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Thread: 4/0AWG on 250A Breaker - Is 240.4(B) Applicable?

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    4/0AWG on 250A Breaker - Is 240.4(B) Applicable?

    I believe I already know the answer to this, but wanted to confirm. We have an application where we have to downsize a cable to terminate at an MCCB in a switchboard. The change is from a 500MCM XHHW-2 using a sleeve (in-line) crimp reducer at a 250AF/AT breaker in order to allow the cable to terminate. My initial desire was to reduce to the 350MCM size to fit the max lug range of #4-350MCM and to avoid to far of a drop from the primary cable size used for the circuit. The installer however wants to go down to a 4/0AWG instead. As background, the cables were initially oversized due to voltage drop and ambient conditions, thus the 500MCM which is larger than normal cable size for the circuit. The 4/0AWG ampacity takes into account the 75C terminals and ambient conditions to rate it at 230A per NEC table 310.15(b)(16) which is obviously less than the 250AT rating/setting of the breaker. My thought is that the installer will reference 240.4(B) which appears to allow 4/0AWG cable to be placed - on the next size up breaker of 250A based on this ampacity (230A) level. This is considered a feeder circuit to a bypass transfer switch downstream for background.

    Here's the kicker - the installer initially believed the trip to be at 225A (which it's not), and they tried to incorporate a 4" gutter requirement on the SWBD to justify the 500MCM to #4/0AWG drop in the first place for bending and termination purposes per Table 312.6(A) (which its not as well; more like 11").

    My gut is saying that what they're doing meets code. Perhaps they just stumbled into this planned installation by accident. Would appreciate confirmation that my interpretation here aligns with what others understand.

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    As long as the calculated load is not greater than 230 amps then you can use 4/0 with a 250 amp breaker
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    Are we talking Cu conductors ? (I ask as XHHW in Cu is rare around here)
    Is the feeder a tap or transformer secondary conductor ?
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    Quote Originally Posted by augie47 View Post
    Are we talking Cu conductors ? (I ask as XHHW in Cu is rare around here)
    Is the feeder a tap or transformer secondary conductor ?
    Yes sir, Cu conductors; 90C rated, but downsized to 75C ratings to account for CB terminations. Not a XFMR secondary conductor; feeder routes to a room located a considerable distance, well over 25' hence the larger conductor size mentioned. Don't believe considered a tap since the 250A breaker on the high side is connected to 2000A bus in the panel before the CB feeds a 250A Static ABT downstream (contains internal CBs). This then services another distribution panel.

    Regarding load, connected is calculated just above 250A, but demand has been calculated to 180A. Not sure if this changes condition per this NEC section since the breaker has already been sized at 250A. The nominal load is expected to be between 180 to 200A. Any concerns? Again, I would like to get them to install the 350MCM to avoid these questions.

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    Quote Originally Posted by msimms View Post
    Yes sir, Cu conductors; 90C rated, but downsized to 75C ratings to account for CB terminations. Not a XFMR secondary conductor; feeder routes to a room located a considerable distance, well over 25' hence the larger conductor size mentioned. Don't believe considered a tap since the 250A breaker on the high side is connected to 2000A bus in the panel before the CB feeds a 250A Static ABT downstream (contains internal CBs). This then services another distribution panel.

    Regarding load, connected is calculated just above 250A, but demand has been calculated to 180A. Not sure if this changes condition per this NEC section since the breaker has already been sized at 250A. The nominal load is expected to be between 180 to 200A. Any concerns? Again, I would like to get them to install the 350MCM to avoid these questions.
    If you can get any allowable methods in art 220 to calculate that load at no more than 230A, then 4/0 copper is acceptable.
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    Thanks, I appreciate the responses.

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