If I have two I-Command gauges, do I need two power taps?
The simple answer is "no", but there is more to it than you might think.
There is a limit on the power that can be consumed on the NMEA-2000 bus. The NMEA organization mentions this in the initial white paper on NMEA-2000. They just say, "The amount of current delivered by the network cable is limited."
Because of this limitation, NMEA requires identification of the power load for a NMEA-2000 device. They say, "To aid in planning network installations manufacturers are required to specify the power rating for each connected device as a 'load equivalency number.'"
To discover the load equivalency number for an I-Command gauge, we look at the gauge itself. Hmm. Nothing found on the device. Next, we check the User's Guide. On page 9 there is a heading, "Load Equivalency." However, in the paragraph under that heading, we only find the load equivalency of the E-TEC EMM. The load equivalency is "1". Hmm. No load equivalency is given for the gauge.
Let's try Lowrance and their LMF-400 gauge, which is very similar. Hmm. Nothing found in their manual, either.
Let's try NMEA.ORG. They have a database of certified products. Searching for "Lowrance" as the manufacturer, the LMF-400 is found. Guess what? No load equivalency given there, either.
In another white paper, the load equivalency number is defined:
NMEA 2000 Certified devices list the power drawn from the backbone using a Load Equivalence Number (LEN) for use in planning and installation. The LEN represents the integer multiples of 50mA drawn by the device. For example, a device that draws 100mA would specify a LEN of 2, and a device that draws 101mA would specify a LEN of 3. The maximum power that a device may draw from the backbone is 1.0 Amp, or 20 LEN.
Now we know what a load equivalency number (LEN) represents, but we don't know the LEN for the I-Command. But things are not that bad. They're worse. We don't know the maximum LEN loading allowed on the network! I can't find any citation of that value.
Sorry, but there is no way to easily know this answer. The organization that set the standard, NMEA, has not clearly told the boating public (anywhere either I or GOOGLE can find) what is the maximum LEN for a network. However, I did find a secondary source, not NMEA.ORG, but a private individual, who says that for a network built with NMEA LIGHT cable (22-AWG for power) the maximum LEN is 80. This is based on a maximum power current of 4-Amperes. (That's 80 x 0.050-Ampere = 4-Amperes.)
However, a presentation on the NMEA.ORG website contradicts this, and suggests that the maximum power for NMEA LIGHT cable is 3-Amperes. That suggests the LEN limit would be (3-Amperes/0.05-Amperes/LEN) = 60 LEN.
This same presentation goes on to show how in addition to the LEN, the length of the network, the size of the cable, the load on the network, and the power supply voltage must all be considered when calculating power delivery. It is actually quite a complicated calculation. On that basis perhaps it is impossible to specify that a network can have a maximum LEN, as the maximum LEN would have to carefully calculated based on the many variables described.
If we use the lower LEN value to be safe, we can figure a NMEA-2000 network with THIN cable and MICRO connectors--the typical small boat installation--will allow a total power of 60-LEN. If we figure the network is very short, no longer than the boat, and is probably less than 15-feet, we can probably ignore the voltage drop calculations. And we know the power is going to be from a lead-acid 12-Volt battery.
With that simplification, if we could just find the LEN for the I-Command gauge or the LMF-400 gauge, then the question could be answered.
In a practical realm, certainly there will be no problem for just two gauges to be powered from the network bus. If you had twenty gauges, perhaps you'd have to look more carefully at the power consumption.
Also, the NMEA-2000 bus power can be split into segments if, for some reason, you had a very high power load. If heavier cable, called NMEA THICK, is used for the backbone, the power can be fused at 8-Amperes. Essentially as much power as necessary can be provided by use of multiple power taps and segmented backbones. But none of that is necessary for just two I-Command gauges. Multiple power taps are not needed for two I-Command gauges.
I believe the answer can be inferred from the requirement that no NMEA-2000 device have a LEN greater than 20. We can probably assume that the I-Command gauge's LEN is therefore less than 20. Two I-Command gauges would have a total LEN of less than 40. If we figured the maximum LEN for the network was 60, then two I-Command gauges are a suitable load.