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motor frame grounding conductor sizing
7

motor frame grounding conductor sizing

motor frame grounding conductor sizing

(OP)
The NEC states that equipment grounding conductor should be sized according to the rating of the overcurrent device feeding the equipment.

It seems like the logic is that in event of a fault within the motor causing ground current to flow in equipment ground conductor, we want the cable to be protected  over a range of time and currents that is bounded by the protection.  i.e. the motor must trip before the cable melts.  

From that logic, should we be permitted to use much smaller grounding conductors for motors that are protected by sensitive ground fault relaying?

RE: motor frame grounding conductor sizing

You may have a point, but don't count on the NEC to change. Also per 430.52(B), 'the motor branch-circuit short-circuit and ground fault device shall be capable of carrying the starting current of the motor.', so you can't really set the GF for a very sensitive value.
Besides, cable melting would occur from high fault current which would be taken out by the instantaneously-operating short-circuit protection before the GF time delay expired.

RE: motor frame grounding conductor sizing


electricpete—that’s an interesting proposal, but I think that it’s not something that consensus standards folks [NFPA…IEEE] would be willing to buy into.  Taking the conservative approach, with the typical limitation of not needing to be larger than the phase conductors, one aspect that would not be effected by relaying is the magnitude of potential difference between equipment frames until protective relaying operates to clear the fault.  [In the whole scheme of things, equipment grounding/bonding is typically a small fraction of electrical-installation costs.]  

Accepted practices vary by voltage level, but I think most conscientious engineers would review limiting I²t as to not exceed the usual 250°C “emergency” copper-conductor rating, which is probably the weakest link in the chain.
  

RE: motor frame grounding conductor sizing

(OP)
Thx for comments, I'll need awhile to digest them.

By the way, I'm not trying to cut corners on a new installation. Just questioning an existing installation.

RE: motor frame grounding conductor sizing

I think the essential point of grounding-cable sizing has been missed!

Cable sizing is not related to fault-clearing time, but instead, to personnel safety.  The voltage-drop from machine-to-ground must not exceed the touch-potential considered dangerous to personnel!

RE: motor frame grounding conductor sizing

A related point.  Grounding-conductors should not be placed in magnetic enclosures, like steel conduit.  And, if it is, then, the conductor must be bonded where it enters and leaves the conduit!

Thermalstation's comment regarding European practice is as close as one can get to a scientific approach.  Think of the touch-voltage problem as a voltage divider consisting of the phase-conductor and the ground-return path.  The point of connection is the motor's carcass/frame.

RE: motor frame grounding conductor sizing

(OP)
Hi Shortstub - You may be right, but it is still not a clearcut logic to me.

I am trying to discern the basis/logic (and resulting application guidelines) for the NEC requirement which ties ground conductor size to protective device setting.  

Article 250.122 requires ground conductors to be sized according to the "rating or setting of the overcurrent protective device in the circuit ahead of the equipment..."

For 15A rating/setting it requires 14AWG copper
For 20A rating/setting it requires 12AWG copper
For 30A rating/setting it requires 10AWG copper
etc

I honestly do not know what the basis of these numbers are.
Clearly they are not to protect against conductor melting as I suggested since they are so low.

It seems to me that they are likely intended to protect the ground cable itself, since they are almost identical to the protection which would be applied to insulated wire in free air (although I agree ground conductor doesn't need as much protection as insulated conductor since it has no insulation to damage).

Is it your belief that these numbers are based instead upon steady state voltage drop?  If so then shouldn't there be a limit on the length of ground conductor in addition to the size?

A related question for the group. Let's say I have an industrial motor fed from load center with the following protection:
residual-connected ground protection 51G - 10A
Time overcurrent - 200A
Instantaneous - 1200A.
(All three currents are expressed on primary side of CT)
Which of these three settings would we pick as the basis for the ground conductor?

RE: motor frame grounding conductor sizing

(OP)
Correction:
"It seems to me that they are likely intended to protect the ground cable itself from steady state overload..."

RE: motor frame grounding conductor sizing

Electricpete,

The presumption is, that because both the phase- and grounding-conductors have to conform to the protective device size criterion, then their per unit length impedances are nearly equal.  Therefore, the grounding-conductor voltage-drop will always be smaller than the phase conductor's voltage-drop.  Thus, by voltage-divider effect, the touch-voltage will be smaller.

Of course, for the above to be true, then the current flows must be equal in both conductors, i.e., phase and grounding.

RE: motor frame grounding conductor sizing

(OP)
I'll tell you one thing is I am thinking about a power plant environment where we have a grounding grid and equipment safety ground is connected to structures attached to grounding grid... does not have to run back toward the power supply.  Maybe that is different than most equipment covered by the code?

Even if we did have a grounding conductor running the same distance as the phase conductor, what you are suggesting is that a voltage divider which provides approx equal voltage drop in the phase conductor and ground conductor will ensure the voltage does not rise to dangerous levels?  I have a very hard time believing that we are relying on votlage drop in the phase cable to reduce voltage to safe level in event of a fault... it is just not reasonable (would 1/2 of line-to-ground voltage be safe?).  I can envision that we might take the fault current level and multiply by the ground cable impedance to determine voltage which a human may be exposed to, but the table does not account for fault current level or resistance (no lenght specified). Without a lenght it seems like it must be looking at thermal effects, not resistance (My opinion)

RE: motor frame grounding conductor sizing

I believe voltage drop in this discussion (touch potential, or touch voltage) should refer to the voltage between the faulted motor frame and any other grounded equipment in the close vicinity which can be 'touched' simultaneously by personnel, not between the phase and ground return cables at the power source.
The theoretical voltage divider should be between the faulted frame(zero volts) and any nearby grounded metal components(measured as the voltage drop across the grounding connection through which the fault current is traveling). The better the bonding and grounding between these components, the lower the potential between them, and the less risk to personnel near them.
This is also the reasoning behind 'step voltage', and why there is a ground grid to bring the potential of the ground to near-equal values everywhere in a switchyard.

RE: motor frame grounding conductor sizing

(OP)
Hi Dan. Are you saying that you also think the basis for the NEC guidelines is associated with providing a sufficiently large cable to minimize step potential or voltage drop across cable due to fault current, and not thermal considerations?

RE: motor frame grounding conductor sizing

Electricpete,

Im not speaking for DanDel, but it is what I mean!

RE: motor frame grounding conductor sizing

'pete, I believe it is probably a combination of several factors, including both of these you mention. Besides, these two factors(voltage drop and thermal damage from fault current) are two sides of the same coin.
A long time ago, the prevailing school of thought was to isolate all electrical equipment and not ground anything. This was supposed to limit exposure to dangerous conditions. Eventually, grounding and bonding came to be the standard, so that if something went to ground, a solid ground connection would assure that the protective device would trip and de-energize the fault.

RE: motor frame grounding conductor sizing

For a grounding wire, I use as thumb rule half the cooper in the phase wire and always a naked wire, because there is no rubber or PVC to be melted, it makes possible to support very high temp. (350 C)

Also, grounding must be as close as possible to the electrical device.

RE: motor frame grounding conductor sizing

'pete...In Canada, we have a similar ruling. The rationale that CSA provides ( in a book that supplements the Canadian Electrical Code)is the bonding conductor size is determined by 2 factors - big enough to carry the current and big enough that it does not excessively increase the impedance of the ground return ciruit.

RE: motor frame grounding conductor sizing

ielivaz:

Your half-size rule will put you in violation of NEC for many installations.

Regarding the ground-wire insulation:  you'll just end up burning the insulation on your phase conductors instead (you are running that bare ground wire with your phase conductors, aren't you?).  Also, your ground wire will corrode much faster without insulation.  There's no code issue with using bare wire, just maybe not as much advantage as you think.

RE: motor frame grounding conductor sizing

(OP)
Thanks Peebee - you have stated an obvious point that I had overlooked.  If the ground conductor is run with phase cables than it's loading/temperature would need to be controlled just the same as an insulated conductor and it would have to be protected at the same level.

From my perspective it now seems very logical that the basis for these ground conductor ratings are thermal protection of cables.  The other comments are welcome and appreciated but I just don't see how we could hope to address touch/step potential issues without considering the fault current level and length of the ground cable.

More discussion is welcome if you think I'm missing soemthing.
Thx

RE: motor frame grounding conductor sizing


Any edition of IAEI Soares' Book on Grounding is fifty bucks well spent.
   

RE: motor frame grounding conductor sizing

Wow, thanks Pete -- A compliment from you, jbartos or busbar is better than ten eng-tips red stars in my book.

BTW, jbartos, congrats on yet another well-deserved re-election as TipMaster.

RE: motor frame grounding conductor sizing

I'm a little late to the party, but here's my $0.02:

As far the rationale for equipment grounding conductor sizing, my understanding has always been that the primary concern is assuring that sufficient fault current will flow to operate the overcurrent protective device (fuse or breaker).  

The NEC requires that in cases where the phase conductor size is increased to account for excessive voltage drop in a long feeder, the equipment grounding conductor must also be increased in size, in equal proportion.  

It does makes engineering sense that the equipment grounding conductor size could be reduced due to the use of more sensitive ground fault protection.  In fact, the NEC actually *does* allow this in the special case of multiple conductors per phase using pre-manufactured cable such as MC cable. See 250.122(F)(2).  

So, if I had an existing installation where the equipment grounding conductor was too small per NEC, for some reason, I would be willing to argue for a variance based on the use of GF protection, provided that grounding conductor was sized adequately based on the ground overcurrent setting.

But the NEC is never more illogical than it is in Article 250 so it is hard to apply logic.  Unfortunately, it is the one section that local inspectors know the best.  

RE: motor frame grounding conductor sizing

Suggestion: Visit
http://ourworld.compuserve.com/homepages/boyce_smith/reason.htm
for: Sizing and Purposes for Good Grounding Practices
http://www.hendersoncountync.org/inspections/National%20Electrical%20Code%20Changes.htm
for:
Article 250- Grounding

The rationale is in the conductor current carrying capability to trip a protective device. If the branch circuit conductors are very long, the ground conductor has to be upsized to enable passing the sufficiently large current in the ground circuit to trip the protective device. The ground conductor impedance is more important is its sizing than the thermal limitations. The insulated ground conductors inside conduits with current carrying conductors must not have the ground conductor temperature be elevated such that the ground conductor insulation is damaged and current carrying conductor insulation is thermally degraded. The calculations of smaller ground conductor sizes and baseline document do not appear to be readily available.

Peebee, thank you for congrats.

RE: motor frame grounding conductor sizing

(OP)
A ground conductor will have to meet multiple criteria as has been said.  The question is which of those criteria is limiting and which are associated with the article 250 table.  It seems very clear that these table limits are the familiar limits that we apply to phase condctors for THERMAL protection.   It also seems very clear that in most any realisic case where a conductor is sized based on thermal considerations, the conductor will not be so high resistance that it will limit current flow below protective settings in event of a solid ground fault.  (If we postulate a high-resistance fault, then the high fault resistance is limiting current below the protective setting, not the cable).  So thermal is the most limiting and thermal limits are provided in the table.

Thank you jbartos for providing backup to my point in your first link:

"All ground conductors must be of sufficient size to trip the
circuit breaker or fuse of the circuit that it protects, without
overheating 15 amp circuit 15 amp wire, 20 amp circuit 20 amp wire
"

RE: motor frame grounding conductor sizing

(OP)
jbartos - I agree with your statement: "If the branch circuit conductors are very long, the ground conductor has to be upsized to enable passing the sufficiently large current in the ground circuit to trip the protective device"

I would consider this the exception rather than the rule.  Usually the phase conductors may be designed for perhaps 5% voltage drop, it is difficult for me to imagine a case where phase conductors plus ground conductor alone limit fault current below protective settings, but I'm sure it's possible. My point is that I don't believe this scenario has anything to do with the table values in article 250.

RE: motor frame grounding conductor sizing

peebee:

of course I dont run the ground wire with phase wires.

Once the naked wire begins oxiding the CuO acts like a protective layer for the core.

Every one have seen 40+ year old distribution lines exposed to wheater and working.

 

 

RE: motor frame grounding conductor sizing

ielivaz:

If you are not running your ground wire with the phase conductors, then your post has nothing to do with the equipment grounding conductor referenced in electricpete's original question.  Per NEC 250.118, "equipment grounding conductors" are the grounding conductors "run with or enclosing the circuit conductors".  

In addition to those required ground conductors, you can certainly continue to provide any additional ground conductors as you see fit in any way you see fit.  They are not required by NEC and therefore can be of about any type, size, or configuration you please.  The required equipment grounding conductors, however, must meet very specific requirements of Article 250.

RE: motor frame grounding conductor sizing

Suggestion marked ///\\\: ///The NEC revisited has the following:\\\
250-122 (B) – The title of this section has been changed from “adjusted for voltage drop” to “increased in size”.  This essentially means that any time an ungrounded conductor is changed in size the corresponding equipment ground must also be proportionally changed in size also.

///However, there is no specific explanation how to change the ground proportionally. Apparently, if one upsizes a current currying conductor to the next size, the ground is supposed to be upsized to the next size.\\\

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