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Old 09-14-2011, 02:14 PM   #1
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Default Transformer Secondary Over Current Protection

Have a situation where a customer has the following installation

150 KVA 480-208/120, 3 phase Delta/wye transformer with the following

PRIMARY
200 amp circuit breaker over current protection
3/0 copper conductors
SECONDARY
4-200 amp fusible safety switches with 175 amp fuses
2/0 copper conductors

In regards to the secondary over current protection what is the maximum size fuse that can be installed in each device.

the maximum single device would be 600 amps (125% of 416 A=520)

Do we take the 520 amp and divide by 4 or am I totally off base?

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Old 09-14-2011, 08:55 PM   #2
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I had a engineer tell me yesterday they cant add up to more than if you had one overcurrent device. So they should add up to 600 amps. See 450-3b note 2

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Old 09-15-2011, 09:07 PM   #3
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Quote:
Originally Posted by brian john View Post
Have a situation where a customer has the following installation

150 KVA 480-208/120, 3 phase Delta/wye transformer with the following

PRIMARY
200 amp circuit breaker over current protection
3/0 copper conductors
SECONDARY
4-200 amp fusible safety switches with 175 amp fuses
2/0 copper conductors

In regards to the secondary over current protection what is the maximum size fuse that can be installed in each device.

the maximum single device would be 600 amps (125% of 416 A=520)

Do we take the 520 amp and divide by 4 or am I totally off base?
So you are basically doing taps off the feeder? I would say that your conductor on the secondary side for each disco needs to meet the taps rule. Also they would need to sized large enough to meet code with no ocpd on the 2nd side. Just off the top of my head I don't want to walk out to the van and grab my code book. You could fuse each disco a 600 amps with the correct size of conductors as far as the code is concerned, for example 100 main with 5-sp20 cb, 2p30 cb and 2p40 cb all together 170 amps but demand comes into play so 100 amp main will suffice.
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Old 09-16-2011, 11:14 AM   #4
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The transformer secondary (winding) is protected by the primary OCPD in this case. Assuming that each of the 2/0s originate at the transformer itself, there is no limit on how many you can connect and there is no limit on the total rating of the OCPDs that protect the secondary conductors. The protection of the secondary conductors would have to be in compliance with 240.21(C).
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Old 09-16-2011, 02:08 PM   #5
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Quote:
Originally Posted by don_resqcapt19
The transformer secondary (winding) is protected by the primary OCPD in this case. Assuming that each of the 2/0s originate at the transformer itself, there is no limit on how many you can connect and there is no limit on the total rating of the OCPDs that protect the secondary conductors. The protection of the secondary conductors would have to be in compliance with 240.21(C).
240.21(c) puts a lot of limitations on primary only protection. Since it is a delta to wye the secondary would need overcurrent protection. If it is an industrial installation it may not. Am I wrong?
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Old 09-16-2011, 03:37 PM   #6
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Existing installation
Not making any modifications
2008 NEC in effect.
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Old 09-16-2011, 09:59 PM   #7
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240.21(c) puts a lot of limitations on primary only protection. Since it is a delta to wye the secondary would need overcurrent protection. If it is an industrial installation it may not. Am I wrong?
We have two sets of rules for transformer installations. The rules in 450 cover the overcurrent protection for the transformer itself and the rules in 240 cover the protection of the conductors. The transformer in this case does not require secondary protection (the primary device is less than 125% of the rated primary current). The code permits an unlimited number secondary conductors to be connected to the secondary of this transformer as long as the conductors themselves are protected per the rules in 240.21(C).
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Old 09-17-2011, 01:08 AM   #8
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This part what I done in France we do a quick and shortcut with transfomers.

The Primary side as long you have OCPD very close to the transfomer rating { I know you guys can bump up to next size larger so we can do it also }

The secondary side ., Really basically we sized the OCPD to the conductor itself unless you snag a motour load which we have to use the NEC Art 440 {I don't recall excat number but I know it in that area }

But just heeded with the tap rules that can intertwine it a bit if not watch out on it.

Merci,
Marc
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Old 09-17-2011, 11:11 AM   #9
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Quote:
Originally Posted by don_resqcapt19
We have two sets of rules for transformer installations. The rules in 450 cover the overcurrent protection for the transformer itself and the rules in 240 cover the protection of the conductors. The transformer in this case does not require secondary protection (the primary device is less than 125% of the rated primary current). The code permits an unlimited number secondary conductors to be connected to the secondary of this transformer as long as the conductors themselves are protected per the rules in 240.21(C).
You are right, thanks for clarifying. That is the way I was taught. I was doing a power survey with an engineer and I think he was wrong.
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Old 09-17-2011, 11:22 AM   #10
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cool, I miss threads like this..
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Old 09-19-2011, 11:50 AM   #11
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Table 450.3B gives you two options, primary only or both primary and secondary protection. One way or another you are limted to 125% of the transformer rating. If you are concerned about high inrush current on the primary side then you can use both. In that case the primary protection can be up to 250%. Personally my practice is to usually protect both at 100% or the next highest standard rating if it is in between. If I need more power, I just order a larger transformer or buy another one. (I'm a specifier, not a bidder so I don't have to be as concerned about cost cutting as say many contractors who are competing. I don't like wasting money but I do have the luxury of not having to squeeze every drop of blood out of a job.)
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Old 09-19-2011, 01:11 PM   #12
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The question regards transformer taps applying the 25' Tap Rule.
The other issues have been addressed, this is an existing installation.

Last edited by brian john; 09-19-2011 at 01:13 PM.
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Old 09-19-2011, 01:27 PM   #13
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Quote:
Originally Posted by brian john View Post
The question regards transformer taps applying the 25' Tap Rule.
The other issues have been addressed, this is an existing installation.
For the 25' transformer secondary conductor rule, the minimum ampcaity of the secondary conductors on this transformer would be 154 amps.

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