Grounding and Bonding of Separately Derived AC Systems 250.30 Grounding and Bonding of Separately Derived AC Systems Oct 11, 2006
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A separately separately derived system is a premises wiring system with no direct electrical connection to conductors originating from another system [Art. 100 definition and 250.20(D). All transformers! e"cept autotransformers! are separately derived #ecause the primary circuit conductors do not have any direct electrical connection to the secondary circuit conductors. $enerators that supply a transfer switch that opens the grounded neutral conductor would #e considered separately derived [250.20(D) %&' 1.
Grounding vs Bonding — Part of !2 Grounding vs Bonding — Part of !2"ind more from t#is series on Grounding vs Bonding Apr 1, 2005
by
Mike Holt
NEC Trainer / Consultant, Mike Holt Enterprises 921 Articles •
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O EMAIL INSHAE
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COMMENTS 0 All Code references are based on the 2005 NEC. The grounding and bonding requirements in this column apply to solidly grounded systems that operate at not more than 600! such as "20#2$0! "20#20%! and 2&&#$%0. 'f you don(t )or* )ith generators! you may thin* the requirements for separately deri+ed systems ,--s/ aren(t rele+ant to you. o)e+er! that thought may con1ict )ith the reality of your
All Code references are based on the 2005 NEC. The grounding and bonding requirements in this column apply to solidly grounded systems that operate at not more than 600! such as "20#2$0! "20#20%! and 2&&#$%0 . f you dont wor* with generators! you may thin* the re+uirements for separately derived systems (,D,s) arent relevant to you. -owever! that thought may conflict with the reality of your situation. &eople often forget that all transformers! e"cept autotransformers! are separately derived. his is #ecause the primary circuit conductors have no direct electrical connection to the secondary circuit conductors.
$%&"ig. !.$'%& A generator is (onsidered to %e a separately derived system if it #as no dire(t (onne(tion to ot#er system (ondu(tors.
&eople also fre+uently forget that not all generators are separately derived. A generator that supplies a transfer switch that opens the grounded neutral conductor is separately derived [250.20(D) %&' 1 (%ig. 1 at right). his is also the case if theres no grounded neutral conductor. /ut often generator setups dont switch the grounded neutral conductor in the transfer switch. o determine if you have an ,D,! start with the definition given in Art. 100 A premises wiring system whose power is derived from a source of electric energy or e+uipment other than a service. ,uch systems have no direct electrical connection! including a solidly grounded circuit conductor! to supply conductors originating in another system. (Fig. 2).
$%&"ig. 2.$'%& A separately derived system is defined as a )iring system )#ose po)er #as no dire(t ele(tri(al (onne(tion to t#e supply (ondu(tors originating in anot#er system.
System bonding jumper. &art 3 of this series e"plained that you cant ma*e neutral4to case (load side) #onds [250.2 (A)(5). t also discussed the e"ception it provides for ,D,s if you follow the re+uirements of 250.30(A)(1). /onding the metal parts of the ,D, to the secondary grounded neutral terminal 6 #y installing a system #onding 7umper 6 ensures +uic* removal of dangerous voltage from a secondary ground fault. 8ou can also accomplish this #y opening the overcurrent protection device of the secondary circuit [250.2(A)(3). 9verall! the system #onding 7umper •
:onnects the e+uipment grounding conductors of the ,D, (metal case) to the grounded neutral conductor (typically the ;0 terminal).
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,hould #e installed at only one location 6 either at the source or the first system disconnecting means or overcurrent device.
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,houldnt #e installed at #oth the ,D, and the secondary system disconnecting means #ecause it will allow dangerous o#7ectiona#le current to flow on conductive
metal parts of electrical e+uipment! metal piping! and structural steel in violation of 250.>(A) (Fig. 3). ,i> #ased on the area of the largest ungrounded secondary conductor [250.2=(D). ?here you run an e+uipment4#onding 7umper to the secondary system disconnecting means! you must also si> #ased on the area of the largest ungrounded secondary conductor.
$%&"ig. 3.$'%& *%+e(tiona%le (urrent flo)ing on metal parts of a system (reates a personal #a,ard.
?here you run ,D, conductors in parallel! install a secondary e+uipment #onding 7umper in each raceway (or ca#le) and once again si> #ased on the area of the largest ungrounded secondary conductor [250.102(:). Grounding electrode conductor (GEC). %or each ,D,! ground the grounded neutral terminal (typically ;0) to a suita#le grounding (earthing) electrode of a type identified in 250.30(A)(@). ,i> #ased on the total area of the largest ungrounded secondary conductor.
o prevent o#7ectiona#le current flow in metal parts of electrical e+uipment! metal piping! and structural steel! terminate the $: to the same point on the ,D, where you installed the system #onding 7umper. -owever! note the following e"ceptions •
?here the system #onding 7umper [250.30(A)(1) is a wire or #us#ar! you can terminate the $: to the e+uipment grounding terminal #ar or #us on the metal enclosure of the ,D, (Fig. 4).
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?here the ,D, originates in listed e+uipment suita#le as service e+uipment! you can use the $: from the service or feeder e+uipment to the same grounding electrode as the $: for the ,D, 6 if it meets the si
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8ou dont need to ground (earth) ,D,s rated 1*BA (1!000 BA) or less. /ut you must install a system #onding 7umper per 250.30(A)(1) to ensure the clearing of ground faults. :onnect grounding (earthing) electrode tap conductors to the $: without splicing the common $:. nstall the $: per 250.>. he #asic re+uirements mandate that the $: is
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:opper! where within 1= inches of earth [250.>(A).
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,ecurely fastened to the surface on which its carried [250.>(/).
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Ade+uately protected if e"posed to physical damage [250.>(/).
$%&"ig. .$'%& -#ere t#e system %onding +umper is a )ire or %us %ar you (an terminate t#e grounding ele(trode (ondu(tor to t#e e/uipment grounding terminal of t#e derived system.
f you run the $: in metal enclosures! ma*e them electrically continuous from the point of attachment to ca#inets (or e+uipment) to the $: [250.>(). /ond #uilding structural steel and all metal piping to an effective ground4fault current path! per 250.10(D). f you have multiple ,D,s! you can ground the grounded neutral terminal (;0) of each ,D, to a common $:. /ut the $: and grounding (earthing) electrode tap must comply with 250.30(A)()(a) through (c) ( Fig. ). hose re+uirements can #e summari
(a) Common 'EC si(e. he common $: must not #e smaller than 3C0 A?$ copper or 250 *cmil aluminum.
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(#) Tap conductor si(e. ,i> #ased on the largest separately derived ungrounded conductor of the ,D,.
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(c) Connections. a*e grounding (earthing) electrode tap connections at an accessi#le location with a listed connector or an e"othermic weld. 8ou can also use
listed connections to #us#ars not less than E inch F 2 inch. f using aluminum #us#ars! comply with 250.>(A). Grounding (eart!ing) electrode. erminate the $: to a grounding (earthing) electrode as close as possi#le to 6 and prefera#ly in the same area as 6 the system #onding 7umper. he grounding (earthing) electrode must #e the nearest •
etal water pipe electrode! as specified in 250.52(A)(1)! or
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,tructural metal electrode! as specified in 250.52(A)(2). f neither of these electrodes is availa#le! use one of the following
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A concrete4encased electrode encased in not less than 2 inches of concrete! located within (and near the #ottom of) a concrete foundation (or footing) in direct contact with earth. his foundation must contain not less than 20 feet of electrically conductive steel reinforcing #ars (or rods) not less than G inches in diameter [250.52(A)(3).
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A ground ring encircling the #uilding or structure! #uried not less than 30 inches #elow grade. t must contain not less than 20 feet of #are copper conductor not smaller than 2 A?$ [250.52(A)() and 250.53(%).
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A ground rod with not less than = feet of contact with the soil [250.52(A)(5) and 250.53($).
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9ther metal underground systems! piping systems! or underground tan*s [250.52(A)(@).
$%&"ig. 5.$'%& ou (an ground multiple SDSs to a (ommon grounding ele(trode (ondu(tor.
9#serve the %&' in 250.30(@) to ensure +uic* removal of dangerous voltage from a ground fault. /ond metal water piping and structural steel in the area served #y an ,D, to the grounded neutral conductor at the ,D,! per 250.10(D). Grounded neutral conductor. f you install the system #onding 7umper at the secondary system disconnecting means instead of at the source of the ,D,! follow these re+uirements •
Houte the grounded neutral conductor with the secondary conductors! and si> #ased on the largest ungrounded conductor for the ,D,.
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f you install the secondary conductors in parallel! si
8our new *nowledge of ,D, grounding and #onding re+uirements allows you to prevent #oth pro#lems.
