240.4 (b)
- Thread starter cmreschke
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- New Jersey
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- Journeyman Electrician
#4/0 Al is a 180 amp conductor @75° C so the next size up rule would permit that on a 200 amp OCPD if the connected load were 180 amps or less.
If my calculated load were 120 amps, and the owner wanted a 200 anyways, do I need to size my wire to handle the full 200 amps? (250 mcm al) or can I just say 4/0 is fine even though it can only safely carry 180 amps. Remember the load calculation is much less at 120 amps.
To me, that's more a contractual than a NEC question. A 4/0 would be Code compliant with any calculated load up to and including 180 amp but if the owner requires an install for a full 200 amps I could forsee a legal issue.
I guess what I'm getting at is if a smaller wire and breaker fulfills the nec min let's say the 120 amps, 1/0 is required for 125 amp breaker, and customer wants bigger, are we required to install to the maximum potential load 250 mcm with 200 amp breaker or is it still sufficient to go with a 4/0 (180 amps) and use the next size up? We have unknown potential loads.
By increasing the panel size above what is required, are we designing now (nec is not a design manual)? Should we be restricted to only what is required then by the nec for the same reason (nec is not a design manual)?
Thoughts, opinions, or am I just way off base and need a beer lol?
By increasing the panel size above what is required, are we designing now (nec is not a design manual)? Should we be restricted to only what is required then by the nec for the same reason (nec is not a design manual)?
Thoughts, opinions, or am I just way off base and need a beer lol?
Carultch
Senior Member
- Location
- Massachusetts
The NEC only considers the present situation of calculated loads. It doesn't require you or advise you how to plan for future expansion.
If you know the calculated load is never going to exceed 180A, then your 4/0 AL with a 200A main is fine.
If you want to plan for the full 200A to be used at some point in the future, it is my recommendation that you install 200A worth of wire (250 kcmil AL) today, than it is to rip out the 4/0 and replace it with 250 kcmil.
Another thing to consider is that your prescribed load calculation algorithms are conservative, and due to load diversity of when each load operates, the calculations will rarely reflect the actual full load amperes that you are likely to draw. With the maximum peak of demand-interval (15 minute) power data of today's loads, you can make the case to expand your future loads within the 180A limit.
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Just to put it short and sweet:
The NEC requires the wire to be able to carry the calculated load.
If the calculated load is less than the ampacity of the wire and that ampacity is between standard breaker sizes you can round up to the next standard breaker size and that breaker will still be considered to be protecting the wire.
The NEC requires the wire to be able to carry the calculated load.
If the calculated load is less than the ampacity of the wire and that ampacity is between standard breaker sizes you can round up to the next standard breaker size and that breaker will still be considered to be protecting the wire.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Regardless of whether you call this a 180 amp feeder or a 200 amp feeder it's code compliant. If the person paying the bill want his full 200 amps then use the larger conductors.
310 BLAZE IT
Senior Member
- Location
- NJ
You misunderstood 240.4(b). It is effecting only the overcurrent protection device, not the feeder.
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