cable sizing on a 300-amp breaker

[NOPete]

I have a distribution panel feeding a 150kVA 480 to 208/120 transformer. Engineering has designated a 300-amp breaker in the DP for the power supply. They have also designated a 3/C #4/0 cable for the primary from the breaker to the transformer.

My concern is that the #4/0 cable is only rated for 240 amps (@ 75C), and is landed under a 300-amp breaker.

Is there ANY NEC Code reference that allows a cable to be landed under a breaker which has a higher amp rating than the cable...???

 

[texie]

I have a distribution panel feeding a 150kVA 480 to 208/120 transformer. Engineering has designated a 300-amp breaker in the DP for the power supply. They have also designated a 3/C #4/0 cable for the primary from the breaker to the transformer.

My concern is that the #4/0 cable is only rated for 240 amps (@ 75C), and is landed under a 300-amp breaker.

Is there ANY NEC Code reference that allows a cable to be landed under a breaker which has a higher amp rating than the cable...???

No, the breaker is too large. The conductor size is good but the breaker should be a 225.
Slight correction-4/0 @ 75 degree is 230 amp.

 

[kingpb]

You did not mention what was happening on the LV side of the transformer.

I would recommend Table 450.3(B) for review. The 300A would be possible if there is secondary protection.

 

[augie47]

You did not mention what was happening on the LV side of the transformer.

I would recommend Table 450.3(B) for review. The 300A would be possible if there is secondary protection.

Respectfully disagree. Regardless of what you do on the secondary, the primary conductor must be protected by a overcurrent device selected with the conductors amapciy in mind with 240.4(B) taken into account.

 

[kwired]

You did not mention what was happening on the LV side of the transformer.

I would recommend Table 450.3(B) for review. The 300A would be possible if there is secondary protection.

Which may allow 300 amp for the transformer primary protection, the 4/0 conductors still need protected at next next standard OCPD if not an application mentioned in 240.4(G), correct. Only thing mentioned there is secondary tie conductors. That would mean next size up is 250 amps, to use 300 amp device for the transformer one would need to increase conductor size I believe.

 

[kingpb]

No, the breaker is too large. The conductor size is good but the breaker should be a 225.

My position that the breaker can be 300A stands; but this is dependent on the LV side protection in place. I specifically did not mention anything about cable being too small.

215.1(A)(b) says the feeders must be sized to meet the minimum of the load, and protected against overcurrent with ampacities as stated in 310.15. #4/0 AWG is rated for 260A. It will not be operating over 180A, which is less than the value of 230A of 75 deg c column. The load for a 240.4 (B) does apply in that the next size up breaker for 260A is 300A in accordance with Table 240.6(A).

Thus a 300A breaker with #4/0AWG cable is the correct selection* as long as the LV side has protection installed.

 

[david luchini]

My position that the breaker can be 300A stands; but this is dependent on the LV side protection in place. I specifically did not mention anything about cable being too small.

215.1(A)(b) says the feeders must be sized to meet the minimum of the load, and protected against overcurrent with ampacities as stated in 310.15. #4/0 AWG is rated for 260A. It will not be operating over 180A, which is less than the value of 230A of 75 deg c column. The load for a 240.4 (B) does apply in that the next size up breaker for 260A is 300A in accordance with Table 240.6(A).

Thus a 300A breaker with #4/0AWG cable is the correct selection* as long as the LV side has protection installed.

4/0 is rated for 230A at 75deg. It cannot be protected by the 300A c/b. 250A is the largest allowable ocpd.

 

[augie47]

4/0 is rated for 230A at 75deg. It cannot be protected by the 300A c/b. 250A is the largest allowable ocpd.

:thumbsup: (I tried )

 

[david luchini]

:thumbsup: (I tried )

It was a valiant effort.

 

[kingpb]

4/0 is rated for 230A at 75deg. It cannot be protected by the 300A c/b. 250A is the largest allowable ocpd.

The cable is rated for 260A @ 90 Deg C per 310.15 . It is derated to 230A at 75 degC to meet termination temperature requirements. As long as the cable is not going to operate at an ampacity greater than the ampacity in the 75 degC column after all derating factors are applied, then its the correct size. You can see in the curve the cable is protected.

Now for the transformer itself. The 300A thermal mag breaker trip curve is not under the thermal damage curve of transformer. This is often overlooked when selecting the HV side protection. Fuses are actually much better than a thermal mag breaker. I plotted a 250A breaker and the xfmr curve as still within the error band.

 

[kingpb]

I have a distribution panel feeding a 150kVA 480 to 208/120 transformer. Engineering has designated a 300-amp breaker in the DP for the power supply. They have also designated a 3/C #4/0 cable for the primary from the breaker to the transformer.

My concern is that the #4/0 cable is only rated for 240 amps (@ 75C), and is landed under a 300-amp breaker.

Is there ANY NEC Code reference that allows a cable to be landed under a breaker which has a higher amp rating than the cable...???

You can certainly go to engineering and ask about it, but I would imagine they are going to tell you it's ok. I don't necessarily agree with the protection of the transformer, but then I do not have the exact transformer protection curves or the model of the breaker so I can't say one way or the other.

 

[david luchini]

The cable is rated for 260A @ 90 Deg C per 310.15 . It is derated to 230A at 75 degC to meet termination temperature requirements. As long as the cable is not going to operate at an ampacity greater than the ampacity in the 75 degC column after all derating factors are applied, then its the correct size. You can see in the curve the cable is protected.

The ampacity of the conductor is 230A at 75deg C. The next standard circuit breaker size is 250A. You cannot protect the #4/0 at 300A per 240.4(B).

Using the 90deg C ampacity does not comply with 110.14(C).

 

[kwired]

The cable is rated for 260A @ 90 Deg C per 310.15 . It is derated to 230A at 75 degC to meet termination temperature requirements. As long as the cable is not going to operate at an ampacity greater than the ampacity in the 75 degC column after all derating factors are applied, then its the correct size. You can see in the curve the cable is protected.

Now for the transformer itself. The 300A thermal mag breaker trip curve is not under the thermal damage curve of transformer. This is often overlooked when selecting the HV side protection. Fuses are actually much better than a thermal mag breaker. I plotted a 250A breaker and the xfmr curve as still within the error band.

I won't disagree that it isn't likely you will overload this conductor, but you still must size the conductor per the 75C ampacity table if the terminations are 75C, 90C ampacity is only for ampacity adjustments - number of conductors in raceway, ambient temp.. nearly all the time with 600 volts or less.

 

[kingpb]

You cannot protect the #4/0 at 300A per 240.4(B). Using the 90deg C ampacity does not comply with 110.14(C).

The #4/0 AWG ampacity does not exceed the temperature rating at 75DegC which is stated in first sentence of 110.14(C). The fact that the 260A is used for derating purposes meets the intent of the second sentence of 110.14(C). I can always use the higher rated ampacity to start, then derate. As long as the derated value is less than or equal to the amapcity in the 75DegC column, I'm good. (or 60degC if its old stuff)

Certainly you're not trying to say that if I have 4 current carrying conductors in a raceway I have to derate to 80%, but I have to start with the 75degC column ampacity? Because we all know that is not true.

 

[infinity]

What does derating have to do with this? He's feeding a transformer with #4/0 with an ampacity of 230 amps @75° C, next standard size is 250 amps.

 

[kingpb]

I won't disagree that it isn't likely you will overload this conductor, but you still must size the conductor per the 75C ampacity table if the terminations are 75C, 90C ampacity is only for ampacity adjustments - number of conductors in raceway, ambient temp.. nearly all the time with 600 volts or less.

So, if you set the protection on a 150KVA transformer to 250% (450A), your going to provide cable capable of 450A on a 150KVA transformer? Can't say I ever consider a 700KCMIL on a 150KVA transformer before. :rotflmao:

 

[augie47]

The devil may be in the details as the term "cable" is being used.
In most cases I encounter the transformer would have to be wired using a 310.104 "Building wire" and most all of those have a 75° ampacity of 230 amps which would limit you to a 250 amp breaker
Agreed if he has a "cable" with a higher 75° ampacity a higher breaker could be possible but there also may well be an issue in using it for a permanent installation due to the rules in Art 400.
I would venture to say the 95% of the transformer installs most of us see would not allow a 300 amp breaker.

 

[infinity]

So, if you set the protection on a 150KVA transformer to 250% (450A), your going to provide cable capable of 450A on a 150KVA transformer? Can't say I ever consider a 700KCMIL on a 150KVA transformer before. :rotflmao:

It's a design issue but in answer to your question, yes.

 

[david luchini]

The #4/0 AWG ampacity does not exceed the temperature rating at 75DegC which is stated in first sentence of 110.14(C).

A 4/0 ampacity of 230A does not exceed the temperature rating at 75degC. A 4/0 ampacity of 260A DOES exceed the temperature rating at 75degC.

The fact that the 260A is used for derating purposes meets the intent of the second sentence of 110.14(C). I can always use the higher rated ampacity to start, then derate. As long as the derated value is less than or equal to the amapcity in the 75DegC column, I'm good. (or 60degC if its old stuff)

You can derate from 260A. You cannot protect the conductor at 260A.

Certainly you're not trying to say that if I have 4 current carrying conductors in a raceway I have to derate to 80%, but I have to start with the 75degC column ampacity? Because we all know that is not true.

No one has suggested that in this post.

 

[david luchini]

So, if you set the protection on a 150KVA transformer to 250% (450A), your going to provide cable capable of 450A on a 150KVA transformer?

Since that is what Code requires, the answer would be yes.