Another transformer secondary protection question

[mull982]

I have another transformer which is 112.5kVA 480V-208V. The secondary of the transformer has a 400MCM cable that terminates into a 400A main breaker in a panelboard? Am I correct in saying that this 400MCM cable rated for 335A is not large enough to be protected by the 400A breaker?

Is the solution here to change the main to a 350A breaker or increase the cable size to 500MCM?

 

[mayanees]

Yes, and yes!

Yes, and yes!

Mull,
I say your suggested solutions are appropriate. 240.6 dictates 350 as the next standard ocpd, so the 400 won't work for that cable, but it will work for the 380-amp 500 kCM. The xfmr is covered under 450.3(B), and depending on the primary protection, you're safe either way with those choices.
Of course the drawback to the 350-amp breaker selection is less potential for selectivity between the secondary main and it's downstream devices, and of course it will limit the transformer output capacity slightly.
John M

 

[david luchini]

I have another transformer which is 112.5kVA 480V-208V. The secondary of the transformer has a 400MCM cable that terminates into a 400A main breaker in a panelboard? Am I correct in saying that this 400MCM cable rated for 335A is not large enough to be protected by the 400A breaker?

Is the solution here to change the main to a 350A breaker or increase the cable size to 500MCM?

No, that would not be a correct solution. The correct solution would be to change the main breaker to 300A (assuming it will handle the load) or to increase the cable size to 600MCM, per 240.21(C).

 

[mayanees]

.. agreed to the "ocpd can't exceed the rating of the conductors", as opposed to round up to the next higher ocpd. It's not just overcurrent protection, but governed by 240.21c, xfmr secodnary conductors. thanks! 300 amp ocpd

 

[mull982]

No, that would not be a correct solution. The correct solution would be to change the main breaker to 300A (assuming it will handle the load) or to increase the cable size to 600MCM, per 240.21(C).

I dont follow you here.

Basically the reasons you state pretty much say that you cant use the "next size up" rule. Is that not the case? Cannot you not use the next size up rule on these secondary transformer cables?

 

[raberding]

transformer secondary ocp

transformer secondary ocp

last sentence of 240.21(C)
"...240.4(B) shall not be permitted..."

 

[don_resqcapt19]

The secondary conductors must be protected at or below their ampacity. You cannot use the rule in 240.4(B) for these conductors.

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]

How long are the secondary conductors? If you are using the 10 foot rule of 240.21(C)(2), then the ampacity of the conductors (yours are 335 amps) cannot be less than the overcurrent device to which they are connected (you are suggesting 350 amps). That would not work.

 

[topgone]

Here's my take:
Rated Secondary amps = 112,500/(1.732x208) = 312.27 amps

Cable size amp rating = 1.25 x 312.27 = 390.33 amps, therefore, a 600kCMil Cu will do(420 amps).

The "OCPD should not exceed the rating of the conductors" choosen (agreed), so anything below 400 will do.

 

[david luchini]

Here's my take:
Rated Secondary amps = 112,500/(1.732x208) = 312.27 amps

Cable size amp rating = 1.25 x 312.27 = 390.33 amps, therefore, a 600kCMil Cu will do(420 amps).

I'm not aware of any requirement for the secondary conductors to be rated at least 125% of the secondary rated current. 500MCM to a 350A MCB panel, or 350MCM to a 300A MCB panel would be OK if it can carry the load.

 

[mull982]

How long are the secondary conductors? If you are using the 10 foot rule of 240.21(C)(2), then the ampacity of the conductors (yours are 335 amps) cannot be less than the overcurrent device to which they are connected (you are suggesting 350 amps). That would not work.

Conductors are about 5ft. Are you suggesting that there is a provision elsewhere that may allow the conducotrs to be rated less than the breaker they terminate into?

 

[mull982]

I'm not aware of any requirement for the secondary conductors to be rated at least 125% of the secondary rated current. 500MCM to a 350A MCB panel, or 350MCM to a 300A MCB panel would be OK if it can carry the load.

I've typically always seen conductors sized for 125% of secondary as well. Is this the standard however you are just saying it is not necessarily required?

 

[don_resqcapt19]

Conductors are about 5ft. Are you suggesting that there is a provision elsewhere that may allow the conducotrs to be rated less than the breaker they terminate into?

There is no prevision of the NEC that will let you terminate a transformer secondary conductor on a device that has a rating higher than the ampacity of the conductor.

 

[david luchini]

I've typically always seen conductors sized for 125% of secondary as well. Is this the standard however you are just saying it is not necessarily required?

Yes, I'm saying its not required. For instance, if the load on the secondary was calculated so that you needed an ampacity of 280, you could use a 300A c/b with 350mcm secondary conductors. Nothing requires you to use conductors that are at least 125% of the rated secondary current.

 

[topgone]

We have to agree that the provisions on the code are there to guide us all in making safe installations. The code specifically says you should follow the 80% of rated capacity rule, meaning, breakers should be designed to be loaded at no more than 80% of its rating (unless specified as 100%-rated).

Here's the catch:
You will terminate your cable to a breaker. It follows that, since no breaker will be installed with a rating above the cable capacity rating, the cable rating will have to be 125% of the load current you have expected. That is because you have implemented an 80% limit on the load applicable to the breaker rating, you factor-in 125% on the cable sizing calcs (1.25 = 1/0.80). The cable rating should match the breaker capacity, or cable rating never below the breaker capacity.

Example:
FLA = 280 amps load; breaker rating = 1.25*280 = 350 amps; and your cable size = 500MCM(75 deg)
Reasons:

• Using cables sized 500 MCM (380 amps @75 deg. temp) is above the 350 amps circuit breaker rating, i.e. code compliant - that is, your breaker protects your cables before the cables get damaged.
• With an load of 350 amps (125% of expected 280 amps load) on your cable, you will be loading the cables 350/380 = 92% which is safe.
• You are not imposing a load limitation from your source; if it is a transformer, you are allowed to overload transformers for a specified period of time.

This topic has been an exam question, and you have to understand everything written in the code. If you haven't seen a code provision saying 125%, try looking for the "80% rule".

I hope this puts this question to rest. Crystal!

 

[david luchini]

We have to agree that the provisions on the code are there to guide us all in making safe installations. The code specifically says you should follow the 80% of rated capacity rule, meaning, breakers should be designed to be loaded at no more than 80% of its rating (unless specified as 100%-rated).

Here's the catch:
You will terminate your cable to a breaker. It follows that, since no breaker will be installed with a rating above the cable capacity rating, the cable rating will have to be 125% of the load current you have expected. That is because you have implemented an 80% limit on the load applicable to the breaker rating, you factor-in 125% on the cable sizing calcs (1.25 = 1/0.80). The cable rating should match the breaker capacity, or cable rating never below the breaker capacity.

This is all well and good, but the "80% rule" only applies to continuous load. If your load is 295A non-continuous, then a 300A c/b with 350mcm conductors would be fine.

 

[topgone]

This is all well and good, but the "80% rule" only applies to continuous load. If your load is 295A non-continuous, then a 300A c/b with 350mcm conductors would be fine.

Yep, I agree with that proposed design consideration! But it would be wiser to design with an "install and forget" guidance in mind. Even with load growing in time, the owner can sleep well and will be able to plan when to upgrade his electrical installations. It's just me, maybe.

 

[mull982]

We have to agree that the provisions on the code are there to guide us all in making safe installations. The code specifically says you should follow the 80% of rated capacity rule, meaning, breakers should be designed to be loaded at no more than 80% of its rating (unless specified as 100%-rated).

Here's the catch:
You will terminate your cable to a breaker. It follows that, since no breaker will be installed with a rating above the cable capacity rating, the cable rating will have to be 125% of the load current you have expected. That is because you have implemented an 80% limit on the load applicable to the breaker rating, you factor-in 125% on the cable sizing calcs (1.25 = 1/0.80). The cable rating should match the breaker capacity, or cable rating never below the breaker capacity.

Example:
FLA = 280 amps load; breaker rating = 1.25*280 = 350 amps; and your cable size = 500MCM(75 deg)
Reasons:

• Using cables sized 500 MCM (380 amps @75 deg. temp) is above the 350 amps circuit breaker rating, i.e. code compliant - that is, your breaker protects your cables before the cables get damaged.
• With an load of 350 amps (125% of expected 280 amps load) on your cable, you will be loading the cables 350/380 = 92% which is safe.
• You are not imposing a load limitation from your source; if it is a transformer, you are allowed to overload transformers for a specified period of time.

This topic has been an exam question, and you have to understand everything written in the code. If you haven't seen a code provision saying 125%, try looking for the "80% rule".

I hope this puts this question to rest. Crystal!

This is a good point you brought up regarding the 125% breaker secondary size being related to the 80% loading requirement for breakers. So esentially we are saying that the 125% on secondary breaker stems from the fact that in order to use the entire capacity of the transformer full load amps in a continuous loading application the breaker would have to be sized at 125% of this so that at full load capacity on the transformer the breaker would only be loaded to 80%. Is that right?

So we could size the secondary breaker to the same size of the transformer secondary full load, however we would only be able to use 80% of the transformers capacity.

In a hypothetical situation as David points out if all the load is non-continuous then we would have no problem sizing the secondary breaker rated at the size of the transformer secondary capacity.

 

[topgone]

This is a good point you brought up regarding the 125% breaker secondary size being related to the 80% loading requirement for breakers. So esentially we are saying that the 125% on secondary breaker stems from the fact that in order to use the entire capacity of the transformer full load amps in a continuous loading application the breaker would have to be sized at 125% of this so that at full load capacity on the transformer the breaker would only be loaded to 80%. Is that right?

I believed that what I posted is a proper interpretation of a provision of the code. Unless some other posters bring in a newer information, that's just what it is. When a breaker is placed within an enclosure, cooling airflow is restricted; this reduces the ability of the breaker to carry a current to 80% of its ampere rating. When breakers are installed in an electrical enclosure, breakers will trip when a current equal to their rating is placed upon them continuously. A 100%-rated breaker is an exception and because you can load this type of breaker at 100% of its rating, your conductors are going to be sized properly to that 100% amp level, a limitation based on the secondary rating of the transformer, not on whether the loads connected are continuous or not.

So we could size the secondary breaker to the same size of the transformer secondary full load, however we would only be able to use 80% of the transformers capacity.

That's right, the same reasons provided above.

In a hypothetical situation as David points out if all the load is non-continuous then we would have no problem sizing the secondary breaker rated at the size of the transformer secondary capacity.

That is also correct. The breaker sized according to the rated secondary amps (100% of rated trafo amps) will be compliant to the code (the code says "not more than 125% of secondary amps, 100%<125%). Try referring to NEC Art. 450-3(b)(2) and see that you are allowed to use a breaker at a maximum of 125% of transformer rated secondary amps.

 

[david luchini]

This is a good point you brought up regarding the 125% breaker secondary size being related to the 80% loading requirement for breakers. So esentially we are saying that the 125% on secondary breaker stems from the fact that in order to use the entire capacity of the transformer full load amps in a continuous loading application the breaker would have to be sized at 125% of this so that at full load capacity on the transformer the breaker would only be loaded to 80%. Is that right?

I don't believe that the 125% secondary breaker sizing requirement relates to the "80% breaker loading" requirement such that a transformer could be loaded to its "full capacity."

Consider a 75kVA, 480-208/120V xfmr with primary and secondary protection. The primary c/b could be 225A per T450.3(B), and the secondary c/b could be 300A. The 300A secondary c/b allows the transformer to be load to 115% of its capacity for a fully "continuous" load, and to 144% of its capacity for a fully "non-continuous" load (or somewhere between 115-144% for a combination of continuous and non-continuous loads.)

So we could size the secondary breaker to the same size of the transformer secondary full load, however we would only be able to use 80% of the transformers capacity.

This is not correct. You would only be able to use 80% of the transformer's capacity IF the load is completely "continuous" load.

A 100%-rated breaker is an exception and because you can load this type of breaker at 100% of its rating, your conductors are going to be sized properly to that 100% amp level, a limitation based on the secondary rating of the transformer, not on whether the loads connected are continuous or not.

I'm not sure I understand the point you are making here. If the 300A secondary breaker in the 75kVA xfmr example above was a "standard" breaker or if it was a "100% rated breaker," the conductor size would be the same. 240.21(C) requires that the secondary conductors ampacity be at least 300, so 350mcm could be used with either a standard or 100% rated secondary c/b. The only difference is you would be able to load the transformer to 144% of its capacity with fully "continuous" loads using the 100% rated breaker.