18AWG Wire Handle 125VDC, 7Amp

Forgot to mention that the 18 AWG mulit-conductor cable is 10ft long before terminating terminal blocks for field.

rabat, rabat

I'm too tired to berate you for being lazy.

Current Carrying Capacity of Copper Conductors

I did my google research. I already saw this pdf. It does not bare any fruit.

Most things I find online states 10Amps and 300V is what 18AWG wire can handle. I'm wondering if the NEC handbook has something in it that is more offical. I don't have a set with me in the office.

Sorry I couldn't help.

After having a look at the American catalog and website they certainly don't make it easy, the New Zealand website below has more general information (but still not everything you need).

http://www.generalcable.co.nz/NewZealand/default.aspx

Many cable manufacturer websites have the current ratings and derating factors but I don't know of any American ones with AWG sizes (you can use the equivalant European mm2 cable data and scale it for your 0.8mm2 18 AWG cable if you don't have access to the American wiring standards).

As for the current rating of a 18AWG (0.8mm2) multicore cable, the current rating depends on a number of factors (as you probably already know). The specific information can be found in the appropriate American standard, but as a rough guide your cable should have a continuous current rating of 8-17A (dependent on number of loaded conductor cores, cable insulation, mounting or burying method, ambient temperature de-rating, etc).

So, short answer is 7A should be fine but I personally would go up a cable size.

Surprising .. you could not get information on internet.

Here is something:

The formula for current carrying capacity in amperes for copper magnet wire wound into a coil is: d² x 4869.48 (d = diameter in inches). This formula is pretty conservative, and formulas from other sources based on straight lengths of solid or stranded conductors in ambient air may indicate greater current carrying capacity than this one.

http://www.mwswire.com/faqs.htm you will get good information about copper magnet wires.

Normally current in magnet wire should not exceed above 50% of its capacity to avoid overheating and melting.

18AWG is size of single magnet core. The current carrying capacity of the cable will depend up on the number of cores in the cable. Just 18AWG is not sufficient to decide the current carrying capacity of the cable.

Secondly, magnet wire has nothing to do with voltage. The specified voltage is generally insulation failure voltage, when the insulation between two cores will fail and short circuit the cores. Otherwise, keeping the current within limit voltage may be anything for single core magnet wire.

Rabat,

A lot of people think of the building wire section of the NEC when they are looking for ampacity of cables such as the one you describe. That is article 310. However, two other areas are more direct--article 400 for flexible cords and cables, and article 402 for fixture wires. Both of these have ampacities for 18AWG wire. In article 400, for 60°C cables, 2-conductors are 10A and 3-conductors are 7A; for 90°C cables, 3-conductors are 10A; derating for 4-6 conductors is 80% & 7-9 conductors is 70%. In article 402, ampacity is 7A for 18AWG wire.

I no longer work with a U/L panel shop and don't have any of their books. However, I recall that in many instances their allowable panelboard ampacities were higher than the NEC.

I looked at the post from Lynlynch and compared its ampacities to the NEC's allowed overcurrent protection for 14 or 12AWG conductors. I believe that those ampacities should be multiplied by 0.5 for your application, prior to applying any derating for multiple conductors. Therefore, 17*0.5*0.8=6.8 so 7A would be OK to me.

The ultimate answer to your question should be from the view of the temperature rating of the conductor insulation, the allowable voltage drop for your application, and the environment in which it is being used. At the distances involved and 125VDC you quote, I believe voltage drop is OK.

I didn't have time this AM to look carefully at the General Cable catalog, but did see that C2425A was a 80°C cable and that they had versions that were 90°C.

My "final answer" is use 90°C cable and ensure that it has appropriate overcurrent protection.

--John M

I decided to rewire the relay contacts with 14AWG wire. Just a little teadious since we have 60-80 wires to pull and terminate, but better to be safe than sorry.

I consulted with a UL inspector and he just stated the same thing you said. He said do the research or re-wire with 14AWG.

Rather than do a research project to determine and figure out if we were ok. It would take 1hour or 2hour to re-wire the panel.

The UL inspector told you to research the NEC? Jeebus...

The NEC has to do with the INSTALLATION. UL rules have to do with the equipment. They do not have to (and frequently don't) agree. You need to simply look up the UL listed current rating of the wire. Ampacities of Control Circuit wires are in UL508A Table 38.1

Current is current, it knows not from AC or DC. In fact, since technically current in an AC conductor flows mostly on the surface of the conductor, the core serves mainly as a heat sink. With DC, the entire cross sectional area conducts and there are no eddy currents. So if anything, a wire can carry more current with DC than it can with AC. But nobody uses that fact, it provides an added safety factor.

Forgot to add: 18ga in a multi-conductor cable = 7A

Another word of caution: wire doesn't know from AC or DC ampacity, but CONTACTS definitely do! Double check your relay to make sure that it is rated for 125VDC. Just because it may be 300VAC does not mean the DC voltage rating is anywhere near! I have seen 400A AC contactors that have a DC current rating of only fractions of an amp.

I believe, the ampere rating should be written on the shield of your cable; if so, just use it. NEC is more about construction, buildings, etc.

A really useful reference can be found at the following site from Anixter (major supplier of wire & cable):

http://www.anixter.com/AXECOM/US.NSF/TECHNOLOGY/TheWireandCableTechnicalInformationHandbook_WIRECABLE?OpenDocument&Division=DivTab5

This is their "Wire and Cable Technical Information Handbook" - see chapter 7 for your question, Figure 7.3. It applies specifically to electronic / multi-conductor cables, and allows you to determine allowable ampacity depending on gauge & # of conductors in your cable. You didn't mention how many conductors in your multi-conductor cable, but even for 4-5, the derating factor is 0.8 and 18ga. (assuming 60 deg rating of cable) would not allow you 7 amps.