Wire gauge and amperage

[ls02]

What amperage is Romex #8 copper wire? On some charts I see 40 A and on some I see 50 A.

[RBM]

In a NM cable #8 uses the 60 degree figure, which is 40 amp

[jamesgangnc]

Individual wires you could use 8. There are a number of calculators out there on the web that will help with wire selection. Distance is a factor as well. Plus what's at the other end. Motors that start under a load like a big hvac compressor may need bigger wire simply to prevent to much voltage drop at urge current.

[ls02]

The wire is used to connect sub-panel to mail panel. The distance is

3'.

[trader4]

In the case of general loads and Romex, it's 8 gauge for 40. With an HVAC load, the rules change. The circuit has to be wire sized to at least the minimum specd by the manufacturer. The breaker has to be sized between the min and max specd by the manufacturer. This leads to installations that are correct and to code, but that would be a violation of the rules for general purpose branch circuits.

As an example, if you have an AC unit rated for 35 amp min circuit and rated for a breaker of 40amp min, 60 max, you could have a 60 amp breaker using 8 gauge wire. The reasoning to this is that:

A - the AC has a high starting current that doesn't last

B- the AC has it's own built-in over current protection and is the only device on that circuit.

[RBM]

The wire is used to connect sub-panel to mail panel. The distance is

3'.

Run a 3' length of 1" PVC between the panels and use #8 THHN copper on a 50 amp breaker

[gfretwell]

If this is only 3 feet, why cheap out on the wire. Put in #2 and you can set a 100a sub panel. You may never need it but you have it for virtually no additional expense.

[RBM]

A moment after I hit the send button, I had that exact thought

[Art Todesco]

Actually, you could put 100 amps through an 8 gauge wire ... not for very long. It's all a matter of heating and insulation and how many wires are packing into a tight (conduit) space. That's why we have NEC which specifies the current for different sizes and types of wires and how they are routed.

[Evan]

Umm no, the OP asked the rating of #8 Romex wire...

Some have responded that #8 THHN individual wires are rated slightly higher...

Are you unable to actually read the entire thread before adding your own "wisdom"...

~~ Evan

[jamesgangnc]

.

Same response I gave. I just added other factors that might affect wire choice. Now that we know it's 3 feet to a subpanel there's not much reason not to use bigger wire since there would be no noticable cost difference.

[bud--]

Wire have many different kinds of electrical insulation. Some kinds of insulation can be used at various higher temperatures. That allows a higher current rating - higher current produces more heat. Romex is limited to the current for 60 degree insulation, which for #8 is 40A. (There further details you don't want to know about.)

There is a limitation (110.14-C-1) on "ampacity" rating of the wire based on the terminals the wire is connected to.

for circuits 100A and less, or terminals marked #14 through #1, the 60 degree ampacity of the wire is used.

for circuits over 100A, or terminals marked larger than #1, the 75 degree ampacity of the wire is used (unless the wire is rated 60 degrees)

In both cases wires of a higher temperature rating can be used, but the current allowed is limited to the 60 or 75 degree column.

In both cases the wire can be used at a higher current if both terminal devices are marked for a higher temperature.

I expect you know all that, but some may not.

Long time since I looked at a breaker - a new 20A SquareD breaker is marked for 60 or 75 degree wire, which means the 75 degree column can be used for 90 degree rated THHN. If a 50A breaker is similarly marked, the usable rating for #8 THHN is 50 amps, as stated.

Are most new breakers marked for 75 degrees these days? Is this enforced?

Presumably connections to old breakers that aren't marked would hit the

60/75 degree temp limit.

[gfretwell]

The only problem with that is 240.4(D) will limit you to 12ga or larger wire on a 20a breaker so you can't even use the 60c column in

310.16

[bud--]

The only recent breaker I have available is the 20A SquareD. Shouldn't have commented on 75 degree for that one. (They even foot note the 310 tables for up to #10.) The question is whether a 50A breaker is rated for 75 degree wire. If a 20 is I expect a 50A SquareD is.

More generally are breakers 40-100A often rated for 75 degree (or maybe even 90 degree) wire?

Are breakers over 100A rated for 90 degree wire?

Anything general that can be said about lugs and other terminals?

Kind of annoying to use THHN and not be able to use the 90 degree ampacity (other than derating).

Do you tag for wire (larger than #10) used at an ampacity over the terminal rating? (Does anyone take the time to figure out the ratings?)

[gfretwell]

Most breakers are rated 75c. I am not sure I have ever seen a 90c breaker and you will be hard pressed to find a load end device rated

90c. There are a number of straight through connectors rated 90c but I think that is just to avoid derating confusion in condolets.