Wires and Breakers

[Dave e Victoria]

On another post I expressed concern about the idea of changing a breaker size without knowledge of the wire size used in the circuit. There was a reference to an outside article that talked about how much current a given wire size was capable of handling. The article was not well researched and I volunteered to get the facts.

First some philosophy about sizing breakers. I spent most of my career in engineering and manufacturing as an OEM supplier. That is someone who supplies equipment for installation in someone elses vehicle. I believe. equipment should be designed to contain its own problems. That is it should not fail if supplied adequate power. Moreover, no failure should cause failures in the parent vehicle or any other equipment.

Second, Breakers should be sized to protect the wire. Wire current carrying capability depends on how the wire is routed and how long the current lasts. That is, for short times the wire can carry extra load with out failing. For long lengths of wire the voltage can drop over the length of the wire due to inherent resistance in the wire.

The US Department of Commerce publishes a nomograph for selecting proper copper wire sizes depending on current load, wire length and application. I used this nomograph to develop the following table. The table is limited to typical current loads we will find in RV applications. I used 20 feet as a typical length and made sure we were not going to see any voltaage drop. There are three applcations cited. Continuous load for bundled wire, continuous load for free standing single strands of wire and for intermittent load of two minutes maximum.



To use the chart, determine the current load and then select the column for the application. The cell at the intersection contains the MINIMUM wire size.

The table can be used bacwards: if you know wire size you can determine how many amps can be carried.

This table is for copper wire, either solid or stranded, in 12 volt (13.6 volt battery)applications

[H. John Kohl]

Thanks Dave,
That is a good reference to have. I hope Glenn gets it moved over to the Web page.
Cheers,

[ARJ]

There are many sources which do not agree with the above chart, that does not seem to take into account factors such as differences in temperature and insulation properties (which is probably nothing unusual for the Dept of Commerce) .

http://www.wiktel.com/standards/ampacit.htm

http://www.powerstream.com/Wire_Size.htm

http://www.alphawire.com/pages/383.cfm?partner=0&part=0

http://en.wikipedia.org/wiki/American_wire_gauge

http://www.rbeelectronics.com/wtable.htm

[Dave e Victoria]

Allan,
Not sure how you figure the other sources disagree. Your first reference doesn't apply as it is not for twelve volt applications. On the others I sampled 16 gauge wire and found good agreement. The second reference has a column for chassis applications using free air conductors. It has the same result as I posted. The third reference talks about insulation types. It agrees with the chart I posted for the more common insulation types and is conservative for the more unusual insulations. If folks don't know wire size I doubt they will know the type of insulation. Incidently, I use teflon insulation in my aircraft but always use the information I posted as it is more conservative. The Wikipedia reference is almost straight out of the Handbook for Electrical Engineers. (There is a copy on my shelf if a bit dusty. ) Again using the bundled application the 16 gauge numbers agree. The final reference also agrees with my posted chart for bundled applications that are 20 feet long.

The whole point of this exercise is to give someone who is contemplating changing a breaker size an easy reference to determine if the change is reasonable. As to criticising the Chamber of commerce, they are a reliable source as the nomograph they developed is simply a graphical representation of the classic equations in the Electrical Engineers Handbook. I did not post the whole nomograph as it is way over the top for what I was trying to accomplish here.
Dave

[ARJ]

Dave-wasn't this whole discussion based on changing the hydraulic pump breakers to parallel 40 amp breakers?

Therefore the discussion was centered around 6ga wire (Yes?) and it's current carrying capacity which even varies in the examples cited.

[Dave e Victoria]

Alan,
yes it was and the chart I have shows a 6 gauge wire is safe at 2times 40=80 amps IF operation is not continuous. . If the wire were bundled and current was continuous then my chart says 50 amps max and the Engineers hand book says 55. In either case two 40's are not good. One can, however, certainly argue that the pump is never operated for more than 2 minutes. I would conclude the two 40's in parallel are OK if the wire is indeed 6 gauge in this application. Just don't lean on the switch forever and hope the contacts don't fuse.
Dave

[richfaa]

I like Glenn's chart and explaination a lot better than the charts I looked at. Values do vary depending on the application.