# What is the ampacity for 8 AWG wire? 50 or 40

Every electrician I ask tell me different story
This is important since wire are very expensive
I am need to connect an air-condition rated 240 50 Amp 3 wires (2 hot and ground)

According to the NEC
Table 310.16 Allowable Ampacities of Insulated Conductors Rated 0 Through 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F)

Types RHW, THHW, THW, THWN, XHHW, USE, ZW

For 60°C (140°F) is 40 amp

And for 75°C (167°F) is 50 amp

What is this temperature? Is it the temperature in the attic where I put the wire?

Also from the NEC 2002 book
8 AWG THHN, 90°C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60°C. This same 8 AWG THHN, 90°C wire is limited to 50 Amp

Can someone tell me if I can use 8 AWG THHN
My contact: Electrical contractor, Electrician Santa Monica CA, Venice CA, Mar Vista, Culver City Westchester CA
gil@amorelectric.com
310-699-9444

Thank you

 tkrussell Posts: 9,673, Reputation: 3698 Senior Electrical & Lighting Expert #2 Jul 8, 2007, 07:08 AM

The insanity! It is enough to make you run screaming into the night.

This is why electricians get paid so much, and not just to pull wire.

The code is in a logical order, so start by reviewing Section 110.14(C) Temperature Limitations.

You will discover that it is the temp rating of equipment terminations, in subsection#1 and separate wire connectors, in subsection #2, that dictates, one, which type of insulation to use for a specific piece of equipment, and two, what the ampacity of wire will be with that combination of insulation and equipment or termination .

When a job is designed, first to be chosen is the equipment,ie: panelboard, circuit breakers, disconnect, meter sockets, switches, receptacles, etc. Manufacturers will refer to 110.14 to rate their terminations to be UL listed.

Section 110.14 will then dictate to designers and installers how to rate the ampacity of the wire/insulation chosen.

So, before any wire is chosen, you must check the specs of the equipment being installed.

Here is the specs for a popular range receptacle, for example:
Product Datasheet -- HBL9450A

I used a 50 amp to get higher than #10 wire, as there are other limitations on #14, #12, and #10 ampacities per Sec 240.4(D).

Note the temp rating is 75 Deg C, so the wire connected can not be rated at the ampacity not exceeding the 75D C column of Table 311.16. A popular wire is THHN/THWN with a max rating of 90 Deg C, so #8 THHN at 90 Deg C is rated 55 amps, however I can only use #8 THHN at 50 amps because the device is rated 75 Deg C.

You mention a disconnect with a temp rating of 60 Deg C.. Then the same wire #8 THHN, which has a max rating of 55 amps at 90 Deg C, and I can rate it at 50 amps if I connect to the range outlet, can only be rated at 60 D C if I connect to the disconnect you have.

Look at page 56 of the following Square D brochure:
http://ecatalog.squared.com/pubs/Ele...0110PL9401.pdf

This is for a popular 60 amp disconnect, note the rating is a max of 75 Deg C. see footnote under Terminal Data.

OK now let me really confuse you.

Next is the max rating of certain cable assemblies. If you are using Romex cable, AKA NM-B. See Section 334.80.

Even thou NM-B has THHN wire and is rated at 90 Deg C, Sec 334.80 limits the ampacity of NM-B to the 60 Deg C Column of Table 310.16, which, of course, is 40 Amps.

So this is why you get all different answers. Clear as mud. At least to many electricians.

Oh, the attic you mention? Now refer to the Correction Factors at the bottom of Table 310.16. If the area a wire/cable will be installed in will be a high temperature, now apply these multipliers to lower the ampacity of a wire/insulation. If the rating of a certain wire ends up lower, you may need to increase the wire size.

If, for example, the #8 THHN wire you protect with a 40 amp breaker because of the 60 De C terminals at the disconnect must run through an area that the temperature will reach 160 Deg F, ( an exaggeration), multiply 40 amps by .41 and now the amp rating of this wire is only 16.4 amps!

I hope I have been able to clear this up for you without adding more confusion.

So to finally answer your question, no,#8 THHN cannot be used on a 50 amp circuit when using a disconnect that is only 60 Deg C because of Section 110.14 (1) (a) (2).
 gilamor Posts: 8, Reputation: 5 New Member #3 Jul 8, 2007, 01:27 PM
Actually I didn’t copy past the whole paragraph from the NEC book
The whole paragraph is:

Section 110.14(C)(1)(a) requires that conductor terminations, as well as conductors, be rated for the operating temperature of the circuit. For example, the load on an 8 AWG THHN, 90°C copper wire is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60°C. This same 8 AWG THHN, 90°C wire is limited to 50 amperes where connected to a fusible switch with terminals rated at 75°C. The conductor ampacities were selected from Table 310.16. Not only does this requirement apply to conductor terminations of breakers and fusible switches, but the equipment enclosure must also permit terminations above 60°C. Exhibit 110.5 shows an example of termination temperature markings.

My question:
What is the “switch with terminals rated at 75°C “?
Is it the disconnect near the air-condition? Or the breaker on the panel?
I guess I don’t understand why the rating of a terminal would have any effect on the ampacity of the wire?
By the way I am not running an NM cable I am running 3 single conductors THHN (2*8 awg and ground 10 awg) in ¾ flex. And I already run the wire and preferably don’t want to change the wire to 6 AWG
What do you mean by terminations (lugs)
Is it the connection to the breaker and the lugs on the disconnect box?
Also what is the ambient temperature for an attic in sant monica ca

My contact

Electrical contractor, Electrician Santa Monica CA, Venice CA, Mar Vista, Culver City Westchester CA
gil@amorelectric.com

Thank you
 tkrussell Posts: 9,673, Reputation: 3698 Senior Electrical & Lighting Expert #4 Jul 8, 2007, 01:55 PM

What is the “switch with terminals rated at 75°C “?

I used a Square D fusible switch as an example to show a 60 amp disconnect that has 75 Deg C terminals.

Is it the disconnect near the air-condition? Or the breaker on the panel?

Any terminal and wire connector in the circuit is subjected to the rating of the circuit, both the breaker and any disconnect switch must be considered.Don't forget to check the line terminals inside the actual air conditioner

I guess I don’t understand why the rating of a terminal would have any effect on the ampacity of the wire?

Any terminal is mounted and/or supported by some sort of insulating material, ie: plastic, bakelite, etc. It is this material that will be affected by any overheating.

By the way I am not running an NM cable I am running 3 single conductors THHN (2*8 awg and ground 10 awg) in ¾ flex.

OK, I just wanted to expand on the info I was providing, since NM-B is very popular, and to add to the confusion NM-B is furnished with 90 Deg C insulation.

And I already run the wire and preferably don’t want to change the wire to 6 AWG

Then you will need to change to a disconnect that has 75 Deg C rated terminals. You can refer to the one I provided.

What do you mean by terminations (lugs)

Yes, any mechanical device that connects to a bare wire. "Termination" refers to the terminal that a wire is connected to. "Lug" is slang for terminal.

Is it the connection to the breaker and the lugs on the disconnect box?

Yes.

Also what is the ambient temperature for an attic in sant monica ca

Beats the hell out of me. I know I have been in attics in New England that have gotten to over 100 Deg F in the summer.

 gilamor Posts: 8, Reputation: 5 New Member #5 Jul 8, 2007, 04:22 PM
Thank you very much. I actually checked the disconnect it is the FP222R
on page 35
Square D 60 amp 75 degrees c
The wire is 8 AWG 90 c and the max temp in the attic is 104 f or 40 c (it is near the beach never get that hot)
55*0.91=50.05 so I am OK
also the rating for the air-conditioned is about 30Amp maybe when it is starting it takes 50 amp for a short time
 tkrussell Posts: 9,673, Reputation: 3698 Senior Electrical & Lighting Expert #6 Jul 8, 2007, 05:40 PM

Are you knowledgeable about Article 440 Air Conditioning & Refrigeration of the NEC? This explains how to size the branch circuit wiring and overcurrent protection for HVAC.
 gilamor Posts: 8, Reputation: 5 New Member #7 Jul 8, 2007, 09:42 PM
I read the article and didn’t see any violations to the way I am installing the unit. The guy who install the unit told me that fusible disconnect is not required. I will check it tomorrow when installing the disconnect. Should I know something in particular?
 tkrussell Posts: 9,673, Reputation: 3698 Senior Electrical & Lighting Expert #8 Jul 9, 2007, 02:41 AM
No ,nothing in particular, just your comment about it needing 50 amps to start for a short time was curious. Appears you did not realize why a large breaker was needed. Judging by your website, I would think you are familiar with refrigeration characteristics.

Right, a fusible switch is not needed, just an example I chose.

If the breaker in the panel is HACR rated you should be fine. Did not mean to imply a fusible switch is needed.
 gilamor Posts: 8, Reputation: 5 New Member #9 Jul 9, 2007, 08:13 PM
I actually called the manufacture American Standard and they told me to put fusible disconnect. I put a fusible disconnect even though I believe they don't know what they are talking about.
How do you know if the breaker is HACR? By the way after I connected the air conditioned and turn it on full power the air conditioned consume only 13 amp per phase. I don't get it whey the name plate said 50 amp min.
 tkrussell Posts: 9,673, Reputation: 3698 Senior Electrical & Lighting Expert #10 Jul 15, 2007, 06:14 AM
Missed your last response with questions.

You did exactly what should be done, follow the manufacturer's instructions.

A manufacturer will always err on the side of caution, to eliminate product liability issues. Be sure they know what they are doing. I am not sure your being fair to them, since it appears your not sure yourself, and apparently not familiar with HVAC codes.

HACR rating will be found in the manufacturers specifications and "HACR" will be stamped on the circuit breaker. This means that the overcurrent protection device has the characteristics that will allow a refrigeration unit to start. All motors have what is called Locked Rotor Amperage, which is the large amount of current a motor will draw upon starting under load.

All refrigeration compressors start under load, and more so that a fan or fluid pump, due to the immersion in refrigeration gas under pressure.

The LRA for a compressor can exceeding 6 to 10 times the running amps. Have you noted that a compressor can have an overcurrent protection device rated at a minimum of 175% of the nameplate current, up to a maximum of 225%?

Since the codes in Article 440 are and have been a bit confusing, several years ago the manufacturers had been mandated to state the minimum circuit size and the maximum OCPD on their nameplates.

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