- Action: To Do List item A0290 – Replace 25 Amp with 15 Amp circuit breaker and replace wiring to Isotherm 3751 ASU Refrigerator.
- Completed: 17 September 2018
- Cost: $10.95
The Isotherm 3751 ASU SP Water Cooled Refrigeration Installation and Operating Manual was downloaded and reviewed. Additional documentation from their online parts store was also reviewed.
The pertinent information gleamed from the documentation was:
- Fuse: 15 Amp Recommended
- Amps (Max.): High speed - approx. 6.2 Amps - important for determining connecting wire size.
As the refrigerator power wire was to run from the 12V DC circuit breaker panel to essentially the same location under the galley sink - the estimated length of the wire run was about 15 feet. We decided to replace the old wiring since the old wires were not marine rated and were assembled from three separate pieces of wire joined together with butt splices and a plastic screw terminal connector - this increases overall circuit resistance. We would use a continuous run of UL1426 marine rated wire.
To determine the actual size of the wire needed we used the following formula (ref 46 CFR §183.340.(p) and ABYC E-9.14.e.(4)). The 3% allowable voltage drop, instead of 10%, was selected since DC motors operate more efficiently at their rated voltage.
CM = (K x I x L) / E
Where:
CM = Circular mil area of conductor.
K = 10.75 (constant representing the resistivity of copper).
I = Load current in amperes.
L = Length of conductor from the positive power source connection to the electrical device and back to the negative power source connection, measured in feet.
E = Maximum allowable voltage drop at load in volts (For a three percent voltage drop at 12V, E= 0.03 x 12 = 0.36).
We used the value of 10.75 for K, 6.2 amps current draw at 12V for I, 30 feet (15ft x 2) for L, and 0.36 as the 3% voltage drop for E.
CM = (10.75 x 6.2 x 30)/0.36 = 5,554.2 which equates to 12 AWG since it has a CM of 6,530; ref ABYC E-11 (7/15) Table XI. Note - 14 AWG has a CM of 4,110 which is smaller than the calculated value, so the next larger AWG size is needed.
Rechecking the manual, it stated, "For a 12 Volt system, the minimum area of the cable from the battery to compressor must be: 2.5 mm² if the length is less than 2.5 meter; 4 mm² if up to 4 meter; and 6 mm² if up to 6 meter. USA wire size: 12 gauge for 10 ft and under and 10 gauge for lengths over 10 ft." This did not agree with the calculated 12 AWG wire size, why? Using the same formula, solved for I = (CM x E)/(K x L) using the CM value of 6,530 for 12 AWG and L = 20 (10feet x 2). So, I = (6530 x 0.36)/(10.75 x 20) = 10.93 Amps - this value does not agree with the stated maximum 6.2 Amps at high speed. Their recommended wire size and lengths are not accurate.
The recommended fuse size of 15 Amps seemed a bit high for a maximum current draw of 6.2 Amps. Downloaded the Danfoss BD50F compressor datasheet and it also recommended a 15 Amp fuse. Based on the 6.2 Amp max current draw and 150% over current protection being 9.3 Amps, decided a 10 Amp protective fuse is sufficient.
To remedy the incorrect use of the 25 Amp circuit breaker for the Isotherm 3751 ASU SP Water Cooled Refrigeration, it would be swapped with an existing 10 Amp circuit breaker from within the 12V DC circuit breaker panel.
The following picture illustrates the electrical installation diagram.
The following items were purchased to support this installation.
| P/N | Item | Q | Each | Total |
| 12/2 AWG Duplex Tinned Marine Wire (100 Feet - $59.00) | 15 | $0.59 | $8.85 | |
| 12-10 AWG #8 Ring Heat Shrink Splice | 1 | $0.25 | $0.25 | |
| 12-10 AWG #10 Ring Heat Shrink Splice | 1 | $0.25 | $0.25 | |
| 12-10 AWG Female Heat Shrink Disconnect | 2 | $0.30 | $0.60 | |
| Cable/Tie Straps | 10 | $0.10 | $1.00 | |
| TOTAL | $10.95 |
On 17 September 2018, removed the old wiring for both the Isotherm 3751 ASU SP Water Cooled Refrigeration and the fresh water pump as they followed the same wire route. Installed new 12 AWG duplex wiring from the 12V DC circuit breaker panel, 10 AMP circuit breaker, to the refrigerator and connected the wires with marine rated terminal disconnects.
Momentarily powered on the refrigerator and the control panel while in the manual position - the green light illuminated indicating the power was on and the compressor started running. Switched the control panel to automatic and both green and amber lights illuminated. As the boat is out of the water "on the hard", we shut it off after just a few minutes of running.
