OK, now for extra credit:
For 115.46A per
leg (I am not confirming that this is the right answer to the original question since this is still in progress, but use it for my extra credit questions), what would be the minimum
breaker or
fuse size at the head of the
feeder? While we're at it, what would be the minimum single-conductor sizes, temperature ratings, and allowable
wire types of each of the conductors on this 4-wire plus
ground portable feeder cable if it were 50' in length?
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).