240.4B vs 240.4C
- Thread starter darrylb
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Someone may find a flaw in that reasoning, but, it seems fairly descriptive.
There have been some lively discussions here about conductors with a load over their ampacity but withing the "next size" overcurrent device. The discussions revolve around 240.21(B) vs 210.19.
(For example a #6 with a 67 amp load on a 70 amp breaker).
Since there are opinions, the safest route is probably to make sure your conductor's ampacity is adequate for the calculated load and if your over current protective device is over 800 amps, that your conductor ampacity is not less than the OCP rating.
There have been some lively discussions here about conductors with a load over their ampacity but withing the "next size" overcurrent device. The discussions revolve around 240.21(B) vs 210.19.
(For example a #6 with a 67 amp load on a 70 amp breaker).
Since there are opinions, the safest route is probably to make sure your conductor's ampacity is adequate for the calculated load and if your over current protective device is over 800 amps, that your conductor ampacity is not less than the OCP rating.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
No. Conductors are sized on the basis of the calculated load. You first determine the load, then you pick a conductor with at least that ampacity, then you pick an overcurrent device to protect the conductor. There is a set of standard ratings for overcurrent devices. It is in 240.6(A). The difference between the two articles you cited has to do with what you do when you have a conductor with an ampacity value that does not exactly match one of those standard ratings. If the ampacity is less than 800 amps, then you can use an overcurrent device that is the next higher standard rating. If the ampacity is higher than 800 amps, then you use an overcurrent device that is the next lower standard rating.
That means if I have a calculated load of 1630amp, I go to 1600 amp for my overcurrent protection? That would not provide enough protection. Right? Wouldn't you go instead to a 2000 amp?
A tried and true process:
1. Calculate your load
2. Select a conductor to carry that load
3. Select an overcurrent device to protect that conductor
(If the overcurrent device is over 800 amps the conductor must be rated at or above the OCP rating.)
1. Calculate your load
2. Select a conductor to carry that load
3. Select an overcurrent device to protect that conductor
(If the overcurrent device is over 800 amps the conductor must be rated at or above the OCP rating.)
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
You left out "Step Two." Step One is to calculate the load (1630 amps, in your example). Step Two is to select a set of conductors that has at least that ampacity. I might suggest five parallel sets of 400 MCM (ampacity is 5 x 335, or 1675 amps). Step Three is to select an overcurrent device that is no higher than the ampacity value of 1675. I might suggest the next lower standard rating of 1600. But you could get a breaker with a variable setting, and set it at 1675 amps.
When you are over 800, the one thing you cannot have is an overcurrent trip setting that is higher than the ampacity of the selected conductors.
I have to learn to type faster, or arrange to have Gus distracted while I am typing.
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