conductor sizing

[Smart $]

would you mine elaborating on this maybe with an example?

47A load.
28A noncontinuous portion.
19A continuous portion.
75?C terminations.
90?C-rated CU conductors.
100?F ambient.
6 CCC in conduit.

SIZE: 28A + 125% ? 19A = 52A

Minimum #6 @ 65A (#8 is 50A)
Minimum 60A standard OCPD rating

AMPACITY: 47A ? 0.91 ? 80% = 65A

Minimum #6 @ 75A (#8 is 55A)

75A ? 0.91 ? 80% = 55A

#6 @ 55A is protected by 60A OCPD per 240.4(B).

 

[electrofelon]

47A load.
28A noncontinuous portion.
19A continuous portion.
75?C terminations.
90?C-rated CU conductors.
100?F ambient.
6 CCC in conduit.

SIZE: 28A + 125% ? 19A = 52A

Minimum #6 @ 65A (#8 is 50A)
Minimum 60A standard OCPD rating

AMPACITY: 47A ? 0.91 ? 80% = 65A

Minimum #6 @ 75A (#8 is 55A)

75A ? 0.91 ? 80% = 55A

#6 @ 55A is protected by 60A OCPD per 240.4(B).

Thanks. I think I get the same thing as you but you seem to go about it a different way which confuses me. also I guess I dont have to derate continuous loads much. I think I have always done this correctly but the more I am thinking about it the more I am confusing myself.....So it boils down to that you need a conductor that after ampacity adjustment from the 90 degree column, is rated great than or equal to 100% of the non continuous load and 125% of the continuous correct? Then the maximum breaker size (ignoring next size up) is the number you got after derating from the 90 degree column correct?

 

[Smart $]

...you need a conductor that after ampacity adjustment from the 90 degree column, is rated great than or equal to 100% of the non continuous load and 100% of the continuous...

FIFY

Then the maximum breaker size (ignoring next size up) is the number you got after derating from the 90 degree column correct?

No. The minimum breaker rating is noncontinuous plus 125% continuous... same as minimum conductor SIZE.

 

[electrofelon]

FIFY



No. The minimum breaker rating is noncontinuous plus 125% continuous... same as minimum conductor SIZE.

Thanks. Ok so ignoring next size up rule and say I can get any breaker size I want, In your example, 52 amp would be minimum size breaker. What is the maximum size breaker? Is it the lesser of 1) the result of ampacity adjustment from the 90 degree column 2) the 75 degree column ampacity ?

(say non motor, non AC type circuit)

 

[Smart $]

Thanks. Ok so ignoring next size up rule and say I can get any breaker size I want, In your example, 52 amp would be minimum size breaker. What is the maximum size breaker? Is it the lesser of 1) the result of ampacity adjustment from the 90 degree column 2) the 75 degree column ampacity ?

(say non motor, non AC type circuit)

Maximum breaker ratings are limited by circuit rating. [210.3, 210.23 for general applications]

 

[electrofelon]

Maximum breaker ratings are limited by circuit rating. [210.3, 210.23 for general applications]

?? 240.4 states "conductors......shall be protected against overcurrent in accordance with their ampacities specified in 310.15...."

So I get kinda lost in the legalese of 310.15 so when I am derating, what is the ampacity I am protecting with an OCPD? Is it the lesser of the 90 degree column derated value and the 75 degree rating (for terminations assuming they are 75 degree)? Assume a non 240.4(G) install...

 

[Smart $]

?? 240.4 states "conductors......shall be protected against overcurrent in accordance with their ampacities specified in 310.15...."

So I get kinda lost in the legalese of 310.15 so when I am derating, what is the ampacity I am protecting with an OCPD? Is it the lesser of the 90 degree column derated value and the 75 degree rating (for terminations assuming they are 75 degree)? Assume a non 240.4(G) install...

Conductor ampacity is conductor ampacity. That's what the OCPD must protect.

The ampere values you use for terminal temperature limitations are only for the purpose of establishing the minimum conductor SIZE... which has nothing to do with AMPACITY.

Three parts to conductor determination:

• Minimum SIZE
• Minimum AMPACITY
• Maximum-permitted PROTECTION

Only the latter two are interdependent.

 

[Carultch]

Thanks. Ok so ignoring next size up rule and say I can get any breaker size I want, In your example, 52 amp would be minimum size breaker. What is the maximum size breaker? Is it the lesser of 1) the result of ampacity adjustment from the 90 degree column 2) the 75 degree column ampacity ?

(say non motor, non AC type circuit)

Obviously, you have to pick a size breaker that you can find. And you'll have a tough time finding a 52 Amp breaker. You need to round up to a standard size overcurrent device per 240.6, which in this case would be 60A.

The maximum OCPD size would usually be dictated by the load you connect, such that its internal circuits are also protected by it (if needed). This is common for receptacles and HVAC equipment, that you'll have a maximum OCPD.

The breaker size also drives the wire ampacity. The wire must have at least enough ampacity (at conditions of use derate factors) to round up to the next standard size. For instance, if the wire has 53 A after conditions of use, and you are using it in an application requiring 52A, you can use it, and you can consider it protected by the 60A overcurrent device. 240.4(B)

If you want to install a 70A overcurrent device instead of the minimum OCPD of 60A, then you have to have at least 60.0001A of wire. Both in terms of terminaton rating and wire ampacity at conditions of use. I'm not really sure how many nanoamps of wire ampacity you need to exceed 60A in order to use a 70A OCPD. This actually happened to me, where I had a 100A fusible disconnect left over from a previous job that I wanted to use on a 60A application. The minimum fuse size for it is 70A, so therefore I needed at least a #4 wire (with an 0.76 conditions of use derate for being in direct sunlight).

For devices exceeding 800A, you have to have as much ampacity as the overcurrent device rating (if not more). Next size rule doesn't apply. The rationale for this is that the overcurrent devices are much more sparse in the high ratings.

 

[electrofelon]

Conductor ampacity is conductor ampacity. That's what the OCPD must protect.

That is my question: what is ampacity? Is this a correct statement:

Ampacity is either, 1) the value given in 310.15(B)(16) under the column that the insulation is rated at (or required to be rated at by another code article) if there is no derating from conditions of use, or 2) the value given in 310.15(B)(16) under the column that the insulation is rated at multiplied by the applicable derating factors. (assume 310.15(B)(16) is the applicable chart for the installation)

In other words, ampacity is the number from the appropriate chart (i.e. 310.15(B)(16) adjusted for derating factor/conditions of use (if applicable). Correct? Maybe sounds like a stupid question, this thread just really confused me.

The ampere values you use for terminal temperature limitations are only for the purpose of establishing the minimum conductor SIZE... which has nothing to do with AMPACITY.
Three parts to conductor determination:

• Minimum SIZE
• Minimum AMPACITY
• Maximum-permitted PROTECTION

Only the latter two are interdependent.

Got it, makes sense.

I really appreciate the help guys...really... but....Can I get an answer to my question? I word a question in a way that makes sense to me and answering the question directly rather than rewording it, adding additional information, and never really answering it, is the easiest way for me to make sure I have it straight:

What is the maximum size breaker (for Smarts example)? Is it the lesser of:
1) the number resulting after applying the adjustment factors to the 90 degree column ampacity
2) the 75 degree column ampacity (termination limitation) ?

(Ignore any NEC or manufacturer equipment protection requirements, and next size up rule)

 

[Smart $]

That is my question: what is ampacity? Is this a correct statement:

Ampacity is either, 1) the value given in 310.15(B)(16) under the column that the insulation is rated at (or required to be rated at by another code article) if there is no derating from conditions of use, or 2) the value given in 310.15(B)(16) under the column that the insulation is rated at multiplied by the applicable derating factors. (assume 310.15(B)(16) is the applicable chart for the installation)

In other words, ampacity is the number from the appropriate chart (i.e. 310.15(B)(16) adjusted for derating factor/conditions of use (if applicable). Correct? Maybe sounds like a stupid question, this thread just really confused me.

Correct. When in doubt, resort to the Article 100 definition.

Got it, makes sense.

I really appreciate the help guys...really... but....Can I get an answer to my question? I word a question in a way that makes sense to me and answering the question directly rather than rewording it, adding additional information, and never really answering it, is the easiest way for me to make sure I have it straight:

What is the maximum size breaker (for Smarts example)? Is it the lesser of:
1) the number resulting after applying the adjustment factors to the 90 degree column ampacity
2) the 75 degree column ampacity (termination limitation) ?

(Ignore any NEC or manufacturer equipment protection requirements, and next size up rule)

Are you sure? Given the follow up question, I have to wonder.

Your approach can be likened to testimony or lie-detector questioning, where subject is limited to answering only yes or no to a 'gray-topic' question.

Correct (...where a 90?C-rated conductor is used, and under imposed limitations ).

 

[electrofelon]

Your approach can be likened to testimony or lie-detector questioning, where subject is limited to answering only yes or no to a 'gray-topic' question.

Smart $, have you stopped beating your wife? Yes or NO?

When in doubt, resort to the Article 100 definition.

Good call, I forget about those sometimes...

What is the maximum size breaker (for Smarts example)? Is it the lesser of:
1) the number resulting after applying the adjustment factors to the 90 degree column ampacity
2) the 75 degree column ampacity (termination limitation) ?

(Ignore any NEC or manufacturer equipment protection requirements, and next size up rule)

Ok so #1 is the answer to your example. What I meant to say in hindsight, was not "in your example" but "in general." So In general, that statement is valid and the maximum breaker is the lesser of either #1 or #2 right?

 

[Smart $]

Ok so #1 is the answer to your example. What I meant to say in hindsight, was not "in your example" but "in general." So In general, that statement is valid and the maximum breaker is the lesser of either #1 or #2 right?

No.

For applications where Code does not permit a higher rating, the maximum breaker rating is equal to the conductor ampacity value.

 

[electrofelon]

No.

For applications where Code does not permit a higher rating, the maximum breaker rating is equal to the conductor ampacity value.

Are you sure? Read the proposition again.

What is the maximum size breaker (in general)? Is it the lesser of:
1) the number resulting after applying the adjustment factors to the 90 degree column ampacity
2) the 75 degree column ampacity (termination limitation) ?

(Ignore any NEC or manufacturer equipment protection requirements, and next size up rule)

 

[Smart $]

Are you sure? Read the proposition again.

Yes, I'm absolutely certain.

You might be confusing this with the maximum load capacity (in amperes) being the lesser of the two.

 

[electrofelon]

Yes, I'm absolutely certain.

You might be confusing this with the maximum load capacity (in amperes) being the lesser of the two.

Really....so the breaker can be larger than the terminal ampacity (even w/o next size up) as long as load isnt? I may never have known/thought about that....

Example: (assuming every value is a standard size breaker)
#6 thwn-2, say no deratings apply. 65 amp non continuous load, 75 amp breaker ok?
#6 thwn-2, deratings apply and adjust ampacity to 70 amps. 65 amp non continuous load, 70 amp breaker ok?
#6 thwn-2, deratings apply and adjust ampacity to 15 amps. 100 amp breaker ok if Im not getting it inspected? :lol:

 

[Smart $]

Really....so the breaker can be larger than the terminal ampacity (even w/o next size up) as long as load isnt? I may never have known/thought about that....

Example: (assuming every value is a standard size breaker)
#6 thwn-2, say no deratings apply. 65 amp non continuous load, 75 amp breaker ok?
#6 thwn-2, deratings apply and adjust ampacity to 70 amps. 65 amp non continuous load, 70 amp breaker ok?
#6 thwn-2, deratings apply and adjust ampacity to 15 amps. 100 amp breaker ok if Im not getting it inspected? :lol:

No to one of 'em. :angel:

 

[electrofelon]

No to one of 'em. :angel:

thanks for all the help.

 

[david luchini]

Really....so the breaker can be larger than the terminal ampacity (even w/o next size up) as long as load isnt? I may never have known/thought about that....

Example: (assuming every value is a standard size breaker)
#6 thwn-2, say no deratings apply. 65 amp non continuous load, 75 amp breaker ok?
#6 thwn-2, deratings apply and adjust ampacity to 70 amps. 65 amp non continuous load, 70 amp breaker ok?
#6 thwn-2, deratings apply and adjust ampacity to 15 amps. 100 amp breaker ok if Im not getting it inspected? :lol:

I think the section you are looking for is 110.14(C).

You cannot protect a #6 thwn-2 with a 75A breaker where the terminations are rated 75C.

 

[Smart $]

I think the section you are looking for is 110.14(C).

You cannot protect a #6 thwn-2 with a 75A breaker where the terminations are rated 75C.

As electrofelon asked me, are you sure?

(IMO) Breaker rating has no bearing on meeting the terminal temperature limitation. That is solely determined by load (NC+125%C). In the example presented, a 65A noncontinuous load value equals the 75?C column value of #6 copper (also 65A).

What I can agree with is no load can be added to this circuit.

 

[david luchini]

Breaker rating has no bearing on meeting the terminal temperature limitation.

Didn't say it did. I said terminal temperature rating has a bearing on the selection of ampacity, per 110.14(C).

A conductor that has an ampacity of 65 cannot be protected at 75A, as 70A is the next standard size up.