Quote:
Originally Posted by Nicholson58
What you fail to appreciate is that resistance is not constant with a heating element. This initial inrush is high, quickly heating the element and causing resistance to increase dramatically. If there is an inline resistor, the cord, he heater never gets hot enough to increase resistance. The heater current remains high and trips the breaker.

I don't think inrush current is relevant for this application. The temperature coefficient of the materials used is close to zero (0) and is measured in
parts per million/°C. The effects are small. When you consider that the components are at ambient temperature to start with (which is likely cold for a
boat heating application) and the
ramp time is relatively fast it isn't going to matter. Remember all the terms are linked. You can change voltage without changing current or resistance, you can't change power without changing current or resistance.
This is why for circumstances where inrush current is a factor we turn to things like softstart components to buffer inrush current.
Since we started this thread with regard to EU cord sets compared to US let's run some numbers.
We'll use a 1200W ceramic fan heater.
For the European case from I=P/E=1200/240=5A steady state. A 16A
shore power cord uses #14 wire at 2.5Ω per 1000 ft. For a 50' cord that's a 100' (both conductors, out and back) or 0.25Ω. P=I²R=(5A)²(0.25Ω)=6.25W. For simplicity let's double that to account for connectors and connections, so 12.5W steady state. That is 1% of the load.
We can do the same for the US case. I=P/E=1200/117=10.3A steady state. A 30A
shore power cord uses #10 or #8 wire at 0.63Ω per 1000 ft for #8 or 1.0Ω per 1000 ft for #10. For a 50' cord that's a 100' (both conductors, out and back) or 0.10Ω for the smaller #10 wire. P=I²R=(10.3A)²(0.10Ω)=10.6W. For simplicity let's double that to account for connectors and connections, so 21W steady state. That is 1.7% of the load. By the way, 12.7W for #8 wire  almost exactly the same as the EU case.
In neither case are we talking about a lot of power, but it is at least four orders of magnitude (multiple by 10,000) more significant than current (amperes) effects of temperature coefficients for materials. Even then it is only relevant for a positive coefficient. It still isn't enough to make a difference.
What is apparent (without numbers, so I have to depend on
insurance statistics and anecdotal reporting) is that loose connections and
corrosion DO make a difference. We can see that from the number of
boat and
dock fires attributable to shore power cord connections. Remember the power loss in the shore power connection is all turning into heat. Keep raising the heat and
insulation starts to melt which can lead to fire.
Corrosion at sliding connectors (like a plug) causes intermittent connections that can spark and lead to fire.
We do know that time is a factor. This is why you can use a 12VDC cigarette lighter power point to supply 30A long enough to heat a lighter coil (analogous to an inrush current case) but only 6A continuous (so your 90W
laptop with 10%
battery left can lead to smoke behind the lighter plug). It's also why a 70W slow cooker works so well on such little power: enclosed space and time. Small amounts of heat build up over time. In the case of resistive heating the seconds to reach steady state just wouldn't make a big difference even if the temperature coefficient numbers were much bigger.
There are inherent assumptions: that wire sizes are appropriately selected, that connectors are properly installed, that the
installation is properly maintained and corrosion mediated, that the little fan in the heater is insignificant from a power draw perspective.