Tables 250.66 & 250.122

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Rockyd

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Does anyone know how the two tables, 250.66 & 250.122 were derived? The Tables are used a lot but does anyone know if they are from empirical knowledge or engineering studies? How many actually do the load calculations as required per 220.61 to properly size the neutral service entrance conductor, vs. 250.66, and use the greater of the two numbers?
 

Dennis Alwon

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I seriously doubt too many people do a neutral calculation to downsize the neutral at the service. It may be more common on feeders where the neutral cannot be smaller then the egc.
 

Rockyd

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Retired after 40 years as an electrician.
I seriously doubt too many people do a neutral calculation to downsize the neutral at the service. It may be more common on feeders where the neutral cannot be smaller then the egc.

Dennis,

That is not the intent of the code when it comes to 250.66. it directs you to 250.24(C) The intent of 250.24(c) is to establish the minimum size that can be used for a service entrance neutral. That is done buy doing the load Calculation, then comparing it against 250.66 whichever of the two is bigger, that will be the Service entrance neutral size.

Here is commentary from the NEC Handbook (not code, but dollars to doughnut - it's dead on the money fro methodology)

If the utility service supplying the premises wiring system is grounded, the grounded conductor, whether or not it is used to supply a load, must be run to the service equipment, be bonded to the equipment, and be connected to a grounding electrode system. Exhibit 250.8 shows an example of the main rule in 250.24(C), which requires the grounded service conductor to be installed and bonded to each service disconnecting means enclosure. On the line side of the service disconnecting means, the grounded conductor is used to complete the ground-fault current path between the service equipment and the utility source. The grounded service conductor's other function, as a circuit conductor for line-to-neutral connected loads, is covered in 200.3 and 220.61. The exception to 250.24(C) permits a single connection of the grounded service conductor to a listed service assembly (such as a switchboard) that contains more than one service disconnecting means, as shown in Exhibit 250.9.

(1) Sizing for a Single Raceway. The grounded conductor shall not be smaller than the required grounding electrode conductor specified in Table 250.66 but shall not be required to be larger than the largest ungrounded service-entrance conductor(s). In addition, for sets of ungrounded service-entrance conductors larger than 1100 kcmil copper or 1750 kcmil aluminum, the grounded conductor shall not be smaller than 12? percent of the circular mil area of the largest set of service-entrance ungrounded conductor(s).


Mike Holt has been teaching like this since at least 2005 which I will attach to the next post.
 

Rockyd

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Retired after 40 years as an electrician.
Here is an article from EC & M from Mike Holt covering the issue -

Grounding vs Bonding - Part 3 of 12
Mar 1, 2005 12:00 PM, By Mike Holt, NEC Consultant

0 Comments ShareThis4Grounding and bonding at service equipment

Find more from this series on Grounding vs Bonding

All Code references are based on the 2005 National Electrical Code.

Because utilities provide grounded AC services and most facilities have at least one utility service connection, a grounded AC service most likely provides power to your premises wiring system. When you have one, your premises wiring system must have a grounding electrode conductor connected to the grounded service conductor [250.24(A)].

This brings up the question of how to comply with grounding electrode conductor requirements. Because a grounding electrode conductor must connect the grounded conductor to the grounding (earthing) electrode, the question of how expands to include where. Can you make this connection anywhere?


Fig. 1. Make your grounding electrode conductor connection at one of these three points.
Location, location, location. Some inspectors require the grounding electrode conductor to terminate to the grounded conductor terminal at the meter enclosure. Other inspectors require the grounding electrode conductor to terminate to the grounded terminal at the service disconnect.

The Code says you can make this connection at any accessible location, from the load end of the service drop or service lateral up to and including the service disconnecting means [250.24(A)(1)] (Fig. 1). The choice then becomes an engineering decision that balances such factors as installation costs, available space, and maintenance issues.

Connections. In this day of increased demand for uninterrupted power, many facilities are dual-fed. This means separate lines run to the same services; such services are referred to as “double-ended.” If the dual feeds are in a common enclosure (or grouped together in separate enclosures) and they employ a secondary tie, you can use a single grounding electrode connection to the tie point of the grounded conductors from each power source [250.24(A)(3)].

Whether your service is double-ended or not, you must install an unspliced main bonding jumper between the grounded terminal and any metal on the service disconnecting means enclosure. Ensure the bonding jumper and installation comply with 250.28 and 250.24(C), respectively.

Your main bonding jumper is probably a wire or busbar. If you've installed this jumper from the grounded conductor terminal (or bus) to the equipment-grounding terminal (or bus) in the service equipment, the NEC allows you to connect the grounding electrode to the same equipment-grounding terminal (or bus or bar) to which you connected the main bonding jumper [250.24(A)(4)].

Bonding of service equipment. A neutral-ground bond anywhere other than at service equipment is a common cause of power quality problems. Such a bond creates ground loops, which allow undesired current to circulate in the system. Power quality problems often lead to the discovery and removal of such a bond. But don't wait for power quality problems to reveal the bond.


Fig. 2. A grounded neutral conductor must be run to and bonded to each service disconnect.
Another concern makes corrective action imperative. Load side neutral-ground bonds allow objectionable current to flow on conductive metal parts of electrical equipment — thereby violating 250.6(A). This objectionable current can cause lethal electric shock. And it sets the stage for inadvertent flashovers, overheating of equipment, and other problems stemming from the presence of electricity in the wrong place.

So don't make (or allow) a neutral ground connection on the load side of the service disconnect [250.24(A)(5)]. However, exceptions to this rule (250.142) allow you to make such a connection for:


•Separately derived systems if you follow the requirements of 250.30(A)(1).

•Separate buildings if you follow the requirements of 250.32(B)(2).


Grounded conductor. Electric utilities don't typically provide an equipment-grounding (bonding) conductor to service equipment, and they aren't required to do so. Thus, you must run a grounded conductor from the electric utility transformer to each service disconnecting means [250.24(B) and 250.130(A)] (Fig. 2).

The grounded service conductor provides the effective ground-fault current path to the power source winding. This path ensures that opening the circuit protection device will quickly remove dangerous ground-fault voltage from the circuit [250.4(A)(3) and 250.4(A)(5)].


Fig. 3. Very little fault current returns to the source if the earth is the only fault-current return path.
The earth's resistance is too great for it to be an effective bonding jumper. Very little fault current returns to the power source winding if the earth is the only fault-current return path. But let's suppose the earth is your only fault-current return path option. What would the consequences be? For one thing, the circuit overcurrent protection device wouldn't open and clear the ground fault. Consequently, metal parts like metal piping and structural building steel would become — and remain — energized to circuit voltage (Fig. 3). The system then poses a high risk of shock, arc flash, and fire.

You can calculate, for example, the voltage on a metal enclosure due to an open service grounded conductor. Forensic engineers often crank out these kinds of numbers when investigating a fatality case or damage to a facility. It's easier just to comply with NEC requirements and eliminate such a voltage in the first place.

So it's obvious you need a grounded conductor, but how big should it be? Remember, this grounded service conductor serves as the effective ground-fault current path. Thus, you must size it so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. Size the grounded conductor per Table 250.66 — based on the total area of the largest ungrounded conductor. The grounded conductor must also have the capacity to carry the maximum unbalanced current, per 220.61.

To test your understanding of the concept, consider the following scenario:

What's the minimum size grounded service conductor required for a 480V, 3-phase service, where the ungrounded service conductors are 500 kcmil and the maximum unbalanced load is 100A?

The unbalanced load requires a 3 AWG grounded service conductor — rated for 100A at 75?C per Table 310.16 [220.61]. However, the grounded service conductor can't be smaller than 1/0 AWG (Table 250.66). This minimum size requirement ensures the conductor will accommodate the maximum fault current likely to be imposed on it. Thus, the real answer is 1/0 AWG.

If you parallel your service conductors, do you use just the one conductor or do you parallel your grounded conductor the way you parallel the current-carrying conductors? The answer is neither.

First, you must install a grounded conductor in each raceway whenever you parallel your service conductors.

Second, you can't simply divide your grounded conductor into two smaller equal conductors. You would satisfy the requirement that the grounded conductor must have the capacity to carry the maximum unbalanced current per 220.61, but it could also result in a grounded conductor that's too small for a given raceway.

To eliminate such a problem, size each grounded conductor per Table 250.66 — based on the total area of the largest ungrounded conductor in the raceway. Note that regardless of the number you come up with, the grounded conductor in each parallel service raceway can never be less than 1/0 AWG (310.4).

Let's review with another quick quiz:

What's the minimum size grounded service conductor required for a 480V, 3-phase service installed in two raceways, where the ungrounded service conductors in each raceway are 350 kcmil and the maximum unbalanced load is 100A?

The unbalanced load requires only a 3 AWG grounded service conductor, per Table 310.16 (220.61). However, the grounded service conductor in each raceway can't be smaller than 2 AWG (Table 250.66) [250.24(C)(2)]. This ensures it will accommodate the maximum fault current likely to be imposed on it. But ungrounded service conductors run in parallel can't be smaller than 1/0 AWG (310.4), so the answer is 1/0 AWG per raceway.

Properly grounding and bonding service equipment improves safety while eliminating a common cause of power quality problems. You just have to make the right connections in the right places. If you think in terms of providing a low-impedance ground-fault path back to the source, you'll have no problem.



Here's a link
 
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Rockyd

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Nevada
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Retired after 40 years as an electrician.
The rule on grounded conductors- they are figured on 250.66 and are sized per raceway.

The EGC is figured 250.122 per breaker and seems rather large, especially where paralleling raceways use a wire conductor to satifsy the EGC requirement.

To keep the thread on track, the OP wants to know how Tables250.66 and 250.122 were developed....Who, what, when, where, why, and how. Any part would help...

Got anything on that?
 

Rockyd

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Nevada
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Retired after 40 years as an electrician.
To clarify Dennis,

The handbbok reference is taken from NEC PLUS website for the 2011 NEC handbook.

The reference to Mike Holt is a 2005 article, but most of the conceptuality is reasonably close, even though it was 2005 and should help those that aren't "all dialed up tight" on the latest, greatest, codeology. Add some new defs, 250.32, and 250.142 changes, and that 250.30 got turned upside down for a rewrite.
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
Dennis,

That is not the intent of the code when it comes to 250.66. it directs you to 250.24(C) The intent of 250.24(c) is to establish the minimum size that can be used for a service entrance neutral. That is done buy doing the load Calculation, then comparing it against 250.66 whichever of the two is bigger, that will be the Service entrance neutral size.

I know how it is done for services and feeders but I still don't think people do it very often. I know I can downsize my neutral for a 200 amp service to #4 if my calculations allow but how many EC's would do that. Why #4 was chosen for the GEC-- I don't know--- Why #6 for the 200 amp EGC- I don't know.

I realize I didn't answer your question but that is because I don't know the answer as to how the tables were done. I would have to assume that some heavy engineering calculations went into this.

Where are all those engineers-- Charlie, Smart......and the list goes on.
 

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
Rockyd, I don't know how these tables were derived either. But if you're still in Alaska, I suggest you turn on "Ice Road Truckers" and take a break from these tables.:)
 

Rockyd

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Nevada
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Retired after 40 years as an electrician.
LOL Bill and Dennis! Got a bunch of trips over that haul road (Fairbanks to Prudhoe Bay). Not in any hurry to get back on it.

I just find it really funny that everyone blindly goes to the tables and uses them like they are words from on high. I would be intrested in knowing how it came to be?

Hey Dennis, I know a couple of shops that do the calc's and see if there is any changes will be necessary in sizing the grounded service entrance conductors. Trust me, oil companies are real funny about wanting to insure that static is not going to be a problem in the oil patch. They have no problem with using copper, or stepping up the size.

So yes, the OP is still hoping for an engineer, or some others (moderators included) to cough up an answer to the tables 250.66 and 250.122.
 

George Stolz

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Windsor, CO NEC: 2017
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Service Manager
Does anyone know how the two tables, 250.66 & 250.122 were derived?

I don't.

How many actually do the load calculations as required per 220.61 to properly size the neutral service entrance conductor, vs. 250.66, and use the greater of the two numbers?

I don't think it comes up all that often. Generally, the neutral is either the same size as the phase conductors, or a couple sizes smaller when using cable assemblies. The only time it would occur to me to run a neutral smaller than that would be if the neutral load were 0, then I would run a conductor sized per 250.66.

I know Don has a good feel for how much fault current conductors can withstand, I'm sure if you shoot him a PM he'd chime in.

I will say I'm comfortable that the conductor sizes given in 250.66 are probably more than adequate to withstand the fault currents they'd be subjected to.
 

bphgravity

Senior Member
Location
Florida
Both Tables are based on engineered calculations using tested data and potentially realized conditions.

Simply put, Table 250.66 is calculated to provided grounding electrode conductors which will meet the performance criteria outlined in 250.4(A)(1). It is purely based on the ratio between the size of the ungrounded conductors of the system and the grounding electrode conductor. It does not take into account the grounding electrode type. In short, the size of the GEC must be increased in proportion as the potential of imposed voltages on the system increases.

Table 250.122 is calculated to provided equipment grounding / bonding conductors which will meet the performance criteria outlined in 250.4(A)(5) & 110.10. This calculation is based on the ratio between the circuit overcurrent device and the equipment grounding / bonding conductor. Conductors can only withsatnd so much current over a period of time. Like Table 250.66, the table does not take into account specific installations which may result in the need to adjust or correct the table values. In short, the size of EGC must be increased as the potentail of fault current on the circuit increases.

A good comparison would be conductor ampacities. The table is based on baseline criteria and must be corrected for conditions outside the parameters of the baseline. This is why grounding electrode conductors to ground rods can be smaller than table 250.66 or why EGC's must be increased per 250.122(B).
 

don_resqcapt19

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Somewhere I think that I read that the EGC sizing is based on having no more that a 40 volt drop under fault conditions, but I don't know how you could come up with that size without knowing the fault current.
 

Rockyd

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Nevada
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Retired after 40 years as an electrician.
Thanks everyone for weighing in. The ratio, and empirical data gathered sounds right on target. The charts only need fit "most" jobs that we address on a regular basis. There will always be "special exceptions" to the rule (that's part of what engineering covers) but that it works most of the time serves the industry.

I'm with George on the fact that the average field install is done of the chart, or whatever size the "hots" are, minus two. That would be 4/0 gets you a 2/0 grounded conductor, etc.

Fits right in with the theory that I have that we build at a 300% safety factor in our industry. Most stuff we install, could withstand short term operation at 3 times what it's rated.

Charts must be pretty good, when I see a report of an electrical fire, I tend to think a DIY, more than I do looking at our industry.

Edit - Don, I haven't had a chance to see where 70E relates in correlation to the charts, but be something I can also look at.
 
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vanvan

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Hey Rockyd,
Not sure if it's exactly what you are looking for, but in my Soares book on grounding there is a section "Conductor Withstand Rating." Basically stating conductors have to be large enough to safely carry any short circuit/ground fault current long enough for o/c device to open. The time given is 5 seconds. Values were derived from the Insulated Cable Engineers Assoc publication P32-382 (1994). "an insulated conductor with a bolted connection can safely carry one ampere for every 42.25 cir mils for 5 seconds without destroying its validity." I x I x t = short time rating of conductor. They then go on to do some examples with a #8 copper conductor. Pretty interesting stuff for sure!
 

don_resqcapt19

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Illinois
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Hey Rockyd,
Not sure if it's exactly what you are looking for, but in my Soares book on grounding there is a section "Conductor Withstand Rating." Basically stating conductors have to be large enough to safely carry any short circuit/ground fault current long enough for o/c device to open. The time given is 5 seconds. Values were derived from the Insulated Cable Engineers Assoc publication P32-382 (1994). "an insulated conductor with a bolted connection can safely carry one ampere for every 42.25 cir mils for 5 seconds without destroying its validity." I x I x t = short time rating of conductor. They then go on to do some examples with a #8 copper conductor. Pretty interesting stuff for sure!
Here is a Bussmann paper that gives some details on the short circuit withstand of conductors.
 

Rockyd

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Nevada
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Retired after 40 years as an electrician.
That Bussman paper was interesting. Looks like a lot of research has been been put into the intial seign ofthe chart. Enough so that the field installation, and those associatd with, will do good following the charts. Now after 250.66....

Thanks for the information! Will be checking back on it .
 
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