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

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Fer Lefer

Member
gilamor said:
I am need to connect an air-condition rated 240 50 Amp 3 wires (2 hot and ground)
Can someone tell me if I can use 8 AWG THHN
I am running 3 single conductors THHN in ? flex (2*8 awg and ground 10 awg).
Click to expand...

OK, introducing example 3A of NEC-2005:

You have to make TWO different calculations in order to get the answer. One is the ampacity due to the terminals at which is connected the cable. The other one is due to the installation conditions... and No, I'm not going to say nothing about voltage drop :D

Ampacity due to the terminals: Let's first assume that the device's MLA is 50A (covering 440.32 or 440.33). The OCPD should be less or equal than 100A (1.75 x 50 = 87.5 A, 440.22). Let's choose a 70A OCPD (OK, it all depends on the nameplate). A 70A OCPD could be listed as 60/75?C, so we can either columns in the tables...but, we don't have the temperature rating of the air-conditioner. So, we have to choose the conductor from the 60?C column: Is the air-conditioner going to be used more than 3 hours? then, we calculate 1.25x50A = 62.5A.

From Table 310.16, it should be 4 AWG (70A)

If the air-conditioner is going to be taken as a non-continuos load, then we choose a 6 AWG (55A), enough to carry 50A.

Ampacity due to installation conditions:

In this part of the calc?s is where we ask about how are the wires and conduit going to be installed: 3 wires in conduit, and let's assume that we're going to be between 31-35?C (87-95?F). So, we take into account the derating factors: # conductors <4 so the derating factor due to the number of wires is 1.00 (no derating) and for a THHN wire (90?C), the temperature derating factor is 0.96, from table 310.16. This ampacity shall be: 50A/(1.00 x 0.96) = 52.08A. Selecting from the 90?C column, we see that 8 AWG, can take this (55A)

Bad news, amigo m?o: we have to choose the largest one between 4 AWG and 8 AWG (or 6 AWG and 8 AWG). And there is another calc for the neutral... :rolleyes:

This is just an application of this example 3A as a guide, The next link guides to a clearer explanation. Hope it will be useful: :wink:

http://www.iaei.org/subscriber/magazine/03_d/magazine_03d_hartwell.htm

Saludos
 
J

JohnConnolly

Senior Member
Location
Phoenix AZ
Fer Lefer said:
OK, introducing example 3A of NEC-2005:

You have to make TWO different calculations in order to get the answer. One is the ampacity due to the terminals at which is connected the cable. The other one is due to the installation conditions... and No, I'm not going to say nothing about voltage drop :D

Ampacity due to the terminals: Let's first assume that the device's MLA is 50A (covering 440.32 or 440.33). The OCPD should be less or equal than 100A (1.75 x 50 = 87.5 A, 440.22). Let's choose a 70A OCPD (OK, it all depends on the nameplate). A 70A OCPD could be listed as 60/75?C, so we can either columns in the tables...but, we don't have the temperature rating of the air-conditioner. So, we have to choose the conductor from the 60?C column: Is the air-conditioner going to be used more than 3 hours? then, we calculate 1.25x50A = 62.5A.

From Table 310.16, it should be 4 AWG (70A)

If the air-conditioner is going to be taken as a non-continuos load, then we choose a 6 AWG (55A), enough to carry 50A.

Ampacity due to installation conditions:

In this part of the calc?s is where we ask about how are the wires and conduit going to be installed: 3 wires in conduit, and let's assume that we're going to be between 31-35?C (87-95?F). So, we take into account the derating factors: # conductors <4 so the derating factor due to the number of wires is 1.00 (no derating) and for a THHN wire (90?C), the temperature derating factor is 0.96, from table 310.16. This ampacity shall be: 50A/(1.00 x 0.96) = 52.08A. Selecting from the 90?C column, we see that 8 AWG, can take this (55A)

Bad news, amigo m?o: we have to choose the largest one between 4 AWG and 8 AWG (or 6 AWG and 8 AWG). And there is another calc for the neutral... :rolleyes:

This is just an application of this example 3A as a guide, The next link guides to a clearer explanation. Hope it will be useful: :wink:

http://www.iaei.org/subscriber/magazine/03_d/magazine_03d_hartwell.htm

Saludos
Click to expand...


I see a lot of words but did you answer his question? :)
 
M

megloff11x

Senior Member
The NFPA gave me a disclaimed opinion once that you must go by worst-case temperature derating. Why they think this is a 30C/86F world is beyond me. Attics can easily hit 140+ F. It was 116F in Vegas last week. I can't imagine an attic there.

Personally I think they should make the nominal table ratings for 50C/122F, and make a new derate table from there, that way those who miss this key point will still have something closer to safely designed.

THERE'S NO LAW AGAINST FATTER WIRES other than cost and maybe it won't fit on the terminals. It's cheaper than what the fire will cost.

I won't get started on how many motors I've had where the terminals are two or more wire sizes smaller than their UL listed current rating requires.

Matt
 
F

Fer Lefer

Member
JohnConnolly said:
I see a lot of words but did you answer his question? :)
Click to expand...

And you had to quote them all!!!!! :D

Didn't I answer? My fault: you need, at least a 4AWG or a 6AWG to wire your air-conditioner (Depending on many things!)

The ampacity of a 8AWG THHN (90?C), connected to a device that doesn't say anything about its temperature rating, shall be taken from the 60?C column: 40A if it feeds a non-continuous load, or 40/1.25 = 32A if connected to a continuous one.

And the ampacity of a 8 AWG THHN (90?C), , due to the installation conditions? 55A x 0.96 x 1.00 = 52.8A

:grin: Served!
 
infinity

infinity

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You can't really properly answer the question since he didn't say what the 50 ampere rating is. Is this the MCA or the maximum OCPD size?
 
stickboy1375

stickboy1375

Senior Member
Location
Litchfield, CT
infinity said:
You can't really properly answer the question since he didn't say what the 50 ampere rating is. Is this the MCA or the maximum OCPD size?
Click to expand...

I bet its the maximum breaker, and the minimum is a 25 amp breaker... so # 10 THHN would be fine... but it's just a guess since I can't see the damn thing...
 
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J

JohnConnolly

Senior Member
Location
Phoenix AZ
And the ampacity of a 8 AWG THHN (90?C), , due to the installation conditions? 55A x 0.96 x 1.00 = 52.8A
Click to expand...

Isn't 52.8 more than 50?



Don't answer, I'll just get more confused.

I am going to take up drywall so I don't have to think anymore.
 
infinity

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
JohnConnolly said:
Isn't 52.8 more than 50?



Don't answer, I'll just get more confused.

I am going to take up drywall so I don't have to think anymore.
Click to expand...


You're basing this on the assumption that were talking about the maximum OCPD and not the minimum conductor ampacity. For AC units the MCA is used to size the conductors not the max OCPD.
 
radiopet

radiopet

Senior Member
Location
Spotsylvania, VA
hmm...I have seen this question before...lol....This unit should have a nameplate on it with the Mininum circuit size and maximum OCPD size listed on it.
 
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