In looking for some 43.13 in html, I found this thread. Just had to
comment...
1A was quoted, 1B is current and the relevant passage is in Section 5,
11-66.b

According to AC 43.13-1B -- "The voltage drop in the main power wires from
the generation source or the battery to the bus should not exceed 2 percent
of the regulated voltage when the generator is carrying rated current or
the battery is being discharged at the 5-minute rate. The tabulation
shown in table 11-6 defines the maximum acceptable voltage drop in the
load circuits between the bus and the utilization equipment ground."

Two different current paths are being addressed; one from the generation
source to the bus, and a second from the bus to the load. Dissecting that
sentence a bit to get two, one for the generation source and one for the
battery.

1) The voltage drop in the main power wires from the generation source to
the bus should not exceed 2 percent of the regulated voltage when the
generator is carrying rated current.

2) The voltage drop in the main power wires from the battery to the bus
should not exceed 2 percent of the regulated voltage when the battery is
being discharged at the 5-minute rate.

Two totally different statements. The current in the gen/alt circuit
should never exceed it's charging capacity; hence that limitation, however
the discharge current from the battery to the bus (in a fault condition)
could approach that 5-minute rate they speak of. That's the rate where
you have a dead battery in ~5 minutes and approximates the battery
ampacity*12. For most of the small batteries, say a size 35, the 1C rate
is in the low 20's. The Concorde 25 size is rated 21Ah; the 35 size at
25Ah. 21*12=252. That's a lot of current! It's not exactly linear
either, the actual current is less, but I haven't seen too many published
12C rates.

So the first sentence says you cannot exceed 2% drop from the generation
source to the BUSS, at rated current. For most of us with 14V (pick your
reference) systems that is ~ 0.28V. That statement is for the circuit to
the generator/alternator, only and is not too difficult to comply with
with a 50 or 60A source. Even with the 100-150mV or so of drop across the
gen/alt breaker, you still have a bit over 100mV of drop in a wire to work
with. Fifty amps through ten feet of AWG-2 is about 85mV. This is the
easy part to comply with.

That second one is the killer, especially for those with an aft mounted
battery. Assume the 5-minute discharge rate isn't really 252A, but (for
ease) only 200A. Ohms law R=E/I 0.28V/200A=0.0014 ohms. That's only a
bit over eight feet of the #2 cable, and if you have an ammeter shunt in
the battery cable, subtract another 50mV from your budget and shorten up
that cable even more.

The last statement, "The tabulation shown in table 11-6 defines the
maximum acceptable voltage drop in the load circuits between the bus and
the utilization equipment ground." clearly states voltage drops between
the bus and equipment ground; effectively from the breaker unique to the
equipment to the ground connection for the piece(s) connected to that
breaker (powered from the bus). For most loads these are easy; 0.5V
continuous and 1.0V intermittent, for 14V systems. Only high current
loads are much of a problem; heated pitot tubes, gear/flap motors,
landing/taxi lights and such. Most all under 20A.

So back to the question, acceptable bus voltage. The comments on
regulator stability are prime, especially for charging, and (I think), the
intent of the question, as the AC reads "2 percent of the regulated
voltage". What is the regulated voltage? As was posted earlier,
depending on temperature Concorde suggests 13.7514.75V so that 0.28V
should be decent estimate for a 14V system. Measure your alt/gen terminal
voltage (under load directly across the terminals) and to be legal per
43.13 your bus should not be more than 0.28V less, and your battery should
charge...

My apologies for my long (and first) post. I hope to create no ire from
the group and hopefully contribute something of value for those
interested.

Ron
'73 BL17-31ATC