Secondary Transformer Protection
- Thread starter blackowl
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Senior Member
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- New York, 40.7514,-73.9925
Re: Secondary Transformer Protection
What size is the xfmr and what size is the primary protection? We will be refering to Table 450.3(B).
Also see 240.21(C)(1).
[ May 30, 2003, 08:56 AM: Message edited by: ron ]
What size is the xfmr and what size is the primary protection? We will be refering to Table 450.3(B).
Also see 240.21(C)(1).
[ May 30, 2003, 08:56 AM: Message edited by: ron ]
Re: Secondary Transformer Protection
The primary protection is a 1000 amp circuit breaker, transformer is 750 KVA. I believe this is 110% and secondary protection would not be needed. However, This is a delta/wye transformer. Section 240-21(c)(1)states the secondary conductors are not considered to be protected by the primary overcurrent protective device. This run is under 10' long. If I can comply with all of the requirements of 240-21(c)(2)can I consider these secondary conductors protected?
The primary protection is a 1000 amp circuit breaker, transformer is 750 KVA. I believe this is 110% and secondary protection would not be needed. However, This is a delta/wye transformer. Section 240-21(c)(1)states the secondary conductors are not considered to be protected by the primary overcurrent protective device. This run is under 10' long. If I can comply with all of the requirements of 240-21(c)(2)can I consider these secondary conductors protected?
Re: Secondary Transformer Protection
Blackowl, there are only two conditions where the transformer secondary conductors are not required to have protection.
1. A transformer with a 2-wire primary and a 2-wire secondary, and the primary is protected the in accordance with 450.3.
2. A 3 phase, delta-delta-connected transformer having a 3-wire, single voltage secondary, provided its primary is protected in accordance to 450.3.
Since you have a delta/wye transformer you need secondary protection. Refer to 240.4(F), 240.21(C), and 450.3.... Dereck
Blackowl, there are only two conditions where the transformer secondary conductors are not required to have protection.
1. A transformer with a 2-wire primary and a 2-wire secondary, and the primary is protected the in accordance with 450.3.
2. A 3 phase, delta-delta-connected transformer having a 3-wire, single voltage secondary, provided its primary is protected in accordance to 450.3.
Since you have a delta/wye transformer you need secondary protection. Refer to 240.4(F), 240.21(C), and 450.3.... Dereck
- Location
- Connecticut
- Occupation
- Engineer
Re: Secondary Transformer Protection
Bwyllie, I would have to disagree with your reading of 240.21.C.2 allowing secondary conductors without an OCPD. Instead, I think 240.21.C.2(1)(b) indicates that the secondary conductors are allowed to be protected by the OCPD at the termination of the conductors. For this reason, the ampacity of the conductors is required to be not less than the rating of the OCPD at the conductor termination. Derek was correct when he indicated that there are only two conditions that do not require transformer secondary conductor protection.
Dave
Bwyllie, I would have to disagree with your reading of 240.21.C.2 allowing secondary conductors without an OCPD. Instead, I think 240.21.C.2(1)(b) indicates that the secondary conductors are allowed to be protected by the OCPD at the termination of the conductors. For this reason, the ampacity of the conductors is required to be not less than the rating of the OCPD at the conductor termination. Derek was correct when he indicated that there are only two conditions that do not require transformer secondary conductor protection.
Dave
Re: Secondary Transformer Protection
Section 240-21 (c)TRANSFORMER SECONDARY CONDUCTORS states that conductors shall be permitted to be connected to a transformer secondary, sithout overcurrent protection at the secondary, as specified in (1) through (4).
240-21 (c)(2) states "Where the length of secondary conductor does not exceed 10 ft and complies with all of the following". These conductors can be limited to less than 10' and I believe I can satisfy the three requirements. It is my "Interpretation" that I will not need a "Main Circuit Breaker" in this switchboard and main lugs will suffice.
Section 240-21 (c)TRANSFORMER SECONDARY CONDUCTORS states that conductors shall be permitted to be connected to a transformer secondary, sithout overcurrent protection at the secondary, as specified in (1) through (4).
240-21 (c)(2) states "Where the length of secondary conductor does not exceed 10 ft and complies with all of the following". These conductors can be limited to less than 10' and I believe I can satisfy the three requirements. It is my "Interpretation" that I will not need a "Main Circuit Breaker" in this switchboard and main lugs will suffice.
Re: Secondary Transformer Protection
Puzzling uh. I tried your method some time ago and got dinged by 240.4(F). I understand the confusion of 240.21(C)(2), it conflicts with 240.4(F). The problem is the secondary-to-primary voltage ratio.
For example lets say we have a 83KVA, 480 delta input, and 208/120Y secondary. We fuse the primary at 125 amp breaker for a 100 FLA, We size the secondary conductors for 230 amps FLA. A 120 line to neutral load could draw up to 400 amps before the primary side OCPD operated, which could cause dangerous overloading of the secondary conductors.
At least that is my take on the subject.. Dereck
[ May 30, 2003, 04:31 PM: Message edited by: dereckbc ]
Puzzling uh. I tried your method some time ago and got dinged by 240.4(F). I understand the confusion of 240.21(C)(2), it conflicts with 240.4(F). The problem is the secondary-to-primary voltage ratio.
For example lets say we have a 83KVA, 480 delta input, and 208/120Y secondary. We fuse the primary at 125 amp breaker for a 100 FLA, We size the secondary conductors for 230 amps FLA. A 120 line to neutral load could draw up to 400 amps before the primary side OCPD operated, which could cause dangerous overloading of the secondary conductors.
At least that is my take on the subject.. Dereck
[ May 30, 2003, 04:31 PM: Message edited by: dereckbc ]
Re: Secondary Transformer Protection
Owl bring up an interesting point if he can comply with 240.21.C2.
C2.1.b states that the secondary conductors must be not less that the rating of the device at the termination of the secondary conductors and if he is connecting to a
400 amp panel and the bus is rated at 400 amps
and the conductors are at least equal to 400 amps,
doesn't he meet these requirements?
I would prefer to install a main breaker.
[ May 30, 2003, 05:33 PM: Message edited by: bob ]
Owl bring up an interesting point if he can comply with 240.21.C2.
C2.1.b states that the secondary conductors must be not less that the rating of the device at the termination of the secondary conductors and if he is connecting to a
400 amp panel and the bus is rated at 400 amps
and the conductors are at least equal to 400 amps,
doesn't he meet these requirements?
I would prefer to install a main breaker.
[ May 30, 2003, 05:33 PM: Message edited by: bob ]
Re: Secondary Transformer Protection
Bob maybee, but how do you get around 240.4(F)? Which states: Single phase (other than 2-wire) or multiphase (other than delta-delta 3-wire) transformer secondary conductors shall not be considered to be protected by the primary OCPD.
That takes me back to what I said were the only two conditions where a primary could protect the secondary.
Bob maybee, but how do you get around 240.4(F)? Which states: Single phase (other than 2-wire) or multiphase (other than delta-delta 3-wire) transformer secondary conductors shall not be considered to be protected by the primary OCPD.
That takes me back to what I said were the only two conditions where a primary could protect the secondary.
- Location
- Illinois
- Occupation
- retired electrician
Re: Secondary Transformer Protection
Dereck,
I don't think that 240.4(F) enters into this. These are tap conductors and I belive that as long as the ampacity of the conductors between the transformer and the panel have an ampacity at least equal to the panel rating that 240.21(C)(2) will permit this installation. The language in 240.21(C)(2) is poor, but the word "device" would apply to the panel. I'm not sure if this was really the CMP's intent, but it is what the words say. This section does not require any specific protection for these tap conductors.
Don
Dereck,
I don't think that 240.4(F) enters into this. These are tap conductors and I belive that as long as the ampacity of the conductors between the transformer and the panel have an ampacity at least equal to the panel rating that 240.21(C)(2) will permit this installation. The language in 240.21(C)(2) is poor, but the word "device" would apply to the panel. I'm not sure if this was really the CMP's intent, but it is what the words say. This section does not require any specific protection for these tap conductors.
Don
Re: Secondary Transformer Protection
Don I understand, and have gone through the arguments before. Won a few, lost a few. But after I got snake bit a few years ago by an AHJ with his interpetation, I have just always provided seconday protection as the code certainly permits this approach to be empolyed.
Don I understand, and have gone through the arguments before. Won a few, lost a few. But after I got snake bit a few years ago by an AHJ with his interpetation, I have just always provided seconday protection as the code certainly permits this approach to be empolyed.
brian john
Senior Member
- Location
- Leesburg, VA
Re: Secondary Transformer Protection
On the same issue, we have had a running disagreement with the AHJ and between ourselves in the office.
In the following installation what is the maximum allowable overcurrent protection and minimum allowable conductor size to comply with the NEC.
45 KVA with primary and secondary overcurrent protection. Copper conductors.
Connected secondary load is calculated at 100 amps.
On the same issue, we have had a running disagreement with the AHJ and between ourselves in the office.
In the following installation what is the maximum allowable overcurrent protection and minimum allowable conductor size to comply with the NEC.
45 KVA with primary and secondary overcurrent protection. Copper conductors.
Connected secondary load is calculated at 100 amps.
- Location
- Illinois
- Occupation
- retired electrician
Re: Secondary Transformer Protection
Brian,
What are the system voltages and where is the secondary OCPD located in relation to the transformer?
Don
Brian,
What are the system voltages and where is the secondary OCPD located in relation to the transformer?
Don
brian john
Senior Member
- Location
- Leesburg, VA
Re: Secondary Transformer Protection
It is the little facts left out that cause major problems, sorry
480 delta primary 208/120 secondary, the primary overcurrent protection and secondary overcurrent protection are located adjacent to the transformer,
Fused safety switch primary, main CB in a 200 amp rated bus panel secondary.
It is the little facts left out that cause major problems, sorry
480 delta primary 208/120 secondary, the primary overcurrent protection and secondary overcurrent protection are located adjacent to the transformer,
Fused safety switch primary, main CB in a 200 amp rated bus panel secondary.
Re: Secondary Transformer Protection
Primary OCPD: 125A (250% of FLA(135A))
Secondary OCPD: 150A(125% of FLA(156A))
The size of the wire depends of the type of load, linear vs non-linear load, is the neutral considered current carrying? K-Rated for 200% neutral also?
Primary OCPD: 125A (250% of FLA(135A))
Secondary OCPD: 150A(125% of FLA(156A))
The size of the wire depends of the type of load, linear vs non-linear load, is the neutral considered current carrying? K-Rated for 200% neutral also?
- Location
- Illinois
- Occupation
- retired electrician
Re: Secondary Transformer Protection
Maximum permitted primary OCPD 125 amps with #2 copper. Maximum permitted secondary OCPD is 175 amps with 2/0 copper. Based on the 100 amp load you could use a 100 amp secondary OCPD with #3. You could also use a primary OCPD of 60 amps and #6 wire based on the calculated load.
don
Maximum permitted primary OCPD 125 amps with #2 copper. Maximum permitted secondary OCPD is 175 amps with 2/0 copper. Based on the 100 amp load you could use a 100 amp secondary OCPD with #3. You could also use a primary OCPD of 60 amps and #6 wire based on the calculated load.
don
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