View Full Version : O:T:Cable ampacity

The Plc Kid
June 9th, 2009, 03:23 PM
Today when selecting some mc cable for a new project i was looking at the ampacity tables for the cable and the way it is listed they show the ampacity of the cable increases with temperature rise?

I am just checking myself because i am still very green but i thought it was the other way around. That ampacity decreases with temp rise?

here is the document i was looking at http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet263

Does southwire have it wrong or do i have it wrong?

June 9th, 2009, 03:41 PM

At first glance you are right

BUT you gotta look at them funny little marks in these tables and read fine print below.

In South wire they call for a 3 cond #1 at 130 amp at 75C and 150 at 90C.
The reason current is higher is that the 90 column is for cable rated at 90. the reason for this is the insulation is more temperature tolerant ie wont melt as easy.

Go to NEC and compare the same conductor in conduit vs in free air. Current allowed in free air is higher - MORE COOLING basically.

ALSO go to NEC and compare same size wire for TW and THHN.

ALSO IF for that compressor
Spend a few more bucks use conduit ideally rigid. If you lose a conductor you should be able to pull out old and install new conductor. With cable you have to dig the whole mess up and buy new cable.
I've made a few bucks on direct buried cable and not very many on buried conduit. Leads me to believe conduit is less prone to breakage and conductor breakdown. Conduit is NOT immune to large trackhoes, buldozers nor dynamite but you cannot have everything I guess.
Dan Bentler

The Plc Kid
June 9th, 2009, 04:17 PM

We are going to put pvc or rigid conduit in the ground from the load side of the disconnect but the mc cable is going from the mcc through a wall and then run horizontal across the wall for about 15 feet to an LB that goes into the back of the outdoor disconnect.

I would use conduit but it would be very,very difficult to use conduit where this has to run.

I would not say it is a tight space but it is too tight of a space to work with conduit.

I was involved as a helper wiring some very large and well known buildings in atlanta ,georgia and we did mc cable direct into the slab no conduit. I just hope it never fails or the basket ball court will have to come up.

June 9th, 2009, 04:25 PM
Interior with existing conduit ducting piping does get to be a challenge. In this case sometimes cable is the only choice
UNLESS you have a power bender and you are GOOD at it.


June 9th, 2009, 05:50 PM
Kid, lemme see if I can help explain.

The amperage capacity does not rise as temperature [of the cable] rises.

Rather, there are different amp capacity listed vs temperature of the cable and all it is connected to.

So, looking at the 90 C column you may see 100 amps, 75 C column you may see 80 amps, 60 C column you may see 50 amps.

The cable itself can carry 100 amps and it's temperature in free air will not exceed 90C (if air is 30C), but if any thing it is connected to is only rated at 60C, then you have to limit (derate) the maximum current usage to 50 amps.

You have to use the lowest temperature in all parts that the circuit is used in, from main breaker to utilization device.

BTW, I have not said temperature RISE,as I think it is the actual temperate in the tables.

Hope that helps a bit.

June 10th, 2009, 06:28 AM
If you cannot use conduit, I'd strongly suggest going with a continuously armored and jacketed cable!