Cable size on continuous load

[kjbob]

If you could be so kind to clear up a feeder size question for me.

Disregarding voltage drop:

If I have a 200A continuous load (480V) on a 250A breaker, how do I size this feeder?

a. 125% of the load
b. Just to cover the 250A breaker
c. 125% of the 250A breaker

Appreciate any help you give me on this question.

Ken

 

[Hameedulla-Ekhlas]

If you could be so kind to clear up a feeder size question for me.

Disregarding voltage drop:

If I have a 200A continuous load (480V) on a 250A breaker, how do I size this feeder?

a. 125% of the load
b. Just to cover the 250A breaker
c. 125% of the 250A breaker

Appreciate any help you give me on this question.

Ken

Breaker size = 200*1.25 = 250 Ampere
Your feeder should be based on 250 ampere with voltage drop and derating consideration.

 

[kjbob]

Thanks Ham

What if my breaker on that particular scenario was 300A?

Then what do I use for my cable size, 200A x 1.25 or 300A breaker?

Regards

 

[Hameedulla-Ekhlas]

Thanks Ham

What if my breaker on that particular scenario was 300A?

Then what do I use for my cable size, 200A x 1.25 or 300A breaker?

Regards

If you want 300 A breaker to install then base your cable or feeder according to 300 A breaker with consideration of derating and voltage drop.

If want to install new breaker and feeder than go with 250 ampere and base your feeder on 250 ampere breaker with voltage drop and derating.

 

[kjbob]

Thanks Hammy... you da man

 

[Hameedulla-Ekhlas]

Thanks Hammy... you da man

check NEC-2008 also artical 220.40 page 118 for more information

 

[winnie]

Ham,

I disagree slightly.

If you have a 200A then you need a conductor with a minimum ampacity of 250A and a breaker with a minimum rating of 250A (with the exception of '100%' rated breakers, ignored here). Additionally the conductor must be considered protected by the breaker.

You can take advantage of 240.6(A) standard ampere ratings. This means that if you had a conductor of 255A ampacity, you could protect this conductor with a 300A breaker, as long as the conductor is correctly sized for the load.

Thus you could have a 200A continuous load, 255A rated conductors, and a 300A breaker, and be legal.

With regard to sizing for voltage drop, I would use the actual load, not the breaker rating. For purposes of voltage drop, the 'actual load' might be the actual continuous load (given as 200A), or it might be the actual startup current which might greatly exceed 300A, depending on the particulars of the application.

-Jon

 

[Smart $]

Ham,

I disagree slightly.

If you have a 200A then you need a conductor with a minimum ampacity of 250A and a breaker with a minimum rating of 250A (with the exception of '100%' rated breakers, ignored here). Additionally the conductor must be considered protected by the breaker.

You can take advantage of 240.6(A) standard ampere ratings. This means that if you had a conductor of 255A ampacity, you could protect this conductor with a 300A breaker, as long as the conductor is correctly sized for the load.

Thus you could have a 200A continuous load, 255A rated conductors, and a 300A breaker, and be legal.

What you are pointing out is that the first consideration is always minimum circuit ampacity (mca or MCA). Everything else falls into place thereafter. There are also situations where motors are involved and the breaker only serves as short-circuit and ground-fault protection, and is rated higher than is generally required to protect the conductors. In these cases, the motor overload control system is what protects the conductors.

With the example in the OP, 200A continuous * 125% + 0 * 100% = 250A mca. Both conductor and breaker must be rated equal or greater than this value. In general, this is true for all circuits: service, feeder, and branch. The most common exception is where multiple motors or motor and non-motor combination loads exist in one assembly, and the manufacturer has predetermined MCA and maximum OCP.

Maximum OCP also comes into play in situations other than noted above. That discussion is specific to the type load or loads involved.

 

[Hameedulla-Ekhlas]

Ham,

I disagree slightly.

If you have a 200A then you need a conductor with a minimum ampacity of 250A and a breaker with a minimum rating of 250A (with the exception of '100%' rated breakers, ignored here). Additionally the conductor must be considered protected by the breaker.

You can take advantage of 240.6(A) standard ampere ratings. This means that if you had a conductor of 255A ampacity, you could protect this conductor with a 300A breaker, as long as the conductor is correctly sized for the load.



For purposes of voltage drop, the 'actual load' might be the actual continuous load (given as 200A), or it might be the actual startup current which might greatly exceed 300A, depending on the particulars of the application.

-Jon

With regard to sizing for voltage drop, I would use the actual load, not the breaker rating.

Yes, ofcourse in the question he has also asked about sizing the feeder and that is why I have point out to take consideration of voltage drop. For voltage drop calculation I am sure he knows that he will take only maxiumum load current not breaker size current.

Thus you could have a 200A continuous load, 255A rated conductors, and a 300A breaker, and be legal.

If you have such a breaker that has more current capacity than cable then any time someone connects extra load to the feeder, the breaker will not trip and it will affect the cable.

I have seen myself such a incident in a house. The owner of house want to connect more load but the breaker was tripping. He went change the breaker size with increase ampere capacity. Then you know what happen, the breaker did not trip but the wire insulation melted and fault occurred.

yes, in motor it is true that the breaker size current capacity is higher than cable. It is only because of start current and start current I think continues for few second or milli second and it does not affect the cable.

 

[ggunn]

Ham,

I disagree slightly.

If you have a 200A then you need a conductor with a minimum ampacity of 250A and a breaker with a minimum rating of 250A (with the exception of '100%' rated breakers, ignored here). Additionally the conductor must be considered protected by the breaker.

You can take advantage of 240.6(A) standard ampere ratings. This means that if you had a conductor of 255A ampacity, you could protect this conductor with a 300A breaker, as long as the conductor is correctly sized for the load.

Thus you could have a 200A continuous load, 255A rated conductors, and a 300A breaker, and be legal.

With regard to sizing for voltage drop, I would use the actual load, not the breaker rating. For purposes of voltage drop, the 'actual load' might be the actual continuous load (given as 200A), or it might be the actual startup current which might greatly exceed 300A, depending on the particulars of the application.

-Jon

It seems to me that you'd want the ampacity of the cable to be higher than the rating of the breaker rather than the other way round.

 

[One-eyed Jack]

It seems to me that you'd want the ampacity of the cable to be higher than the rating of the breaker rather than the other way round.

What Winnie was pointing out was there are some situations that would allow what appears to be an oversized OCP. His example was at the extreme but it could still be code compliant. Your suggestion would be ideal but more costly than the "MINIMUM" required by NEC.

 

[ggunn]

What Winnie was pointing out was there are some situations that would allow what appears to be an oversized OCP. His example was at the extreme but it could still be code compliant. Your suggestion would be ideal but more costly than the "MINIMUM" required by NEC.

I got that, but no matter what the code says I could get away with, I'd want my wire to be able to carry the trip current of the breaker.

 

[mbeatty]

In a design aspect, I would say that as long the selected breaker, or other fault protection device, is sized per Table 430.52, and the wires are sized based on the full load current of the circuit with the appropriate multipliers and derates, then it should not be a problem when a short circuit or ground fault condition occurs.

 

[ggunn]

In a design aspect, I would say that as long the selected breaker, or other fault protection device, is sized per Table 430.52, and the wires are sized based on the full load current of the circuit with the appropriate multipliers and derates, then it should not be a problem when a short circuit or ground fault condition occurs.

I'm not sure I agree with that. If you get a load spike of some kind that is not a dead short but loads the circuit to just below the trip point of the breaker (like maybe a motor in a bind but not stopped), and the breaker trip point is higher than the ampacity of the conductors, wouldn't that be a problem for the conductors?

 

[Hameedulla-Ekhlas]

In a design aspect, I would say that as long the selected breaker, or other fault protection device, is sized per Table 430.52, and the wires are sized based on the full load current of the circuit with the appropriate multipliers and derates, then it should not be a problem when a short circuit or ground fault condition occurs.

430.52 is only used for motors due to their start current which is much more than normal current. Start current is also normal in Ampere but Short circuit current goes to kA which is too large than normal current.

In a design aspect, I would say short circuit calculation must be done separatly to check each avaliable kA shortcircuit current at each node and should be compared to breaker kA.

 

[LarryFine]

If you get a load spike of some kind that is not a dead short but loads the circuit to just below the trip point of the breaker (like maybe a motor in a bind but not stopped), and the breaker trip point is higher than the ampacity of the conductors, wouldn't that be a problem for the conductors?

It depends on the duration (and magnitude) of said load spike.

 

[benaround]

I'm not sure I agree with that. If you get a load spike of some kind that is not a dead short but loads the circuit to just below the trip point of the breaker (like maybe a motor in a bind but not stopped), and the breaker trip point is higher than the ampacity of the conductors, wouldn't that be a problem for the conductors?

No it would not, the motor overloads would open the motor circuit, this would relieve the

feeder of the additional load. The circuit that 'Jon' discribes is not abnormal and is a safe

installation because the continious load is 'known'.

 

[winnie]

I guess the real question is the validity of using the 'round up rule' for protecting conductors.

Yes, if you have a 255A conductor protected by a 300A breaker, there is a chance of some sort of high resistance fault or fault in a load which could allow 290A to flow through the conductor, causing the conductor to overheat without tripping the OCPD. I don't know what the chance of this is; presumably low enough since code permits such an installation.

-Jon

 

[TT009]

I have seen myself such a incident in a house. The owner of house want to connect more load but the breaker was tripping. He went change the breaker size with increase ampere capacity. Then you know what happen, the breaker did not trip but the wire insulation melted and fault occurred.

This is a good reason why if you don't know what you are doing, consult a professional. Either ask an opinion of an electrician or hire them.

240.4(B) does allow you to up size the breaker unless there is 10awg or less connected to it then you have to follow 240.4(D)

 

[iwire]

If you have such a breaker that has more current capacity than cable then any time someone connects extra load to the feeder, the breaker will not trip and it will affect the cable.

I think most of us understand that. :grin:

I have seen myself such a incident in a house. The owner of house want to connect more load but the breaker was tripping. He went change the breaker size with increase ampere capacity. Then you know what happen, the breaker did not trip but the wire insulation melted and fault occurred.

In that case the NEC was not followed and it could lead to a hazard.

But there are sections in NEC that allow the breaker or fuse to have an ampacity greater than the conductors.

yes, in motor it is true that the breaker size current capacity is higher than cable. It is only because of start current and start current I think continues for few second or milli second and it does not affect the cable.

With the NEC when wiring a motor the breaker often provides only short circuit or ground fault protection of the conductors. The additional motor overload devices protect both the motor and the conductors.


Take a look at 230.90 Exception 3, it allows the over-current protection of service conductors to exceed the rating of the conductors.

Here is a picture I have posted many times, showing a fairly typical service in my area. In this one the conductors in the riser (top left heading up) are rated around 250 amps, with over-current protection of 600 amps. (six 100 amp breakers)