cable sizing on a 300-amp breaker

[kingpb]

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

Please provide exact code references that support your position.

 

[david luchini]

Please provide exact code references that support your position.

240.4 Protection of Conductors

Conductors...shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G.)

 

[Julius Right]

It depends on what is the cable run. If the cable run in free air the ampacity will be 360 A [in a cable tray single conductor cable one diameter clearance] or in triangle keeping 2.15 diameters clearance 287A. Art. 392.80 Ampacity of Conductors (2) Single-Conductor Cables.(c) and (d)

 

[infinity]

It depends on what is the cable run. If the cable run in free air the ampacity will be 360 A [in a cable tray single conductor cable one diameter clearance] or in triangle keeping 2.15 diameters clearance 287A. Art. 392.80 Ampacity of Conductors (2) Single-Conductor Cables.(c) and (d)

Is that usable with 75° C terminals on each end?

 

[david luchini]

Is that usable with 75° C terminals on each end?

That's the 75deg ampacity.

 

[NOPete]

The 300 amp breaker feeds the 150kVA transformer. The secondary from the transformer feeds a lighting panel with a 600 amp main breaker.

No other protection is provided -- no adjustable trip settings, etc...

 

[electrofelon]

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.

And that is the big negative in providing primary protection at 250%: Many seem to jump on the idea of setting the primary as high as possible to minimize the likelihood of tripping on energizing, but it can be a huge cost adder when you have to get a 400 or even 600 A disco and that much ampacity. (not saying it isn't prudent sometimes, just worth taking a moment to think about).

 

[topgone]

And that is the big negative in providing primary protection at 250%: Many seem to jump on the idea of setting the primary as high as possible to minimize the likelihood of tripping on energizing, but it can be a huge cost adder when you have to get a 400 or even 600 A disco and that much ampacity. (not saying it isn't prudent sometimes, just worth taking a moment to think about).

The thing is the selection of 250% is only done to account for the inrush, and that's not going to last a longtime! If you intend to protect the primary cable from overcurrents, that should be covered by the secondary protection--> 125% of FLA. IMHO, the primary breaker is only protecting the transformer from short circuit faults. That said, you don't size the primary cable up to 250%, just the respective primary current at full-load.

 

[iceworm]

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:

That's about the same reaction I had 15 years ago when one of the kids pointed out the error of my ways.

And one of the reasons I tend to size primary protection at 125% - unless it is something screwie like a non-ventilated. Yep got caught by one of them once.

My response was, "Thanks kid. Glad I learned that before I got any older."

Following other posters (credit to augie - he was the first. The rest of you just stole it and didn't give credit):
480V, 150KVA, FLA = 180A. X 1.25 = 226A

225A CB and 3/0CU or
250A CB and 4/0CU

And if you had to have 250%, then
450A CB and parallel 3/0CU
The electricians I work with would rather pull 500s or smaller

Just sayin :roll:

The Worm

 

[texie]

The 300 amp breaker feeds the 150kVA transformer. The secondary from the transformer feeds a lighting panel with a 600 amp main breaker.

No other protection is provided -- no adjustable trip settings, etc...

I don't recall that you mentioned the secondary conductor size. Keep in mind that with a 600 amp OCPD on the secondary you are using the next size up rule as stated in Note 1 of Table 4503(B) and thus you will require secondary conductor ampacity of at least 600 amp as transformer secondary conductors do not qualify for the next size up rule.
The 4/0 primary conductors are fine but as stated the 300 amp OCPD needs to be 225 or as others have stated a 250 would also be allowed and may be preferable with modern high inrush transformers we have today.

 

[infinity]

That's about the same reaction I had 15 years ago when one of the kids pointed out the error of my ways.

And one of the reasons I tend to size primary protection at 125% - unless it is something screwie like a non-ventilated. Yep got caught by one of them once.

My response was, "Thanks kid. Glad I learned that before I got any older."

Following other posters (credit to augie - he was the first. The rest of you just stole it and didn't give credit):
480V, 150KVA, FLA = 180A. X 1.25 = 226A

225A CB and 3/0CU or
250A CB and 4/0CU

And if you had to have 250%, then
450A CB and parallel 3/0CU
The electricians I work with would rather pull 500s or smaller

Just sayin :roll:

The Worm

I think that you meant #4/0.

 

[electrofelon]

The thing is the selection of 250% is only done to account for the inrush, and that's not going to last a longtime! If you intend to protect the primary cable from overcurrents, that should be covered by the secondary protection--> 125% of FLA. IMHO, the primary breaker is only protecting the transformer from short circuit faults. That said, you don't size the primary cable up to 250%, just the respective primary current at full-load.

But protecting the primary conductors with the secondary OCPD is not an acceptable method - if that's what you are saying.

 

[kwired]

240.4 Protection of Conductors

Conductors...shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G.)

I tried to point out earlier that only secondary ties are mentioned in (G), and that there is nothing outside the general rules for the primary conductors.

With motors you can have larger branch circuit device than conductor ampacity, but there also is motor overload protection requirements which also will protect conductors from overloading. You don't have that with a transformer.

 

[mayanees]

I tried to point out earlier that only secondary ties are mentioned in (G), and that there is nothing outside the general rules for the primary conductors.

With motors you can have larger branch circuit device than conductor ampacity, but there also is motor overload protection requirements which also will protect conductors from overloading. You don't have that with a transformer.

Actually you do have overload protection for the primary conductors, which is the secondary ocpd. Much the same way that overloads protect motor conductors at ampacity.

But it's still clear to me that I have to protect the primary conductors of a transformer at ampacity, unless the voltage exceeds 1000 Volts, where it's then done all the time.

 

[iceworm]

I think that you meant #4/0.

And I think you are right.

Dumb keyboard:ashamed:

 

[kwired]

Actually you do have overload protection for the primary conductors, which is the secondary ocpd. Much the same way that overloads protect motor conductors at ampacity.

But it's still clear to me that I have to protect the primary conductors of a transformer at ampacity, unless the voltage exceeds 1000 Volts, where it's then done all the time.

I did think about that, but there is no wording specifically allowing the primary conductor to have higher than the overcurrent protection required by the general rule of 240.4.

 

[kingpb]

240.4 Protection of Conductors

Conductors...shall be protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or required in 240.4(A) through (G.)

And the #4/0 is protected accordingly. The transformer max current is 180A, the cable is rated for 230A at 75 deg C. The cable meets the requirements of serving the transformer.

The question is on protecting the cable. So the issue is protection selection.

The cable, without damage, is capable of continuously carrying 260A; according to 310.15. The same 310.15 as you state is required by 240.4. The exceptions of (A) thru (G) have no bearing and 240.4 ONLY says according to ampacities of 310.15, not according to a specific temperature column(s).

The only way the cable will ever see more current than 180A is during overload (non-normal) or fault (non-normal). The only condition for overload is on the load side of the secondary protection and to overload the transformer would also cause the secondary breaker to trip, thus removing the load and consequently "protecting the transformer" although protecting the transformer is a secondary result. The cable would not be damaged, nor would the terminals be damaged because the cable itself, even at 25% overload would only be 225A, which is below the 75degC value of 230A. So, a non-issue.

So, it boils down to protection of the cable under a fault condition. The 300A breaker (or 250A for that matter) is only going to trip on on a faulted condition between the terminals of the load side of breaker and looking through the transformer to the LV line side of the breaker. The fault current that will be seen will be such that, the breaker will trip before it ever reaches an ampacity, according to cable damage curves, that will damage the cable.

So the argument that you can't use the #4/0AWG on a 300A breaker because it won't be protected according to the NEC; is not a valid argument without specific references, and although it is not a popular interpretation does not make it wrong.

Now, selection of the 300A breaker to protect the transformer, without looking at the actual transformer damage curves is an issue, but not part of this topic.

 

[augie47]

I think the problem you may encounter is most inspectors are going to look at things in a simplistic form. 110.14(C) tells us "Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment
termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7)."
Your circuit breaker will likely be 75° termination so, as an inspector, I'm going to looking at the 75° rating of 4/0 of 230 amps and the fact that 240.6 shows a 250 amp breaker to be the next standard size and, as David, has pointed out 240.4 tells us to protect the conductor at its ampacity (next size up rule taken into account)

In my world, I would listen intently to your explanation, look at your 4/0 and the 300 amp breaker and issue a rejection.

Good luck !

 

[david luchini]

And the #4/0 is protected accordingly. The transformer max current is 180A, the cable is rated for 230A at 75 deg C. The cable meets the requirements of serving the transformer.

The question is on protecting the cable. So the issue is protection selection.

The cable, without damage, is capable of continuously carrying 260A; according to 310.15. The same 310.15 as you state is required by 240.4. The exceptions of (A) thru (G) have no bearing and 240.4 ONLY says according to ampacities of 310.15, not according to a specific temperature column(s).

The only way the cable will ever see more current than 180A is during overload (non-normal) or fault (non-normal). The only condition for overload is on the load side of the secondary protection and to overload the transformer would also cause the secondary breaker to trip, thus removing the load and consequently "protecting the transformer" although protecting the transformer is a secondary result. The cable would not be damaged, nor would the terminals be damaged because the cable itself, even at 25% overload would only be 225A, which is below the 75degC value of 230A. So, a non-issue.

So, it boils down to protection of the cable under a fault condition. The 300A breaker (or 250A for that matter) is only going to trip on on a faulted condition between the terminals of the load side of breaker and looking through the transformer to the LV line side of the breaker. The fault current that will be seen will be such that, the breaker will trip before it ever reaches an ampacity, according to cable damage curves, that will damage the cable.

So the argument that you can't use the #4/0AWG on a 300A breaker because it won't be protected according to the NEC; is not a valid argument without specific references, and although it is not a popular interpretation does not make it wrong.

Now, selection of the 300A breaker to protect the transformer, without looking at the actual transformer damage curves is an issue, but not part of this topic.

I've given you the references. You are choosing to ignore them. A cable with an ampacity of 230A is not properly protected by a 300A c/b.

240.4(B) allows the conductor to be protected by an ocpd having the next higher standard overcurrent device rating. The next higher standard device rating from 230A is 250A. Therefore, a 250A c/b is the largest OCPD that can be used to protect the 4/0 conductor.

 

[iceworm]

... So the argument that you can't use the #4/0AWG on a 300A breaker because it won't be protected according to the NEC; is not a valid argument without specific references, and although it is not a popular interpretation does not make it wrong ... .

You have already been give most of the references. I will iterate:
Art 450 does not discuss the conductor overcurrent protection except for 450.3 (Informational Note 1):
450.3 overcurrent protection

450.3 Overcurrent Protection. Overcurrent protection of transformers shall comply with 450.3(A), (B), or (C). As used in this section, the word transformer shall mean a transformer or polyphase bank of two or more single-phase transformers operating as a unit

Informational Note 1: See 240.4, 240.21, 240.100, and 240.101 for overcurrent protection of conductors.​

You might use 240.21.B.3
(3) Taps Supplying a Transformer [Primary Plus Secondary Not over 7.5 m (25 ft) Long]

Which does allow reducing the conductor size under certain conditions. I don't recall ever having this application - but you could.

Other than that, the NEC requires the conductors are to be protected per 240.4.

Put in for a code change. Delete the length limitation on 240.21.B.3. Justification is it is just a waste of copper to oversize the primary conductors for high inrush transformers.
(Don't even think of suggesting me to do this. None of the few interactions I have had with code panel members have not come out well - in fact most have come out somewhat worse than poor)

the worm