Excerpted from Grounding Electrical Distribution Systems for Electrical Safety by Eustace C. Soares copyright 1966.   Published by the Marsh Publishing Company, Inc., P.O. Box 630, Wayne,  New Jersey 07470 (no longer in business. ) Edited and updated to the 1999 NEC® by Gerald Newton, November 13, 1999
 
 
 
 

When a metallic conductor enclosure is used as an Equipment Grounding Circuit Conductor it must have continuity as well as the required conductivity to pass enough current to facilitate the operation of the overcurrent devices.  In other words, the current that will flow in the ground fault circuit should be the minimum that will clear the circuit in a reasonable time.  A minimum current value of about 300 percent of the full-load rating of the overcurrent device may be considered a reasonable standard if reference is to overcurrent devices having the time-current characteristics of standard fuses and inverse-time circuit breakers.

If we can find a practical and simple method of determining the impedance of the single-phase circuit involved when a ground fault occurs we can get a reasonable answer as to whether the metallic enclosure will make an acceptable Equipment Grounding Conductor or whether it will be necessary to use an Equipment Grounding Circuit Conductor of copper or aluminum.  When an iron conduit is a part of the electric circuit, which it will be when a ground fault occurs, there will be a large increase in both the resistance and reactance of the circuit and moreover both the resistance and reactance will vary considerably with the amount of fault current.

Laboratory tests have disclosed that when a single-phase current flows in a conductor within an iron conduit the impedance of such a circuit is approximately equal to the impedance of the conduit itself.  The size of the conductor within the conduit has relatively little effect on the circuit impedance. Also, despite the fact that although there are many parallel paths external to the conduit, the current flowing in all the parallel paths will be very small, and under normal conditions would be under 10 percent.

Two other factors are to be taken into account in estimating the ground fault current flow.  They are the effect of the conduit couplings in increasing the impedance of the circuit and the volts dropped across the point of the fault.  If conduit couplings are installed wrench tight, as required by the Code, the increase in impedance of the conduit with couplings is about 50 percent more than the impedance for a straight run without couplings.  Limited test data has shown that if red lead is used as a joint sealer better results are obtained.  This may and probable is due to the lubricating effect of the sealer which allows the joint to be drawn up tighter with the same effort.

Assume a 200 foot run of 3" conduit with 500 kcmil conductors on a 208Y/120-volt circuit protected by 400 ampere overcurrent devices.  On a ground fault the current that will flow will thus be E/Z.  With a 50-volt drop at the fault and Z = 0.02970 (See Table III in the original work), the current that will flow will be about 2350 amperes.  The use of a 3" conduit as the Equipment Grounding Circuit where 400 amperes overcurrent devices are used is thus satisfactory for this run.

A simpler method of determining if the conduit or metallic enclosure will perform satisfactorily is to first calculate the minimum desired fault current flow (5 times the overcurrent device rating) which in this case is 5 x 400 or 2,000 amperes.  Then on the basis of 70 volts available for a 120-volt-to-ground circuit calculate Z, which will be found to be 0.035.  From Table III in the original work the impedance of a straight run of 3" conduit carrying 2,000 amperes is 0.099 Ohms per 1,000 feet.  To that figure add 50% to include a safety factor.  That will give us an impedance of 0.01485 ohms per 100 ft.  The impedance value that will cause 2,000 amperes to flow in that circuit was found to be 0.035.  since 235 ft. of 3" conduit carrying 2,000 amperes will have an impedance of 0.035 we will have deduced that to have a minimum current flow of 2,000 amperes in a ground fault we can install up to 235 ft. of 3" conduit when a 400-ampere overcurrent device is involved.

If 4" conduit was used instead of 3" and the overcurrent device rating did not change but remained at 400 amperes it will be found on reference to the table with some interpolation and using the same calculation, that 260 ft. of 4" conduit could be installed and provide a satisfactory Equipment Grounding Circuit conductor.

For any circuit and any size conduit we can thus derive for any size overcurrent device the maximum safe length of conduit which will allow a fault current to pass which will be sufficient to facilitate the operation of the overcurrent device.  Should the circuit length exceed the maximum safe length as calculated then it will be necessary to add a metallic (copper or aluminum) Equipment Grounding Circuit conductor in parallel with the conduit sizing that conductor per Table 250-122.

It would be neither practical nor desirable to substitute a copper conductor for the conduit but rather add the copper conductor and connect it in parallel with the conduit to form an Equipment Grounding Circuit of two conductors in parallel; the copper conductor and the conduit. It would be desirable to connect the copper conductor and the conduit together at convenient practical intervals, about every 100 ft. or less.  That would reduce the length of the circuit through which the ground fault current may flow on the conduit alone.  The Code permits an Equipment Grounding Circuit conductor to be bare or insulated.  However, if the conductor is bare there may be some arcing between the bare conductor and the interior of the conduit at points other than the point at which the ground fault occurs.  Such arcing may damage the phase conductors without adding to the proper functioning of the ground fault circuit if the installation has been properly made.  That makes a strong case for the use of insulated Equipment Grounding Circuit conductors when installed in metallic enclosures.

If aluminum conduit was used instead of steel conduit for the same conditions cited above, 500 kcmil copper conductors, 3" conduit and a 400-ampere overcurrent device the circuit run could be about 900 ft. long and the aluminum conduit would provide a satisfactory Equipment Grounding Circuit conductor.  A 3" aluminum conduit has a DC resistance of about 0.0088 ohms/M' and 500 kcmil copper cable has a DC resistance of 0.0222 ohms/M'.

Other conductor enclosures include flexible metal conduit, various metal raceways, including wireways and busways.

Although the Code allows flexible metal conduit to be used as an equipment grounding conductor with some limitations, some engineers feel that flexible metal conduit is not suitable as an Equipment Grounding Circuit conductor and thus such conduit should include a metallic Equipment Grounding Circuit conductor or be provided with a bonding jumper.  The Code requires that the various metal raceways used for equipment grounding shall be so constructed that adequate electrical and mechanical continuity of the complete system be assured.  However, owing to the various joints involved it is well for the engineer, electrician, and inspector to investigate such conductor enclosures to be assured that their impedance is sufficiently low to function along the lines cited above in the event of a ground fault.

For wireways and busways the Code does not specify directly that adequate electrical and mechanical continuity must be assured; but obviously it is just as essential for those conductor enclosures as for any other conductor enclosures.  The engineer, inspector, and electrician should examine the Equipment Grounding Circuit conductor (the metal enclosures) to be assured it will function properly in the event of a ground fault.  Where wireways and busways have steel enclosures it will be found that there may be enough cross section but it is questioned it the proper electrical connection between lengths is accomplished.

In the case of the average busway, up to 1500 amperes rating, there is enough steel in the enclosure to provide an acceptable Equipment Grounding Circuit if proper conductivity is assured at the joints.  For busways of higher rating it is doubtful if the steel enclosure is heavy enough and for all sizes of busways the matter of good electrical connection at the joints must be checked carefully.  Many busways have aluminum enclosures.  In such busways the enclosure has sufficient conductivity but good electrical connections at the joints also must be checked.

The importance of maintaining good electrical continuity and conductivity of the Equipment Grounding Circuit cannot be overemphasized.  If conductivity is not adequate a ground fault may never clear automatically as it should, or the time of clearing may be so long that much damage is done.  On the other hand, if we do not have continuity of the Equipment Grounding Circuit we will have absolutely no connection, resulting in a high enough impedance, that the fault will not clear automatically and dangerous voltages are liable to appear on the system.

In the above discussions it is assumed that a conductor enclosure has been properly installed with good tight joints which will provide a permanent and continuous electrical circuit WHEN IT IS FIRST INSTALLED.  However, time will take its toll and will tend to destroy the continuity that was begun with.

The safety of an electrical system will thus depend on how long we can expect the Equipment Grounding Circuit to remain permanent and continuous.  The answer will vary depending on the TYPE of installation.

For design purposes we can break the types into two categories:
 

1. Where little corrosion will exist and where we can reasonably expect that the Equipment Grounding Circuit to remain permanent and continuous for a period of fifty years.
2. Where corrosion in varying degrees will exist and where the permanency of the Equipment Grounding Circuit can be questioned.

In the second category we could include all industrial plants where a metallic Equipment Grounding Circuit sized per Table 250-122, must be run parallel with and within the conductor enclosure so as to ensure continuity if that circuit is broken owing to corrosion sooner or later.

Whether to use a metallic Equipment Grounding Circuit per Table 250-122 or to rely on the conductor enclosure to serve as the Equipment Grounding Circuit is a question that must be answered by the design engineer, electrical contractor, electrician, and electrical inspector.  The installation should be reviewed to see whether it falls into category No. 1 or category No. 2 as outlined.  A selection of the category in which the installation falls will determine how the Equipment Grounding Circuit is to be designed.

Faults are classified as short circuits or ground faults.  In a "Short Circuit" the fault may be from one phase conductor to another or from one phase conductor to the grounded conductor or neutral.  For either condition the maximum value of fault current is dependent on the available capacity of the system at the point of the fault.  The maximum value of short-circuit current from line to neutral would be approximately 20% of the maximum available from phase to phase.  The available capacity of the system, called the available interrupting current (A.I.C.), thus plays a large part in determining the maximum short-circuit current, which is further limited by the impedance of the arc when one is established, plus the usually very low impedance of the conductors to the point of the short circuit.  In the case of a "Ground Fault"  we have a fault from phase to the conductor enclosure (wire to conduit, wire to motor frame, etc.).  In this case the only part that the available capacity of the system plays usually is to maintain the voltage.  It is a rare case, in services 1200 amperes or larger, where the impedance of the fault circuit, including the impedance of the arc, is low enough to permit much more than normal full-load current of the system to flow in the ground fault circuit when the circuit is improperly grounded.  While conditions may arise where a ground fault current will exceed the full load rating of the overcurrent device the maximum ground fault current may not reach five times that full load rating.  It is reasonable to assume that a ground fault current five times the rating of the overcurrent device is necessary in order to operate the overcurrent device in the minimum time to prevent extensive damage to the electrical components of the circuit. The available fault current is assumed in most calculations to be infinite at the utility supply.  The available fault current decreases the farther away from the utility supply the fault occurs.  For transformers the available fault current at the secondary terminals  is calculated by multiplying the full load secondary current by the impedance of the transformer in percent divided into 100.  For a transformer with an impedance of 5 percent the multiplier is 20.  Standard equipment, panelboards, safety switches, service equipment, etc. have an AIC rating of 10,000 amperes.  When the fault current is more than 10,000 AIC more expensive equipment with a higher AIC rating is required.  One way to lower the AIC is to install the service at a greater distance from the utility supply transformer, use a higher impedance transformer, use smaller parallel conductors or use current limiting fuses.

 

Maximum Length of Steel Conduit That May Safely be Used as an Equipment Grounding Circuit Conductor based on a clearing Ground Fault Current of 500% of Overcurrent Device Rating.
Circuit is 120 volts to ground; 50 volts drop at the Arc.

Conduit Size In.	Conductors AWG No. /kcmil	Overcurrent Device Rating Amps.	Fault Clearing Current 500% of OC Device Rating	Approx. Conduit Impedance Ohms/C ft. including  Couplings	Length of Conduit Run
1/2	3-#12	20	100	0.198	350
	4-#10	30	150	0.135	345
3/4	4#10	30	150	0.098	315
1	4-#8	45	225	0.0924	335
	3-#4	85	425	0.0612	266
1 1/4	3#2	115	575	0.0459	265
1 1/2	3-#1	130	650	0.0402	265
	3-#2/0	175	875	0.0339	220
2	3-#3/0	200	1000	0.0273	255
	3-#4/0	230	1150	0.0261	230
2 1/2	3-250 kcmil	255	1275	0.0207	265
	3-350 kcmil	310	1550	0.0189	235
3	3-500 kcmil	380	1900	0.0159	230
	3-600 kcmil	420	2100	0.0143	230
3 1/2	3-700 kcmil	460	2300	0.0131	265
	3-800 kcmil	490	2450	0.0122	230
4	3-900 kcmil	520	2600	0.0114	235
	3-1000 kcmil	545	2725	0.0108	235
5	3-1500 kcmil	625	3125	0.0091	245
	3-1750 kcmil	650	3250	0.00895	240

 

Maximum Length of Electrical Metallic Tubing  That May Safely be Used as an Equipment Grounding Circuit Conductor based on a clearing Ground Fault Current of 500% of Overcurrent Device Rating.
Circuit is 120 volts to ground; 50 volts drop at the Arc.

EMT Size In.	Conductors AWG No. /kcmil	Overcurrent Device Rating Amps.	Fault Clearing Current 500% of OC Device Rating	DC Resistance EMT Ohms/C ft. including  Couplings	Approx. Impedance EMT Ohms per C ft.	Length of Conduit Run
1/2	3-#12	20	100	0.097	0.262	265
	4-#10	30	150	0.097	0.204	225
3/4	4#10	30	150	0.064	0.173	265
1	4-#8	45	225	0.064	0.134	230
	3-#4	85	425	0.045	0.088	185
1 1/4	3#2	115	575	0.031	0.063	190
1 1/2	3-#1	130	650	0.0253	0.049	220
	3-#2/0	175	875	0.0253	0.042	190
2	3-#3/0	200	1000	0.0199	0.034	205
	3-#4/0	230	1150	0.0199	0.033	180
2 1/2	3-250 kcmil	255	1275	0.00659	0.022	245
	3-350 kcmil	310	1550	0.00659	0.0198	225
3	3-500 kcmil	380	1900	0.00536	0.0161	225
	3-600 kcmil	420	2100	0.00536	0.0153	215
4	3-900 kcmil	520	2600	0.00362	0.0119	225
	3-1000 kcmil	545	2725	0.00362	0.0114	225