600SL
Well-known member
I will be installing a 8500W heater on a 40A CB. According to this it looks like I can use #10 THHN or THWN-2. But according to Wire & Cable Your Way its only good for 30 A. So which is it?
Wire Ampacity , Temperature Ratings.
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theoldwizard1
Well-known member
First time I have seen a table that allows 25A on a 14AWG wire ! (yes, THWN-2 or XHHW-2 in conduit) So what does the * say ?
The terminals that your wire connects to have to be good for the same temperature you use to rate your wire.
You aren't going to find 90° terminations for this. Hopefully you will have 75° terminations and you can use that column.
You aren't going to find 90° terminations for this. Hopefully you will have 75° terminations and you can use that column.
Chuckster in NJ
Well-known member
THIS! ^^^^^
READ the specs on all your equipment and lugs.
Been that way for decades.First time I have seen a table that allows 25A on a 14AWG wire ! (yes, THWN-2 or XHHW-2 in conduit) So what does the * say ?
The * points to a code item that states ocp can't be greater than 15, 20, 30a respectively. Motor and welder ckts excluded.
It should state the spec in the instructions or on the terminals. I wouldn't expect lower than 75c in a heater........but check.
Worst case, wire with 60c rated conductors and you're covered.
theoldwizard1
Well-known member
So if it says the Over Current Protection device (breaker/fuse) has a lower limit, than why publish this ????
micromind
Well-known member
I will be installing a 8500W heater on a 40A CB. According to this it looks like I can use #10 THHN or THWN-2. But according to Wire & Cable Your Way its only good for 30 A. So which is it?
This is table 310.60 in the code.
240.4(D) modifies this table in that it states; Small Conductors. Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent protection shall not exceed 15 amps for 14AWG, 20 amps for 12AWG and 30 amps for 10AWG copper.
240.4(E) is about tap conductors, part of this is why we can get away with using smaller wire to supply light fixtures.
240(G) is about HVAC, motors and a few others, again, allowing smaller conductors than the breaker is rated for.
As noted, the 60C or 75C column is used almost all of the time but the 90C column can be used when derating for multiple wires in a conduit.
Side note......334.80 states that the ampacity of type NM cable (Romex) is based on the 60C column.
micromind
Well-known member
Table 250.122 in the code states that a #10 CU can be used as a ground for circuits up to and including 60 amps.
100 amps is #8, 200 amps is #6, 6000 amps is 800MCM.......
For things like lights and receptacles you are limited with #12 to 20A and the same for 15A and 30A circuits.
There are things that does not apply to so the table is reasonable. Run a wire to an air compressor and you are not limited to the 60* ampacity.
The code is not intended to be a instruction manual...
because these are the numbers you use when derating for temperature, etc.
What if it doesn't?
The installation details assume certain conditions in place. It is up to you as the installer to understand the existing hardware limitations for the branch circuit you intend to install. Vintage of the existing equipment can vary by user.
If the stars line up, then you can use a smaller conductor. If not, use a larger conductor.
Instructions "always" expect installation by a qualified electrician who knows this.
Ampacity derating is explained along with max ocp values permitted. Agreed, a little convoluted, but it is written well enough that it can be followed.
As stated NEC 70 is not a training manual.
Search the the web for this topic and credible sources have white papers/tech docs explaining the logic.
As stated NEC 70 is not a training manual.
Search the the web for this topic and credible sources have white papers/tech docs explaining the logic.
wyliesdiesels
Well-known member
you will not find 9ga wire. you need #8 THWN wire as previously stated (44a rated or higher) and yes #10 grnd is kosher. If you go with NM-b you will need #6