, but the article cited talks about conductors operated above 2000 volts, not the insulation rating of the conductor. Article 100 says
This says to me that they're asking for shielded conductors where the phase to phase voltage is over 2kV. The exception goes on to exclude 2400V systems from the shielding requirement, under the conditions that they mention.
For the example that the poster asked about, of course, nothing changes. He needs shielded cable.
Voltage (of a circuit). The greatest root-mean-square (rms) (effective) difference of potential between any two conductors of the circuit concerned.
This says to me that they're asking for shielded conductors where the phase to phase voltage is over 2kV. The exception goes on to exclude 2400V systems from the shielding requirement, under the conditions that they mention.
For the example that the poster asked about, of course, nothing changes. He needs shielded cable.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
OK. I yield to greater knowledge and to greater force of logic. It's getting close to lunchtime here. Please pass the crow.
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