Calculating the Amp Capacity on the Fronius Buss Bar
By Tim McKernan, Fronius Applications Engineer
The Fronius IG Plus has six terminals for input from the solar array. Each one of these terminals is rated
to handle 20A. When the array is pre-combined and the combined input will exceed 20A, the installer
must employ the Fronius Buss Bar for IG Plus. This product is a six tined copper bar that is designed to
fit into all six terminals at one time and split the amperage evenly among all terminals. The dimensions for
the Buss Bar are given in millimeters below.
The buss bar has 6 points with an area of 8mm2. You can see that they are 4mm x 2mm. This is the
smallest point on the Buss Bar and the limiting component for current flow. The NEC 2008 has a table in
section 310.21 that shows the current rating for various wire sizes of bare copper. Using the following
formula where “n” is the wire gauge in question we can calculate out the area of any size wire:
An = 0.012668mm2 x 92 36-n
AWG
8
6
4
2
1/0
Area in mm2
8.36
13.3
21.15
33.63
53.5
/ 19.5
Amps
98
124
155
209
282
2/0
3/0
4/0
67.4
85
107
329
382
444
The 8mm2 of the buss bar are not a true wire size but we can extrapolate what the NEC would likely give
in terms of current carrying capacity for such an area.
This graph plots the points and shows a trend line that is very linear. If we use the equation and apply it
to a conductor with 8mm2, the amperage capacity would be 109A. This value is higher than what the
NEC code says is acceptable for a wire with a larger area e.g. 8AWG at 98A. Unfortunately, the NEC
does not have a value for a 10AWG wire so we cannot try to find a suitable value between those two
points. Therefore, I will change the equation so that the trend line goes directly though the 8AWG 98A
mark. I’m going to do this by changing the “b” value in the linear equation y = mx + b. I’m doing this
because the *R2 is so high that I feel the slope is accurate. With a new b value of 62.539 and running the
equation for an area of 8mm2 we find that the Buss Bar should be able to take 90.7A.
*R2 is the squared correlation coefficient. This value shows how linear the data is. A value of 1 would be
a completely linear data set. If this value deviates too much, a linear approximation would not be fitting
for trend line modeling. The data would be better modeled a polynomial of a higher order or
exponentially.
Buss Bars are available for purchase from your distributor. Please note that two Buss Bars are required
per inverter. The Fronius part number for the Buss Bar is 42,0201,2923.