STEVETERRY
Well-Known Member
Extra credit attempt part II…or maybe it's part one, since I kind of went out of order...
In a purely logical sense the minimum circuit protection rating at the head of the feeder is 115.46 amps, however this is far from a nice, even number typical of fuse/breaker ratings (at least at currents this high) AND unpractical and poorly thought out. The size of the protection at the head of the feeder is determined by the current requirements of the “utilization equipment” and more importantly the ampacity of the feeder used. It is always ok to use circuit protection rated for less current than the conductor ratings however not the other way around.
As I established in my previous post, if you let the 115.46 A represent an 80% load, you would want to plan for 144.33 amps at the hypothetical 100% load. Since the feeder is 50 feet in length the exceptions mentioned in my other post are not applicable, thus 520.53(H)(2) applies, imposing a minimum current-carrying conductor size of 2 AWG. Table 400.5(B) lists 2 AWG single conductor type SC 90 degree C rated cable as having an ampacity of 190 amps. All of that being said, I would guess that the most appropriate short-circuit protection rating for the head of this feeder would be 150 amps per leg. This rating is well inside the conductor’s rating and allows enough head-room and safety margin to be safe and practical.
As I mentioned in my other post, 520.53(O)(2) requires that the neutral have 130% the ampacity of the ungrounded conductors –
(190 amps x 1.30)=247 amps.
90 degree C rated 1/0 AWG cable handles up to 260 A per 400.5(B).
As for the size of the equipment grounding conductor, in the real world it would likely be the same as the supply conductors used in the rig as specified above, however technically it is unusual that it needs to be as large. 250.122(A) specifies use of table 250.122 for determining grounding conductor size. At 200 amps short-circuit protection a 6 AWG COPPER grounding conductor is required. This is also the minimum size allowable for a grounding conductor in this situation as per 520.53(H)(2).
As for the type of cable, NFPA 70 520.53(H)(1) requires use of “extra-hard usage cords or cables”. NFPA 70 table 400.4 lists a number of cables use extra-hard usage including PPE, SC, SCE, SCT, SE, SEW, SEO and W among a few others unlikely to be used in our field. The above selected are reflected in the draft document Derek linked to (BSR E1.18-1). That document also specifies that all feeder used in our field is to have a 90 degree C temp rating. This seems like common sense as it allows more amperage on smaller (read: lighter) conductors.
As a shameless attempt to score more points on this question, for a 50 foot long set of feeders the maximum number of interconnections allowable on these feeders is three (per run). This means 25’ long feeder cables are the shortest you’ll be lugging out of the bottom of the road case. This is specified in 520.53(J).
I could not have said it better myself. One point is that industry convention has evolved to:
#2 AWG 100A
2/0 AWG 200A
4/0 AWG 400A
These ampacities are reflected in the draft of E1.18. They provide a good general purpose overhead to deal with voltage drop.
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ST