240.21(c)(6)
- Thread starter bwilson
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: 240.21(c)(6)
This is how I read it: The ratio of secondary voltage to primary voltage is 208/480, or 0.433. Multiply this by the ampacity of a 4/0 copper (230 amps), and you get 99.7 amps. This is way more than one third the rating of the 100 amp primary OCPD. Also, the secondary OCPD rating of 225 amps will protect a 4/0 secondary conductor. You do not need to reduce the secondary OCPD.
This is how I read it: The ratio of secondary voltage to primary voltage is 208/480, or 0.433. Multiply this by the ampacity of a 4/0 copper (230 amps), and you get 99.7 amps. This is way more than one third the rating of the 100 amp primary OCPD. Also, the secondary OCPD rating of 225 amps will protect a 4/0 secondary conductor. You do not need to reduce the secondary OCPD.
- Location
- Wisconsin
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- PE (Retired) - Power Systems
Re: 240.21(c)(6)
When the transforner is a wye secondary the rating of the primary OCPD has no relation to and provides no protection to the secondary conductors. The secondary protection is sized to protect the secondary conductors at their begining or to protect the load at their end (per tap rules).
If secondary protection exists and is not larger than 125% of the FLA then the primary OCPD can be sized larger than 125%.
In this case the secondary conductors are correctly protected by the 225A breaker (regardless of length) which is less than 125% of the secondary FLA (208A), the primary breaker can even be larger than the 100A breaker. The fact that both breakers are sized smaller than the maxium allowed is a design issue.
When the transforner is a wye secondary the rating of the primary OCPD has no relation to and provides no protection to the secondary conductors. The secondary protection is sized to protect the secondary conductors at their begining or to protect the load at their end (per tap rules).
If secondary protection exists and is not larger than 125% of the FLA then the primary OCPD can be sized larger than 125%.
In this case the secondary conductors are correctly protected by the 225A breaker (regardless of length) which is less than 125% of the secondary FLA (208A), the primary breaker can even be larger than the 100A breaker. The fact that both breakers are sized smaller than the maxium allowed is a design issue.
charlie
Senior Member
- Location
- Indianapolis
Re: 240.21(c)(6)
The secondary of a transformer may be protected by the primary overcurrent protection only if the primary protection is sized for the secondary and if the configuration is a 3? wye-wye or a 1? single winding on the secondary side. In most cases, this is not true and protection of the secondary conductors must be provided.
The secondary of a transformer may be protected by the primary overcurrent protection only if the primary protection is sized for the secondary and if the configuration is a 3? wye-wye or a 1? single winding on the secondary side. In most cases, this is not true and protection of the secondary conductors must be provided.
Re: 240.21(c)(6)
They are the same formula, but rearranged?
?02
(Sec. Conductor Amps)(Vsec/Vpri) >= (1/3)(Pri OCP)
?05
(Sec. Conductor Amps) >=(Vpri/Vsec)(1/3)(Pri OCP)
OK now I am confused.
Why do we need 240.21C2, if we have 240.21C6?
Looks like C6 is actually MORE lenient. What have I been missing all these years?
They are the same formula, but rearranged?
?02
(Sec. Conductor Amps)(Vsec/Vpri) >= (1/3)(Pri OCP)
?05
(Sec. Conductor Amps) >=(Vpri/Vsec)(1/3)(Pri OCP)
OK now I am confused.
Why do we need 240.21C2, if we have 240.21C6?
Looks like C6 is actually MORE lenient. What have I been missing all these years?
Re: 240.21(c)(6)
75kVA transformer, 480-208/120, 4/0cu secondary
Per example, OCPpri =100A
2002 we get 99.67>=33.3
2005 we get 230>=76.9
Lets size the primary at 250%, OCPpri =225A
2002 we get 99.67>=75
2005 we get 230>=173
If we use OCPpri=299A
2002 we get 99.67>=99.67
2005 we get 230>=230
Pretty rare day we have to limit ourselves to 10'?
I have to be missing something
75kVA transformer, 480-208/120, 4/0cu secondary
Per example, OCPpri =100A
2002 we get 99.67>=33.3
2005 we get 230>=76.9
Lets size the primary at 250%, OCPpri =225A
2002 we get 99.67>=75
2005 we get 230>=173
If we use OCPpri=299A
2002 we get 99.67>=99.67
2005 we get 230>=230
Pretty rare day we have to limit ourselves to 10'?
I have to be missing something
Re: 240.21(c)(6)
When ever someone has called me with regard to a 75kva,75kva, 480-208y/120 transformer and has a problem nuisance tripping the first thing I ask is why the 100a breaker? The answer usually is that it is the largest breaker that can be installed in the panel which is a 1" per pole panel. It was never a consideration that there would be a need for a breaker greater than 100a, that 100a was enough.
I've had more application issues because of that 100a breaker limitation.
Without the 25' tap rule you are allowed to go to a 125a rated breaker which they can't even do unless their panel allows the installations of a 150a frame breaker which in most cases they can not.
20/20 hind sight would now be that the panel should have been ordered with a subfeed breaker dedicated to feed the transformer or a panel with provisions for up to a 250a frame for a 225a rated breaker while applying that 25' tap rule.
Incidentally, regarding the 25', am I to understand that the 25' can be a measurement taken from the pri. OCPD to the sec. OCPD and not be limited to the distance to the transformer?
What is you interpretation?
When ever someone has called me with regard to a 75kva,75kva, 480-208y/120 transformer and has a problem nuisance tripping the first thing I ask is why the 100a breaker? The answer usually is that it is the largest breaker that can be installed in the panel which is a 1" per pole panel. It was never a consideration that there would be a need for a breaker greater than 100a, that 100a was enough.
I've had more application issues because of that 100a breaker limitation.
Without the 25' tap rule you are allowed to go to a 125a rated breaker which they can't even do unless their panel allows the installations of a 150a frame breaker which in most cases they can not.
20/20 hind sight would now be that the panel should have been ordered with a subfeed breaker dedicated to feed the transformer or a panel with provisions for up to a 250a frame for a 225a rated breaker while applying that 25' tap rule.
Incidentally, regarding the 25', am I to understand that the 25' can be a measurement taken from the pri. OCPD to the sec. OCPD and not be limited to the distance to the transformer?
What is you interpretation?
Re: 240.21(c)(6)
The lengths in this article are applied to conductor length physical distance is something else. 240 applies requirements for the protection of conductors.
The formula for secondary conductors is pri ocpd X .333 x (pri over sec).
In the case of a 100 Pri OCPD for a 480 step down to 208, the conductor size should be 33A X 2.3 = 75.9A.
The 2002 language yielded inconsistent conductor sizes or so the party line has stated
Charlie
The lengths in this article are applied to conductor length physical distance is something else. 240 applies requirements for the protection of conductors.
The formula for secondary conductors is pri ocpd X .333 x (pri over sec).
In the case of a 100 Pri OCPD for a 480 step down to 208, the conductor size should be 33A X 2.3 = 75.9A.
The 2002 language yielded inconsistent conductor sizes or so the party line has stated
Charlie
- Location
- Illinois
- Occupation
- retired electrician
Re: 240.21(c)(6)
Why would you use the 10' rule? Because there is no minimum size conductor required in the 10' rule.(in the 2002 code, I think there is a 10% rule in the 05) The 25' tap rule has a minimum conductor ampacity of 1/3 of the primary OCPD times the ratio of the secondary to primary voltage. The 25' tap rule also requires a single OCPD at the load end of the tap conductors to protect the conductors at their ampacity. The 10' rule only requires termination at a "device" that has a rating equal or less than that of the tap conductors.
Don
Why would you use the 10' rule? Because there is no minimum size conductor required in the 10' rule.(in the 2002 code, I think there is a 10% rule in the 05) The 25' tap rule has a minimum conductor ampacity of 1/3 of the primary OCPD times the ratio of the secondary to primary voltage. The 25' tap rule also requires a single OCPD at the load end of the tap conductors to protect the conductors at their ampacity. The 10' rule only requires termination at a "device" that has a rating equal or less than that of the tap conductors.
Don
Re: 240.21(c)(6)
Charlie, My concern with regard to the size of the Pri. OCPD is that the inrush of the transformer is overlooked which should be in the neighborhood of 10x the FLA for 150degC unit. That value can be anywhere from 20% higher with 115degC transformers to 60% with 80degC and K13 transformers.
When the common magnetic calibration of a breaker is usually 10x its rating +-20% nuisance tripping can become a big issue when you are backed into a corner where 100a is the maximum you can install.
It is nice to get lucky and not have a problem but then the complaints start with "I think I have a defective transformer because it trips the breakers when I energize it."
Charlie, My concern with regard to the size of the Pri. OCPD is that the inrush of the transformer is overlooked which should be in the neighborhood of 10x the FLA for 150degC unit. That value can be anywhere from 20% higher with 115degC transformers to 60% with 80degC and K13 transformers.
When the common magnetic calibration of a breaker is usually 10x its rating +-20% nuisance tripping can become a big issue when you are backed into a corner where 100a is the maximum you can install.
It is nice to get lucky and not have a problem but then the complaints start with "I think I have a defective transformer because it trips the breakers when I energize it."
Re: 240.21(c)(6)
I need to qualify where I'm coming from.
The original post (to me) was addressing 240.21(C) (6) which is permissive language for secondary tap conductors from taransformers which essentially is limited to conductor lengths not greater than 25'.
240.21(C)(3) allows the secondary conductors to be up to 25' in length if the transformer is installed in a industrial location. The 2002 NEC adopted language similar to transformer feeder taps (240.21 (B)(3)) allowing 25' lengths of secondary conductors 24.21 (C)(6), in locations other than industrial thus the calculation thats been discussed. The 2005 Code has modified the manner that these secondary conductors minimum ampacity is determined.
The primary overcurrent device for transformer is just that "for the transformer" and it is established under the application of Article 450.
I have no doubt that initial excitation current will cause nusince tripping as well as other issues encounter in field installations.
The code is a minimum standard.
I need to qualify where I'm coming from.
The original post (to me) was addressing 240.21(C) (6) which is permissive language for secondary tap conductors from taransformers which essentially is limited to conductor lengths not greater than 25'.
240.21(C)(3) allows the secondary conductors to be up to 25' in length if the transformer is installed in a industrial location. The 2002 NEC adopted language similar to transformer feeder taps (240.21 (B)(3)) allowing 25' lengths of secondary conductors 24.21 (C)(6), in locations other than industrial thus the calculation thats been discussed. The 2005 Code has modified the manner that these secondary conductors minimum ampacity is determined.
The primary overcurrent device for transformer is just that "for the transformer" and it is established under the application of Article 450.
I have no doubt that initial excitation current will cause nusince tripping as well as other issues encounter in field installations.
The code is a minimum standard.
Re: 240.21(c)(6)
Don is absolutely right in pointing out the two main differences in C2 and C6.
I guess my point is, in a "standard" hookup of a transformer where:
1. you would be terminating to a panel with a main, or a fusible disconnect anyway, and
2. the formula in question is so "liberal" that it would not come into play in a "standard" arrangement.
By standard I mean OCP's and conductors sized at 125% FLA's.
[ February 01, 2005, 04:57 PM: Message edited by: engy ]
Don is absolutely right in pointing out the two main differences in C2 and C6.
I guess my point is, in a "standard" hookup of a transformer where:
1. you would be terminating to a panel with a main, or a fusible disconnect anyway, and
2. the formula in question is so "liberal" that it would not come into play in a "standard" arrangement.
By standard I mean OCP's and conductors sized at 125% FLA's.
[ February 01, 2005, 04:57 PM: Message edited by: engy ]
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