Sizing of main breaker

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ernric43

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I have 2 production lines each with a non-continuous load of 1,000 amps and a continuous load of 300 amps.
If I run a main feed for 1,000 amps to a J Box and split it to (2) 500 amp disconnects and then use the disconnects to feed (2) 500 amp main breaker distribution panels, assuming all wiring is sized for the breakers. am I violating code?


Do I have to feed the line at the non-continuous load +%25 max load?

Does NEC care if I underfeed a device? Doesn't this overprotect the system or device?

Thanks,
Rich
 
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ernric43 said:
If I run a main feed for 1,000 amps to a J Box and split it to (2) 500 amp disconnects and then use the disconnects to feed (2) 500 amp main breaker distribution panels, assuming all wiring is sized for the breakers. am I violating code?
Click to expand...
Yes you would be. My reference is NEC 210.19(A)(1).
ernric43 said:
Do I have to feed the line at the non-continuous load +%25 max load?
Click to expand...
You need to provide conductors capable of serving a total of 1375 amps (100% of non-continuous plus 125% of continuous) for each of your production lines (2750 amps total).
ernric43 said:
Does NEC care if I underfeed a device? Doesn't this overprotect the system or device?
Click to expand...
Yes it does care. There is some merit to the thought that overprotecting is better than underprotecting. If you could have a load as high as 1300 amps, and if you supply it with a condutor that can only handle 500 amps, and if you protect that cable with a 500 amp breaker, your protection concept may seem, on the surface, to be acceptable. But it will frequently result in an overcurrent trip of one or both of your breakers, and that is not something that should be designed into a distribution system. In my Navy days the way they said it was that we should never force a protective device to perform its function. Let it do its thing during an abnormal or even emergency situation, but don't make it do its thing as an expected event during normal operations. Forcing a protective action would risk having that action fail on you, thus creating a very dangerous situation.

 
I could use some help with this one.
charlie b said:
My reference is NEC 210.19(A)(1).
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Thinking about it again, I realized that that article only deals with branch circuits. The OP is talking about a feeder. I don't want to look in 225, because this is not an "outside feeder." I looked around the code book, and could not find a similar statement (i.e., size the conductors for 100% of non-continuous plus 125% of continuous) being applyied to feeders. It is late in a long day, but I can't find it. Does it exist? Do I need a shot of bourbon to clear my mind?

 
215.2(A)
215.2 Minimum Rating and Size.(A) Feeders Not More Than 600 Volts.

(1) General.
Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. Conductors shall be sized to

carry not less than the larger of (a) or (b).


(a) Where a feeder supplies continuous loads or any
combination of continuous and noncontinuous loads, the
minimum feeder conductor size shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.


(b) The minimum feeder conductor size shall have an
allowable ampacity not less than the maximum load to be
served after the application of any adjustment or correction
factors.


 
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Main Breaker

Main Breaker

augie47 said:
215.2(A)
215.2 Minimum Rating and Size.(A) Feeders Not More Than 600 Volts.

(1) General.
Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. Conductors shall be sized to

carry not less than the larger of (a) or (b).


(a) Where a feeder supplies continuous loads or any
combination of continuous and noncontinuous loads, the
minimum feeder conductor size shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.


(b) The minimum feeder conductor size shall have an
allowable ampacity not less than the maximum load to be
served after the application of any adjustment or correction
factors.


Click to expand...

Is the Main Breaker considered the Feeder. Each branch circuit is protected per 215.2. Let me ask this another way. Let's assume that I have a 4,000 amp industrial switchgear with two 1,500 amp breaker and two 500 amp breakers. The entire switchgear is drawing 800 amps. I install another small production line that requires a 500 amp breaker but the switchgear is out a room. Can I add an additional section to the switchgear to allow for expansion?

To me this is the same question. Can the breakers in a distribution panel exceed the ampacity of the main breaker?
 
You can load the switchgear to 4000A continuous. Because of load diversity, it is very common for large switchgear to have breaker ratings that add up to more than the main rating. All that matters is that each individual breaker can carry the running load, and at the same time the total load on the switchgear does not exceed the 4000A.
 
kingpb said:
You can load the switchgear to 4000A continuous. Because of load diversity, it is very common for large switchgear to have breaker ratings that add up to more than the main rating. All that matters is that each individual breaker can carry the running load, and at the same time the total load on the switchgear does not exceed the 4000A.
Click to expand...

That's what I thought but how is this different than the previous question?

If all branch circuits for the production line are protected appropriately and the total load does not exceed the 500 amp rating of the main breaker where is the problem? Per an ANSI article that I read there are five ways to size the main breaker and one of them is based on continuous load.

I'm not trying to argue the point, I'm just trying to make sure that I get and understand the answer.
 
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