If I have a circuit that draws 21amps what size breaker would I use 20 amp 25 amp 30 amp or 40 amp also is there a book that can help me with these kind of questions

[QUOTE=mike albrecht;3168383]if I have a circuit that draws 21amps what size breaker would I use 20 amp 25 amp 30 amp or 40 amp also is there a book that can help me with these kind of questions a math book with electrical calculations or wire calculations.

Is that a continuous draw or peak draw? You should only load your breaker at 80% of its rating even though it is designed to handle 100%, NEC wants only 80% for safety. You need a 30 amp breaker. http://static.schneider-electric.us/docs/Circuit%20Protection/0600DB0101.pdf You may also wish to copy/paste this http://www.groverelectric.com/howto/08_How%20to%20Calculate%20Breaker%20Wire%20Size%20&%20Wattage.pdf for future reference.

if i have a circuit that draws 21amps what size breaker would i use 20 amp 25 amp 30 amp or 40 amp also is there a book that can help me with these kind of questionsIt all depends on the circuit and what it is used for. If this is a general use receptacle circuit then it cannot be on a larger than 20A breaker.

What is on this circuit?

Is that a continuous draw or peak draw? You should only load your breaker at 80% of its rating even though it is designed to handle 100%, NEC wants only 80% for safety.Sorry, but this is simply not true. There is no "80% for safety" rule, or even rule of thumb.

A properly wired circuit and breaker is designed to handle 100% of it's capacity. There are rules that limit use to 80%, such as water heaters, electric heat and continuous loads. But there is no blanket 80% rule.

If you look at general 15, 20 and 30 amp circuits you'll see that the wire is already over sized compared to it's actual ampacity.
See 240.4(D)

if i have a circuit that draws 21amps what size breaker would i use 20 amp 25 amp 30 amp or 40 amp also is there a book that can help me with these kind of questions

You might want to describe the specifics (i.e. what type of circuit this is, what appliance(s) are being used, etc.) so that the best answer can be given.

Usually, wire size to breakers is as follows:
20 amp: 12 AWG
30 amp: 10 AWG
40 amp: 8 AWG
However, this all depends on the circumstances of the circuit.

Here is more info for you to read/save http://www.fs.fed.us/database/acad/elec/greenbook/3_basicdesigns.pdf

Here is more info for you to read/save http://www.fs.fed.us/database/acad/elec/greenbook/3_basicdesigns.pdf
No thanks. There is so much misinformation in there it's not funny.
I like to get my facts from the source, not some summary someone wrote.

See Table 210.24 for more accurate summary information.

210.23 reads "(1) Cord- and Plug-Connected Equipment. The rating
of any one cord- and plug-connected utilization equipment
shall not exceed 80 percent of the branch-circuit ampere rating
unless listed and marked to inform the user of the necessity
for providing an individual branch circuit.
(2) Utilization Equipment Fastened in Place. The total
rating of utilization equipment fastened in place, other than
luminaires (lighting fixtures), shall not exceed 50 percent of
the branch-circuit ampere rating where lighting units, cordand
plug-connected utilization equipment not fastened in
place, or both, are also supplied. (ROP 2-1, 2-142a)
(B) 30-Ampere Branch Circuits. A 30-ampere branch
circuit shall be permitted to supply fixed lighting units with
heavy-duty lampholders in other than a dwelling unit(s) or
utilization equipment in any occupancy. A rating of any one
cord- and plug-connected utilization equipment shall not
exceed 80 percent of the branch-circuit ampere rating." How would we understand this to not be 80%.

Read it again, and don't use logic. You canot use logic when reading the NEC.

It says:
The rating
Of any one cord- and plug-connected utilization equipment
Shall not exceed 80 percent of the branch-circuit ampere rating

This particular 80% limitation is for any ONE PIECE OF EQUIPMENT plugged into a general use circuit. NOT the whole circuit.

Here is the NEC Table 210.24:
http://code.necplus.org/pdfs/7011/Table_210.24.pdf

See where it says "Maximum Load"?

I guess we need the OP to more clearly tell us what he has on this circuit. I see your point about one cord/plug but when you said there was no 80%, I thought I had read about one. It may or may not apply but it does exist.

My point was that there is no blanket across the board 80% rule, like that site you linked says, and many other people say. It is simply not true.

Sure, there are plenty of other specific 80% rules out there, but none like at the beginning of that forestry service site.

I just saw this,


Read it again, and don't use logic. You canot use logic when reading the NEC.

So true.

Some confusion comes from the NEC where it states "shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load." that is applied in various locations. This statement comes from the UL listing of circuit breakers.

From the 2003 UL white book

CIRCUIT BREAKERS, MOLDED-CASE AND CIRCUIT BREAKER ENCLOSURES (DIVQ)

MAXIMUM LOAD
Unless otherwise marked, circuit breakers should not be loaded to exceed 80 percent of their current rating, where in normal operation the load will continue for 3 hours or more.


This is difficult to apply to the typical general purpose lighting circuits. Since no one really knows how the circuit will be used or rarely used more than 3 hours.

From NEC 2008 Edition:

Article 210
II. Branch-Circuit Ratings

210.19 Conductors — Minimum Ampacity and Size.

(A) Branch Circuits Not More Than 600 Volts

(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.

And:

210.20 Overcurrent Protection.
Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies
With 210.20(A) through (D).

(A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.


While Table 210.24 (http://code.necplus.org/pdfs/7011/Table_210.24.pdf) states Maximum Load under each circuit rating column, each column refers to specific paragraphs of Section 210.23 A thru D

210.23 Permissible Loads.
In no case shall the load exceed the branch-circuit ampere rating. An individual branch circuit shall be permitted to supply any load for which it is rated. A branch circuit supplying two or more outlets or receptacles shall supply only the loads specified according to its size as specified in 210.23(A) through (D) and as summarized in 210.24 and Table 210.24.

(A) 15- and 20-Ampere Branch Circuits.
A 15- or 20- ampere branch circuit shall be permitted to supply lighting units or other utilization equipment, or a combination of both, and shall comply with 210.23(A)(1) and (A)(2)

(1) Cord-and-Plug-Connected Equipment Not Fastened in Place.
The rating of any one cord-and-plug-connected utilization equipment not fastened in place shall not exceed 80 percent of the branch-circuit ampere rating.

(2) Utilization Equipment Fastened in Place.
The total rating of utilization equipment fastened in place, other than luminaires, shall not exceed 50 percent of the branch circuit ampere rating where lighting units, cord-and-plug connected utilization equipment not fastened in place, or both, are also supplied.

This is why we always need to know the purpose of the circuit.


Unless the circuit is a general purpose lighting circuit, the circuit should not be loaded more than 80%.

Thanks TKS, a little more complicated than much of what I deal with. It would help if the OP would answer my questions.

Sometimes, and I am not sure how to solve this, often we, those answering questions, end up conversing with each other, which is fine, but tends to get off topic, or a difference of opinion, or interpretation of Code, and the person seeking help is bored with those going back and forth, or just goes away.

And I am guilty also.

Somehow we need to figure out how to answer questions and have our discussions elsewhere, and the get them best answer.

Any idea?

Anyone?

This goes on in many other forums also, and I wish I had the solution.

You may have noticed, I try to answer each question with more questions before I try to give the solution.

Can we try that?

Don't everyone answer or comment at once.

I'm not the original poster, but I very much enjoy and learn from the expert back and forth discussions.

In my first post I asked a question but to this day never received an answer from the OP. Yes that is a good approach but I think I need to stop responding when the OP does not come back with more info.