240-21 Transformer Secondary Conductors-NEC 2008

5/26/2018 240-21 Transformer Secondary Conductors-NEC 2008

Transformer Secondary Conductors

Dont let secondary rules get you wound upFind more articles aboutTransformersFeb. 1, 2008Mike Holt| Electrical Construction and Maintenance

Similar to feeder taps, covered in the last issue, transformer secondary conductors can be every bit asconfusing. Let's take a closer look at 240.21!" to help clear up any misconceptions.

Basic rules

#s $ith feeder taps, you can't use the %ne&tsi(eup)!*+ rule provided in 240.4-" for any transformersecondary conductor. ormally, you can use the ne&t highest overcurrent protective device )!*+" abovethe ampacity of the conductors being protected, but not $ith transformer secondary conductors. /he rule isthe same because the physics are the same. #s $ith feeder taps, the si(ing of that transformer secondaryconductor depends on its length and application.

Scenarios

hen $e demystified feeder taps in the previous issue, $e presented the information in an arrangement

that differs from $hat you see in the !. e also described an easy $ay to pick the correct scenario fromthe five possible. /hat same tip applies to transformer secondary conductors, $hich also have fivescenarios.

Scenario 1 Secondary conductors from a feedertapped transformer.

Scenario 2 )utside secondary conductors.

Scenario 3 Secondary conductors not over 10 feet long.

Scenario 4 Secondary conductors 10 to 23 feet long.

Scenario 5 Secondary conductors 10 to 23 feet long, industrial installation.

/o pick the right one, ans$er three easy uestions

1. 5s the transformer outside6 !hoose Scenario 2.2. 5s the secondary conductor under 10 feet long or bet$een 10 and 23 feet long6 !hoose Scenario 7 or

4, but choose Scenario 3 if it's an industrial installation.

7. 5s the transformer supplied by a feeder tap6 !hoose Scenario 1.

Scenario 1 Secondary conductors from a feeder!tapped transformer

/his one is simply a reference to 240.21-"7", so apply 240.21-"7" if you have a tapsuppliedtransformer.
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Scenario " #utside secondary conductors

Fig$ 1 %nder certain conditions& outside secondary conductors can be of unlimited lengt'& wit'out o(ercurrent protectionat t'e point t'ey recei(e t'eir supply$

)utside secondary conductors can be of unlimited length, $ithout overcurrent protection at the point theyreceive their supply, if they Fig$ 1"

#re suitably protected from physical damage in a race$ay or manner approved by the authority having

8urisdiction #9:".

/erminate at a single circuit breaker or a single set of fuses" that limit the load to the ampacity of the

conductors.

#lso

/he )!*+ for the ungrounded conductors must be an integral part of a disconnecting means or

located immediately ad8acent to it.

/he disconnecting means must be located at a readily accessible location near the point of entrance of

the conductors.

Scenario ) Secondary conductors not o(er 1* feet long

Fig$ "$ Depending on ampacity& secondary conductors can be installed up to 1* feet long& wit'out o(ercurrent protection att'e point t'ey recei(e t'eir supply$

;ou can install secondary conductors up to 10 feet long, $ithout overcurrent protection at the point theyreceive their supply, if they have an ampacity that is not less than Fig$ ""


/he calculated load per #rt. 220.

/he rating of the device supplied by the secondary conductors or the )!*+ at the termination of the

secondary conductors, and

)netenth the rating of the )!*+ protecting the primary of the transformer" multiplied by the primary

tosecondary transformer voltage ratio.

#lso

Secondary conductors must not e&tend beyond the s$itchboard, panelboard, disconnecting means, orcontrol devices they supply.

Secondary conductors must be enclosed in a race$ay.

)vercurrent protection is not reuired on the secondary side of the transformer to protect the secondaryconductors, but overcurrent protection is reuired for branchcircuit panelboards. /his )!*+ must be onthe secondary side of the transformer, and typically it's $ithin the panelboard. /he


/he sum of the ratings of the secondary )!*+s doesn't e&ceed the ampacity of the secondary

conductors.

/he secondary )!*+s are grouped.

/he secondary conductors are protected from physical damage by being enclosed in a manner

approved by the #9: such as $ithin a race$ay".

Test yourself

Fig$ +$ T'e primary o(ercurrent de(ice can protect t'e secondary conductors of a "!wire system if t'e primary de(ice doesnot e/ceed t'e (alue determined by multiplying t'e secondary conductor ampacity by t'e secondary!to!primary (oltageratio$

o$, let's see if you can put all this together to solve a practice problem. Auestion hat is the minimumsi(e 13foot secondary conductor reuired for a 2$ire, 4=0B to 120B indoor transformer rated 1.3kB# inan office installation Fig$ +"6

-ased on those t$o easy uestions, you select Scenario 4. /hen, you si(e the secondary to comply $ithScenario 4 reuirements.

/o calculate the primary )!*+ si(e, first $e need to calculate the primary current. /o do that, $e need tokno$ a couple of things

B# C 1,300B#

C 4=0B

o$ $e can calculate primary current

*rimary current C B# D C 1,300B# D 4=0B C 7.17#

o$ $e can calculate primary protection. *er 430.7-", it has to be 1>EF of the primary current. So, 7.17#G 1.>E C 3.22#. e can't apply the ne&t si(e up rule, so $e need a 3# )!*+.

/o calculate the secondary conductor si(e

+etermine the primary to secondary voltage ratio 4=0B D 120B C 4.

Hultiply the primary )!*+ si(e by 1I7 3 G 1I7 C 1.>E.

Hultiply the t$o values together 4 G 1.>E C >.>E#.

Look in the >0J!, /able 710.1> for the smallest conductor $ith an ampacity larger than >.>E#.

#ns$er 14#K.

5f you $ant more practice, try changing the transformer in the e&ample above, and try different scenarios.


si(e secondary conductors, stop to ans$er a couple of easy uestions. /hen, select the correct scenariofor your installation.

ith some transformer secondary conductors, you can use the primary )!*+ to provide the overcurrentprotection 8ust as you use the feeder )!*+ to protect the feeder tap conductors". -ut, there's a catch ;oucan do this only for t$o specific transformer configurations. /hose are

Singlephase transformer $ith 2$ire single voltage" secondary.

7phase deltadelta transformer $ith 7$ire single voltage" secondary.

#nd

/he )!*+s for those transformers must be si(ed per #rt. 430.

/he secondary conductors must meet the si(ing minimum reuired by the installation type described in

240.21!".

Documents

240-21 Transformer Secondary Conductors-NEC 2008