Transformer Breaker Question

[staticcontrol]

If I have a 75kva transformer 480v primary 208v secondary. This is 90 amps primary and 208 amps secondary. Can I use a 100 amp breaker to protect the primary and 200 amp to protect the secondary? Code states not to exceed 125% of 9 amps or more.

Do I interpret this as "long as I don't exceed 125%"?

Or do I have to use 125amp primary and 300 amp secondary.

I'm eager to get this resolved. Thank you.

static

 

[charlie b]

Your primary current is 90 amps. That is more than 9. So you use the leftmost column under "primary." You are using primary and secondary protection. So the limit is 250% of the rated primary current. 2.5 times 90 is 225. That is a standard size rating, so the max you can use is 225 on the primary.

Your secondary current is 208 amps. That is more than 9. So you use the leftmost column under "secondary." The limit is 125% of the rated secondary current. 1.25 times 208 is 260. That is not a standard size rating, and the next higher is 300. So the max you can use is 300 on the secondary.

That said, these are maximums, and there are no minimums. I would probably use 150 primary and 300 secondary.

 

[staticcontrol]

Thanks Charlie.

This is what I got with the exception of 125amp on primary. If you don't mind another question. A customer is requesting 200amps. (he's no electrician of course) I assume that the only direction is what we have discussed? Any such thing as an install of a 200 amp main on secondary powered from transformer in your opinion?

 

[Npstewart]

I typically always use a 250A breaker on the secondary side of a 75KVA Xfrmr @ 208-3.

 

[david luchini]

Any such thing as an install of a 200 amp main on secondary powered from transformer in your opinion?

I typically use 225A on a 75kVA transformer secondary. But I don't see anything wrong using a 200A.

 

[Pierre C Belarge]

there is no code restriction to installing a smaller overcurrent device.

 

[chris kennedy]

Your secondary current is 208 amps. That is more than 9. So you use the leftmost column under "secondary." The limit is 125% of the rated secondary current. 1.25 times 208 is 260. That is not a standard size rating, and the next higher is 300. So the max you can use is 300 on the secondary.

You can apply 240.4(B) to secondaries?

240.21(C) Transformer Secondary Conductors.
A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

 

[charlie b]

Forgot about that. Thanks, Chris. You are right, you have to go down from 260, not up. The max would then be 250.

 

[david luchini]

Your secondary current is 208 amps. That is more than 9. So you use the leftmost column under "secondary." The limit is 125% of the rated secondary current. 1.25 times 208 is 260. That is not a standard size rating, and the next higher is 300. So the max you can use is 300 on the secondary.

You can apply 240.4(B) to secondaries?

Forgot about that. Thanks, Chris. You are right, you have to go down from 260, not up. The max would then be 250.

I think Charlie was right in the first place, though I don't have the code in front of me. The question was about protecting the transformer, not the secondary conductors. From memory, Table 450.3 allows the "next standard higher size" on the 125% secondary protection, with the primary protection not exceeding 250%, so a 300A c/b - the next standard size from 260A, would be allowed to protect the transformer secondary.

The protection of the secondary conductors is not allowed to use 240.4(B). So if a 300A c/b is used on the transformer secondary, the conductors must have an ampacity of least 300Amps (350MCM or larger.)

 

[kingpb]

As soon as I can figure out why I can't attach a document, I will show a TCC that shows why the NEC guidelines on Transformer protection as misleading and does not protect the transformer.

In this case, unless the HV side (480V) has an adjustable magnetic trip capability, the inrush when the transformer is energized will trip the breaker. Needs to be a 10X. Also, the LV side (208V) at 250A rating protection curve will race to trip between the LV and HV sides. This in itself is not a problem, except they are both above the transformer thermal damage curve. So except for protecting the cable during a fault, the "protective devices" are not doing to much protecting of the transformer.

 

[skeshesh]

As soon as I can figure out why I can't attach a document, I will show a TCC that shows why the NEC guidelines on Transformer protection as misleading and does not protect the transformer.

In this case, unless the HV side (480V) has an adjustable magnetic trip capability, the inrush when the transformer is energized will trip the breaker. Needs to be a 10X. Also, the LV side (208V) at 250A rating protection curve will race to trip between the LV and HV sides. This in itself is not a problem, except they are both above the transformer thermal damage curve. So except for protecting the cable during a fault, the "protective devices" are not doing to much protecting of the transformer.

So true. It's very annoying when OCPD selection has been done (and in some cases already purchased) early in the process, and then at the very last stages while a few weeks from construction a coordination study is needed and they expect everything to be fully coordinated.

 

[zip1]

I think Charlie was right in the first place, though I don't have the code in front of me. The question was about protecting the transformer, not the secondary conductors. From memory, Table 450.3 allows the "next standard higher size" on the 125% secondary protection, with the primary protection not exceeding 250%, so a 300A c/b - the next standard size from 260A, would be allowed to protect the transformer secondary.

The protection of the secondary conductors is not allowed to use 240.4(B). So if a 300A c/b is used on the transformer secondary, the conductors must have an ampacity of least 300Amps (350MCM or larger.)

I'm not sure I understand this point. 240.4(B) can not be applied to the transformer secondary. Wouldn't this limit the breaker to 250 Amp as charlie b noted?

 

[steve66]

I'm not sure I understand this point. 240.4(B) can not be applied to the transformer secondary. Wouldn't this limit the breaker to 250 Amp as charlie b noted?

I think David's point was "240.4(B) can NOT be applied to the conductors on the transformer secondary."

450 rules are for the transformer, 240 rules are for the conductors. So completely ignoring the transformer itself, if you have a 300 amp breaker on the transformer secondary, the conductors between the secondary and that breaker must be rated for at least 300 amps. 500KCM wire going to a 400A breaker would not be allowed.
Steve

 

[david luchini]

I'm not sure I understand this point. 240.4(B) can not be applied to the transformer secondary. Wouldn't this limit the breaker to 250 Amp as charlie b noted?

As Steve noted, section 240 rules are for conductors and 450 rules are for transformers. They are two separate issues, but both must be considered when feeding a transformer.

Section 450 allows protection of the 75kVA transformer, by either primary protection alone, or primary and secondary protection. For primary alone, the protection cannot be more than 125% of the primary rated current, but is allowed to go up to the next standard size. In this case, the primary current is 90.2A, 125% of that is 112.7, the next standard size is 125A. So with a breaker not exceeding 125A on the primary, there is no limit how large the secondary breaker can be. It can be 400A, 500A, whatever.

For primary and secondary protection, the primary breaker can be not more than 250% of the primary rated current, which is 225.5A. 225A would be the largest allowed breaker, as you cannot go up to the next standard size on the primary in the primary-secondary protection case. The secondary breaker protection cannot be more than 125% of the secondary rated current, but is allowed to go up to the next standard size. So the rated secondary current is 208.2A x 1.25 = 260.2A, which can go up to 300A.

Regardless of whether you use primary only or primary-secondary transformer protection, the secondary conductors must be protected per 240.21(C). If the breaker is 250A, the conductors must be at least 250mcm, if the breaker is 300A, the conductors must be at least 350mcm.

If you used a 400A secondary breaker, on the primary only protection, the secondary conductors must be at least 600mcm, or with a 500A secondary breaker the conductors must be at least 900mcm, or more realistically, two sets of 250mcm.

Oddly enough, if the 75kVA transformer in the example was protected by a 225A primary breaker and a 300A secondary breaker in a breaker enclosure, with secondary conductors from the transformer to breaker, and another feeder from the breaker to a panel 30' away, the secondary conductors must be 350mcm, but the conductors from the breaker to panel can be 300mcm in accordance with 240.4(B).

 

[ramsy]

As soon as I can figure out why I can't attach a document

Links to other URL's work best for me.

 

[kingpb]

ok, I think I got it, thanks to ramsy pointing me in the right direction.

Someone please post if this works; I'm a newb at the picture url link stuff:

http://i815.photobucket.com/albums/zz74/kingpbk/75KVA%20Xfmr/75kva.jpg

 

[mull982]

When sizing feeders for a transformer do you size the feeders off of the rated primary current alone, or must you add 125% to this primary current to size the feeders?

If the 125% is not required by code, is it a good practice?

 

[david luchini]

When sizing feeders for a transformer do you size the feeders off of the rated primary current alone, or must you add 125% to this primary current to size the feeders?

If the 125% is not required by code, is it a good practice?

You don't feed the transformer based on its rated primary current, you feed it based on the primary circuit breaker size that you have selected by 450.3.

If you used the 125% primary only protection, and used a 125A primary c/b (on the 75kVA transformer) you could feed it with #2AWG (assuming no derating or voltage drop, etc.) If you used the 250% primary with secondary protection and used a 250A primary c/b, then you could feed the transformer with #4/0AWG.

 

[david luchini]

In this case, unless the HV side (480V) has an adjustable magnetic trip capability, the inrush when the transformer is energized will trip the breaker. Needs to be a 10X. Also, the LV side (208V) at 250A rating protection curve will race to trip between the LV and HV sides. This in itself is not a problem, except they are both above the transformer thermal damage curve. So except for protecting the cable during a fault, the "protective devices" are not doing to much protecting of the transformer.

Looking at your jpeg, I think you're generalizing a little bit. You don't need to have an adjustable magnetic trip capability on the primary c/b, though it might be required in the example you show. The coordination depends on the specific transfomer and circuit breaker that you are using.

I agree that the NEC rules for transformers don't necessarily protect the transformer, but, if you do proper coordination, you probably could find a transformer/breaker combination that would protect the transformer from through-faults.

 

[mull982]

You don't feed the transformer based on its rated primary current, you feed it based on the primary circuit breaker size that you have selected by 450.3.

If you used the 125% primary only protection, and used a 125A primary c/b (on the 75kVA transformer) you could feed it with #2AWG (assuming no derating or voltage drop, etc.) If you used the 250% primary with secondary protection and used a 250A primary c/b, then you could feed the transformer with #4/0AWG.

I believe this would only be the case if the transformer primary OCPD was the same the feeder OCPD (primary breaker located remotely from transmormer)

But what if the primary OCPD was located right at the transformer. Then techincally this could not protect the feeders because this would fall under the tap rule. So then you would have to protect the feeder cables upstream where they origanate. This was the breaker sizing that I was referring to. Since this breaker would simply be protecting the cables. Although I would imagine it should be at least the same size if not bigger than the transformer primary OCPD.