Cable temp

Helping friend on his trawler with a higher amp alternator. The cables are spec'd correctly but we notice when the alternator is putting out max amps, the cables are warm, not hot, just warm. Any problem?

meridian said:

Helping friend on his trawler with a higher amp alternator. The cables are spec'd correctly but we notice when the alternator is putting out max amps, the cables are warm, not hot, just warm. Any problem?

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Warm to the touch "but you can leave your hand on it" is in the 40-50C area. Not a problem if that's as warm as they get at max output. It *could* become a problem if several cables like that get bundled together, especially if it is within engine spaces.

Ken

The cables should be at the same temperature as the engine room.

If you're converting amps into BTUs (heat), you're either loosing volts or amps. This makes the alternator work harder longer which will lead to a shorter life. Agree with FF, same temperature as the engine room.

Ted

O C Diver said:

If you're converting amps into BTUs (heat), you're either loosing volts or amps. This makes the alternator work harder longer which will lead to a shorter life. Agree with FF, same temperature as the engine room.

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Normally I won't argue, but this could send somebody down a path trying to fix a problem that probably doesn't exist.

Speaking as an electrical engineer - all wire (except for superconducting wire) no matter how large has internal resistance (usually spec'd as ohms per foot) and because of this, ALL wire heats up when it is carrying current. If the current is low, it may heat up imperceptibly, but it does heat up. You know those 3% and 10% voltage drop tables we use to help us size wire? That 3% or 10% voltage drop is ALL lost as heat within the wire. When any wire is carrying a significant portion of its rated current for a length of time, it will heat up quite a bit. Within the limits of reasonable voltage drop and the wire's temperature rating this is NOT a problem. The OP stated that it was a "higher amp alternator" "putting out max amps". Even an alternator putting out 100 amps will heat up any wire that is reasonable to attach to it.

Ken

kchace said:

Normally I won't argue, but this could send somebody down a path trying to fix a problem that probably doesn't exist.

Speaking as an electrical engineer - all wire (except for superconducting wire) no matter how large has internal resistance (usually spec'd as ohms per foot) and because of this, ALL wire heats up when it is carrying current. If the current is low, it may heat up imperceptibly, but it does heat up. You know those 3% and 10% voltage drop tables we use to help us size wire? That 3% or 10% voltage drop is ALL lost as heat within the wire. When any wire is carrying a significant portion of its rated current for a length of time, it will heat up quite a bit. Within the limits of reasonable voltage drop and the wire's temperature rating this is NOT a problem. The OP stated that it was a "higher amp alternator" "putting out max amps". Even an alternator putting out 100 amps will heat up any wire that is reasonable to attach to it.

Ken

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Ken, I'm not an electrical engineer, never played one in the movies or on TV. I simply stated the facts. The difference between house (residential, commercial, industrial) wiring and boat wiring is that with house wiring, we care not a lick about the thing generating the power, for our purposes the supply is unlimited, and we rarely run circuits for hours on end where the wires are heating up. In the house analogy, we seldom worry about corroding connectors or a high moisture environment (possible salty).

Now there is a lot the OP hasn't told us. How many amps is the alternator (IMO, high is 200+ amps)? How long is the wire? It may be correct for 3', but not for 23'. Does the alternator have it's own negative cable or is it going through a bunch of iron (engine block) and connections before reaching the battery? Is the negative cable getting hotter than the positive cable? Does either cable run through a series of junctions, battery switches, a dual battery isolator, etc. How warm is warm? If it's warm enough to notice above everything else, it's time to do some proper investigation. Everything may be fine, or this may be a symptom before a big fubar! Think it's a big mistake to tell somebody, "nothing to worry about", without them investigating all the other pieces.

To the OP: Is the cable still flexible, or has it gotten stiff?

Ted

Wow, there is a lot of BS and downright misinformation on this thread. Ken is absolutely right- warm to the touch is ok.


There are two criteria for sizing wire: ampacity and voltage drop. Ampacity is the safe amperage a wire can carry and has nothing to do with length and voltage drop. It is based on keeping the wire insulation below its safe operating temperature which for most boat wiring systems is 105 deg C or about 220 F.


Assuming a 100 amp alternator (in an engine compartment) and not bundled with other wires, it takes a minimum of #6 gauge wire which is safe for 102 amps in those conditions. But I would use #4 or larger for safety and voltage drop.


FWIW a foot of #6 at 100 amps will result in 0.04 V drop or 4 watts. You should certainly be able to feel the effects of 4 watts along a foot of wire, but it shouldn't be too hot to hold your hand on.


Read WM's excellent article on wire sizing here: http://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity


David

djmarchand said:

Wow, there is a lot of BS and downright misinformation on this thread. Ken is absolutely right- warm to the touch is ok.


There are two criteria for sizing wire: ampacity and voltage drop. Ampacity is the safe amperage a wire can carry and has nothing to do with length and voltage drop. It is based on keeping the wire insulation below its safe operating temperature which for most boat wiring systems is 105 deg C or about 220 F.

David

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David,

I'm glad you're so much smarter than me when it comes to boat wiring. Maybe you could explain to me why inverter manufacturers, bow thruster manufacturers, and windlass manufacturers all specify substantial increases in wire size as the length from the power source to the piece of equipment increases. Guess they don't know what you know.

BTW, it would be interesting to know if we're talking about a 100 amp alternator or a 250 amp alternator. Would also like to know how much warmer the cable was than everything else in a warm / hot engine room.

Ted

Ken or Dave,
Have you ever seen calcs for fault currents on boat cables to/from battery banks?
I haven't. Maybe they are different because of battery voltage drop at near-shorted condition. But that can be a lot of heat in the batteries and cables too.

We used to have to check that for some of the industrial circuits I worked on because too big a cable can lead to fault currents that circuit protection couldn't interrupt. Part of balancing ampacity/voltage drop and sizing circuit protection devices.

I've seen a few pics of burned-out battery banks and boards that bring this to mind.

Of course, after hitting "send" I googled "battery fault current calcs". There's a lot of good stuff there!

Ken is absolutely right- warm to the touch is ok.

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So tell us, how warm is OK and how warm is not OK?

It should be imperceptible to the human hand IMO or there is a bad connection or it is undersized.

Isn't heat resistance?

The OP said that the cable was spec'd correctly, which I assume means that they met the ampacity criterion and the voltage drop was acceptable.


If the cable was sized near the minimum gauge for ampacity requirements then it could get a little warm at full current. But to the previous poster's question, how warm isn't the issue for safe wire sizing. The issue is does the wire meet ampacity and voltage drop requirements. These can be determined from the sizing tables.


On the subject of fault current, it is more a consideration in fuse selection, not necessarily wire sizing, although the wire size is an input to the calculations.


David

Warmth can be driven by other factors such as bad connection or corrosion or a cable that has been physically compromised. Any cable that feels warm to the touch should be examined.

The OP stated that the wire got "warm, not hot". I was only stating a rule of thumb that if you can leave your hand on it then it is likely no more than 50C. I know of no common wire that is rated for less than 75C and a lot of wire is commonly rated for a much higher temp. With any circuit that can run at max for a while - like an alternator, or a battery charger, the wires will get warm to the touch and warm under those circumstances is not a problem. Is the heat lost energy? Sure, but at some levels it is unavoidable.

Ken

Ten Deadly Conditions on Boat's Electrical System | West Marine

2. Running fuses continuously at full ratings

When matching circuit protection to the wire it protects, two facts contribute to the complexity of this task:

The amperage at which fuses actually blow, and circuit breakers actually trip, is considerably higher than their nominal ratings, the rating usually marked on the unit.Wire and circuit protection devices heat up dramatically when they carry 100 percent of their rated value for several minutes or more.


Heat from high current can melt wire insulation and fuse blocks.

SEA, Maxi, ATO and AGC fuses, and most circuit breakers, blow or trip at about 130 percent of their rating. ANL fuses blow from 140 percent to as high as 266 percent of their rating. When fuses carry 100 percent of their rated current value, they generate excessive heat. When wires carry 100 percent of their rated current value, they also generate excessive heat. In combination, the heat produced by fuses and wires carrying high current can melt wire insulation and fuse blocks.

This heat generation may become critical when loads run for a considerable time. Large diameter wires take a long time to heat up, so short-term operations like bow thrusters, windlasses, and macerator pumps seldom run long enough for this kind of heating to occur. For example, a 2/0 wire may take 25 minutes to approach its maximum temperature. In contrast, small diameter wires reach near peak temperature in less than ten minutes.

For loads and appliances that run continuously for 10 to 30 minutes, choose circuit protection and wire so that current does not exceed 80 percent of their rating. For more information in this topic, refer to Blue Sea Systems’ Technical Brief:*Choosing Circuit Protection.

I know some resistive heating is normal, but if cable feels warm that sends up a bit of a yellow flag. Could be poor terminal connections, and with Cu having excellent thermal conductivity, the heat can carry a good distance from the source. Also, could alternator be putting out more amps than anticipated?

Depends on how warm, how many amps, and if voltage drop is more than expected.

If an internally regulated unit, any voltage drop is going to reduce charging voltage.

Not a red flag, but still would cause me to check some more stuff out.