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Old 07-08-2015, 08:32   #1
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Using ohmmeter to determine when to replace wiring

I got into a discussion with a friend about boat wiring.

Is it feasible to use a wires resistance, with an ohmmeter, to determine if a particular run of wire needs to be replaced?

He suggested that because some runs of wire were very long it would be difficult to get an accurate ohmmeter reading and instead to use amps?

Your thoughts? AND, does anyone ever actually use this to determine if wire needs to be replaced?

What is your "barometer" to know when it is time to upgrade your wiring? Time?

Thanks!
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Old 07-08-2015, 08:50   #2
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Re: Using ohmmeter to determine when to replace wiring

Quote:
Is it feasible to use a wires resistance, with an ohmmeter, to determine if a particular run of wire needs to be replaced?
Only if the wire is broken

Quote:
What is your "barometer" to know when it is time to upgrade your wiring?
If a visuell inspection tells you

My 2ct

Carsten
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Old 07-08-2015, 08:53   #3
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Re: Using ohmmeter to determine when to replace wiring

Voltage drop should not exceed 5%, measure voltage with the loads turned off, then turn them on. If drop is unacceptable then start figuring it out but otherwise there should be no problems.
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Old 07-08-2015, 09:41   #4
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Re: Using ohmmeter to determine when to replace wiring

I replaced all my wiring when the boat reached its thirtieth year of service. The reasons were several. First, the wire that was available when I built the boat was nowhere as sophisticated and well fabricated as the Ancor cable that replaced the seven strand which I bought at a boatbuilders supply in the seventies. Second, Thirty years ago I had six circuit breakers for my DC circuits. Today, I have sixteen DC circuits. Third, when designing those sixteen circuits, using a new, sophisticated panel with digital meters and such, I was able to create a distributed pathway for the current that was going to flow to the eventual unit. By this I mean, large diameter (gauge) wire from the breaker to a buss, then lighter gauge to the individual unit or a final buss. This allowed me to calculate the total possible load along any stretch of conductor. It also gave me an opportunity to label each individual wire (at each end) to know who's who when exploring my electrical system. Third, because the battery bank, alternator and charger/inverter were all new, every single wire and conductor, and many of the final units, were all brand new with freshly installed terminal connectors, terminal strips, buss bars, and cable fastening, I could assure myself of all the components at the beginning. The result is that tracing circuits is a breeze, all of the connections are visible for occasional inspection, and, when it becomes necessary, I can put my hands on each and every component to replace it should I see evidence of damage or need for modification. Too many boats are struggling with old, inadequate wiring and terminals. Nothing lasts forever.
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Old 07-08-2015, 09:46   #5
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Re: Using ohmmeter to determine when to replace wiring

I would think it might be difficult to get a base line. Most wire specs I've seen show a resistance rating at 1000' of cable. I'm not sure how linear these specs are for different lengths. This link would seem to indicate they are linear.
Cirris Systems - Wire Resistance Calculator & Table
But again it would depend on what wire you started with.
This link from a manf of RV equipment does go into testing resistance in looking for corrosion
http://www.hwhcorp.com/ml40838.pdf
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Old 07-08-2015, 09:49   #6
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Re: Using ohmmeter to determine when to replace wiring

Also as your friend seemed to indicate if your testing on board you would need to calculate the resistance in your test lead extension plus how well the test lead is connected to the wire in question (an alligator clip on a corroded lug may not give you a very accurate picture)
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Old 07-08-2015, 10:00   #7
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Re: Using ohmmeter to determine when to replace wiring

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Originally Posted by Roy M View Post
I replaced all my wiring when the boat reached its thirtieth year of service. The reasons were several. First, the wire that was available when I built the boat was nowhere as sophisticated and well fabricated as the Ancor cable that replaced the seven strand which I bought at a boatbuilders supply in the seventies. Second, Thirty years ago I had six circuit breakers for my DC circuits. Today, I have sixteen DC circuits. Third, when designing those sixteen circuits, using a new, sophisticated panel with digital meters and such, I was able to create a distributed pathway for the current that was going to flow to the eventual unit. By this I mean, large diameter (gauge) wire from the breaker to a buss, then lighter gauge to the individual unit or a final buss. This allowed me to calculate the total possible load along any stretch of conductor. It also gave me an opportunity to label each individual wire (at each end) to know who's who when exploring my electrical system. Third, because the battery bank, alternator and charger/inverter were all new, every single wire and conductor, and many of the final units, were all brand new with freshly installed terminal connectors, terminal strips, buss bars, and cable fastening, I could assure myself of all the components at the beginning. The result is that tracing circuits is a breeze, all of the connections are visible for occasional inspection, and, when it becomes necessary, I can put my hands on each and every component to replace it should I see evidence of damage or need for modification. Too many boats are struggling with old, inadequate wiring and terminals. Nothing lasts forever.
Hmmm - hope those smaller diameter wires coming off your bus are individually fused for their respective rated current.

This is basically the point of fuses / breakers: Even if you short circuit the thing, you'll never draw enough current to start a fire because the fuse or breaker will give.

Say you've got a 30A fuse, and 30A capable wire, feeding your bus. Then from your bus you've got some 5A capable tiny wire going to a lamp. If that lamp shorts, and begins drawing, say, 25A, you aren't going to blow your fuse and that 25A is going to get your 5A capable wire very, very, hot.
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Old 07-08-2015, 10:11   #8
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Re: Using ohmmeter to determine when to replace wiring

Thanks, chris95040, for catching the oversight. I assume you mean like these: (I love Blue Seas hardware).
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Old 07-08-2015, 18:22   #9
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Re: Using ohmmeter to determine when to replace wiring

wire condition can be tested with a megger. but that mostly tests the jacket condition.


otherwise you'd have test voltage with a load.
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Old 07-08-2015, 18:43   #10
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Re: Using ohmmeter to determine when to replace wiring

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Originally Posted by smac999 View Post
......
otherwise you'd have test voltage with a load.
Yes, use a voltmeter.

There are several ways to test if the wiring is OK or not but the simplest, easiest and least painful way is simply using a voltmeter.

What you are trying to establish is the number of volts dropped in the wiring and thus is not available to the device. Rule of thumb is less than 3% lost in wiring, switches, breakers, connectors etc.

The circuit must be turned on and the device operating normally. Measured the supply voltage and the voltage at the device. The difference is the voltage lost in the wiring etc.

If the difference is more than 3% of supply voltage, then the wiring is suspect or undersized or someother part of the circuit is suspect (connectors, switches etc). Simple case of using the voltmeter to hunt out the suspect component.

So if the supply voltage (battery) is say 12.5V, the voltage at the device when operating normally should be greater than 12.125V, say 12.1V
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Old 07-08-2015, 19:00   #11
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Re: Using ohmmeter to determine when to replace wiring

As has been said above: a voltage drop test will show you if the wire is degrading. However if you measure ie 10% drop.... in most people's view an unacceptable loss, but how do you know if the original installation was of a design that allowed this? If that was the case, then no degradation has occurred.
On the other hand, such voltage drop would make the decision for an upgrade anyway.

As an answer on your question: decide what is an acceptable Voltage drop is for you, then measure each circuit under load, replace those circuits that does not meet your standards.
Note, that some voltage drop will occur also in the switches, fuses an connectors, so test them first, as it would be easier to replace those, rather than wiring.
Lastly I think it was mentioned before: a visual check of the copper: black indicates moisture, green stuff (corrosion) on connectors may mean higher resistance, but may be cleaned off in some cases.

Note: that voltage drop will occur both in the supply (positive on most boats) and in the return (negative) line. They can be measured separately, ie by putting the voltmeter on the battery positive and the positive of the appliance, and then on the battery negative and the neg connection of the appliance. Those two measured voltages added up is the total voltage drop for that circuit.
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Old 07-08-2015, 19:14   #12
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Re: Using ohmmeter to determine when to replace wiring

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Originally Posted by Wotname View Post
Yes, use a voltmeter.

There are several ways to test if the wiring is OK or not but the simplest, easiest and least painful way is simply using a voltmeter.

What you are trying to establish is the number of volts dropped in the wiring and thus is not available to the device. Rule of thumb is less than 3% lost in wiring, switches, breakers, connectors etc.

The circuit must be turned on and the device operating normally. Measured the supply voltage and the voltage at the device. The difference is the voltage lost in the wiring etc.

If the difference is more than 3% of supply voltage, then the wiring is suspect or undersized or someother part of the circuit is suspect (connectors, switches etc). Simple case of using the voltmeter to hunt out the suspect component.

So if the supply voltage (battery) is say 12.5V, the voltage at the device when operating normally should be greater than 12.125V, say 12.1V
The only thing could add to that is measure after the c/b, assuming the c/b is good. A c/b has a bit of drop by itself.
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Old 07-08-2015, 19:59   #13
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Re: Using ohmmeter to determine when to replace wiring

I wonder if you couldn't us an IR thermometer to quickly check if a wire was overloaded or spot a bad connection?
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Old 07-08-2015, 20:27   #14
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Re: Using ohmmeter to determine when to replace wiring

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Originally Posted by esarratt View Post
Is it feasible to use a wires resistance, with an ohmmeter, to determine if a particular run of wire needs to be replaced?

...

Your thoughts? AND, does anyone ever actually use this to determine if wire needs to be replaced?

What is your "barometer" to know when it is time to upgrade your wiring? Time?
I'm with the consensus. An ohmmeter will help confirm a problem like an open or a high resistance somewhere in a wire run, but the ohmmeter in most multimeters that you or I have in our toolboxes won't tell you much about the wire itself. Voltage drop across the wire for a known current is much more useful. And of course, visual inspection.

Here's a thought on the topic of wire replacement... if the circuits are now serving loads that have lower current demands, like you've replaced incandescent bulbs with LEDs... then that old wiring, if it was good stuff to begin with, and it's not damaged, has rotting/cracked insulation or corroding, may well serve for a while longer. Maybe. I'd like to hear what some of the pros think about that.

For maximum peace of mind, rewiring with good materials and workmanship can't be beat.

Quote:
Originally Posted by HopCar
I wonder if you couldn't us an IR thermometer to quickly check if a wire was overloaded or spot a bad connection?
Indeed you can, but only when that current's flowing, and ambient heat, sunlight, and even different colours can make a big difference in the IR temp readings.
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Old 07-08-2015, 21:27   #15
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Re: Using ohmmeter to determine when to replace wiring

The problem is that increased resistance resulting from corrosion will be much more noticeable when as current increases. Voltmeter readings are usually taken without current flowing, so would not be a reliable indicator.
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