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section 240.4(B) permits you to use the next higher standard rated fuse or circuit breaker when the conductor rating doesn't match the standard sizes (with certain restrictions). However, 240.4(C) limits this to 800A or less. I have heard that the 800A limit was not always there. My question is when did the 800A limit start? My oldest Code book is 1981 and it is still there in 240-3 exc. 1.
First appeared in the 1962 NEC in Section 240-5,Exception No.1
There was no published report on proposals at that time, and there was no comment made in the NEC Handbook, so where the proposal came from is not known.
So then the restriction was added in 1962. When did the NEC start allowing you to use the next higher standard fuse or circuit breaker setting when your max conductor ampacity did not correspond to a standard size?
The same 240.5.e1 rule also existed in the 1959 code book.
Also, at that time, the largest "standard ampere rating for circuit breakers" was 800 Amps. Fuses had standard ratings up to 6000A and there was no restriction in the '59 code for going to the next higher standard rating...
Then, in the '62 code, there was a change that required an 800A limitation on going to the next higher standard rating, and they combined the wording so that both breakers and fuses were dealt with in the same exception.
Considering what Creighton has said about there not being any comment about the change in the '62 edition, I infer that the code change was done to clean up the disparity in language between the two types of overcurrent devices. Since it doesn't seem that the device technology (breakers vs fuses) was at issue, then I am thinking that it must have been a practical issue of putting a limit on the amount of space taken up by parallel wires. They might have noticed that switchboards weren't allowing enough wire bending space for a larger parallel feeder. In the '59 code, the ampacity of 500MCM THWN wire was 380 amps. Putting them "in multiple" (as they used to refer to parallel conductors then) would provide an allowable "current carrying capacity" (ampacity) of 760A..which would be just fine for that standard 800A overcurrent device.
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