I got a couple of electrical questions I developed as a result of studying some things in the NEC.
- What exactly is a ground ring?
- What is a corner ground?
- What is derating? I heard you must derate in certain times by 80% or something like that? How do you figure it out and why must you?
- Why are electric utilities required to drop a ground every 400 feet?
- Plenum wiring….does this mean running wires inside the metal plenum duct work? Why on earth would you want to do that if so instead of in conduit? What’s the advantage of running inside a plenum anyways?
- The code is drifting towards matching electrical service with an AIC rating. What is an AIC rating?
- The code talks a lot of about 600 volts, voltages at that or above are into a new arena of code requirements. Since I mostly do residential work and occasional light commercial, is there ever a case where I’d see 600 volts being used in a residential application?
Replies
Alot of these questions need some context. There are many possible answers.
"# What exactly is a ground ring?
# What is a corner ground?"
Are you talking about an ground electrode system or equipment grounding conductor?
A ground ring bascially means multiple paths back to some common reference point (the "ground"). In general it is an advantage, but can cause problems with sensitive audio equipment.
It could also mean a type of grounding electrode system.
A corner gorund might mean a type of 3 phase system where one leg of a the detla is grounded.
"# What is derating? I heard you must derate in certain times by 80% or something like that? How do you figure it out and why must you?"
There are several different types of derating. The one that you are talking about is that residential branch & feeder circuits are only rated at 80% of nominal rating for Continous loads. Those are any that are only for 3 hours or more.
Other deratings are required with you have more than 2 equivalent current carrying conductors in a conduit or cables bundeled together. Also if the wiring is in "hot" areas then they need to be derated. The values are all given in the code boooks.
But you also need the wire tables to find the base ampacity rating from which you start the derating.
"# Why are electric utilities required to drop a ground every 400 feet?"
Powerline grounding is to disapate surges (lightning). The farther the surge runs down the wire the more energy that it is going other places than into the ground.
"Plenum wiring....does this mean running wires inside the metal plenum duct work? Why on earth would you want to do that if so instead of in conduit? What's the advantage of running inside a plenum anyways?"
In cold air returns. Most commonly used in commercial work where the whole space above a drop ceiling is a return plenum.
But also used in residential work for fishing new cable and phone lines.
Cables used for plenum work have different jacketing material so that any fumes that they might give off in a fire won't be (as) toxic.
"The code is drifting towards matching electrical service with an AIC rating. What is an AIC rating?"
The interrupting capacity of a breaker. If you try to "turn off" a switch with a very high current flowing through it there will be arcing which generates ionized air and in turn a plasma that will allow the current to continue flowing.
If you have a a dead fault on a circuit you can get such large currents and the breaker has to be able to interrrupt it.
This is not a problem on residential circuit.s
"The code talks a lot of about 600 volts, voltages at that or above are into a new arena of code requirements. Since I mostly do residential work and occasional light commercial, is there ever a case where I'd see 600 volts being used in a residential application?"
Only inside TV's and spark ignition furnaces.
First, the NEC is not written as a manual. It even says so: Article 90.1C - "Intention. This code is not intended as a design specification or an instruction manual for untrained persons". Using the manual without an understanding of electrical theory and some training isn't likely to yeild the right results very easily.
That said I do know a journeyman electrician who essentially trained himself, at least on the accademic side, by getting a copy of the NEC, an illistrated NEC handbook might be best, a copy of the "American Elecrician's Handbook" and a good DIY manual or three.
After studying for several months at night he got a job with a local contractor as a helper. He took a substantial pay cut but moved up quickly. Within a year he was making more than twice his initial pay. The contractor was impressed by his determination and volunteered to certify his 'hands-on' hours and pay the $400 for his journeyman exam. He passed on the first try.
That said:
1 - A ground ring is essentially an extended ground rod. Typically a ring, or rings of heavy copper conductor buried. Some have ground rods dispersed around the ring.
2 - Could be referring to a corner grounded delta configuration for transformers. Pretty rare. Then again this term would depend on the context.
3 - Every conductor/insulation conbination has a limit of how much current it can carry without the insulation melting, failing or falling off. This rating is usually shown for a standard temperature. Usually 30C, 86F.
If the temperature the conductor is operated in is above or below this standard temperature a 'derating' factor is applied to account for the greater ease or difficult of the conductors to give up heat.
Similarly the rating of a conductor can be given in terms of 'free air' or within a cable or conduit. The containment of being surrounded by a conduit and and the presence of more than one 'current carrying conductor' has to be accounted for.
The derate for temperature and the derate for conduit fill are used on top of one another. ie: A conductor with 75C insulation used in a location that is 50C, 122F, has a derating multiplier of .75. So if the conductor normally has a ampacity of 100A, such as a run of #2 copper, the ampacity is derated to 75A. If then this same conductor is within a conduit with three other current carrying conductors, four total outside of shared neutrals and grounds, then a further derating factor of .80 must be applied.
Derate for temperature 100A x .75 = 75A
Derate for conduit fill 75A x .80 = 60A
Which means your #2 copper has to be fused at 60A. The dreaded 'double derate' has bit. Most cases are not so bad but it is something to keep an eye on.
5 - Plenum wiring. Rarely will wiring of any sort be inside of ducts. That which is is usually control or alarm wiring. I have never seen line power circuit conductors run within a duct. Ducts are excluded from use in leu of conduits. But a plenum is not necessarily only inside of a duct. Many commercial buildings have their return air coming through the open are above the ceiling tiles. This too is considered a plenum.
The primary concern with plenum wiring is fire hazards. Both flamability and the results, byproducts, of combustion. Plenum rated cables are designed to be more flame resistant and to produce far fewer toxic byproducts if they do burn.
6 - AIC, Amps Interupt Current. Conductors under load arc when the circuit is broken. This arc is quite powerful. Typically it is 12000 degrees F or ten times as hot as a normal flame at 1200F. This arc is limited in power primarily by the circuit impedance.
Larger conductors have less impedence and so higher currents potentially available. A main breaker near a transformer and connected by heavy conductors have a lot of potential current available and a device used to interupt this current must be rated to withstand the , heat, arc blast and magnetic forces, associated with the interupting this current.
Using a breaker on a circuit capable of producing more than what it is designed for can mean the breaker literaly exploding into a ball of white hot plasma that can vaporize metal, rip buss bars from mountings and turn heavy steel enclosures into shrapnel.
Do not get confused and think a 20A breaker will limit the current to 20 Amps. Most breakers are inverse-time. The higher the overcurrent the the shorter the delay before it operates. But a breaker takes time during operation. Remember electricity is fast. In the time it takes for the breaker to break the circuit the curent can by escallating exponentially. Limited only by the circuit impedance. The higher the current the bigger and stronger the arc.
7 - 600v. If it was a perfect world all 120v circuits would have a steady 120v. In the real world a 120v cicuit varies quite a bit. Very normal this. But on top of this there are spikes.
Older cars used to have points. idea being that current would be run threogh a coil and then the circuit broken. The magnetic energy within the coil has to go somewhere. In the case of the car it leaps in voltage and is routed to the spark plugs.
Run a motor and cut the circuit and usually a voltage spike will be generated. This then travels down the line. This voltage spike may be a hundred or more volts on top of the normal 120v. Suddently the 600v limit doesn't seem so extravigant. Start dealing with 480v cicuits and 600v starts getting close.
The other point is what happens at voltage breakdown. Ever price multimeters? Notice that ones that apparently do the same job vary widely in price? One of the reasons is how high a voltage can the meter safely handle. This is different than what it can read. If my meter is only rated at 500v and I'm reading a 480v circuit when a spike comes down the line the meter can quite literally explode. Once a tiny arc starts the current can run away. A lot of electricians have been hurt in this manner.
600v volts was a compromise. Something the manufacturers could reach at a reasonable cost with the materials available. A rating that is usually adequate to the job of protecting people and property.
And Bill Hartmann;
Excellent reply post by both.
Dave