percentage of current allowed on a 20 amp breaker.

[korthawks]

i am hearing from my supervisor that i can only have 80% of a 20 circuit breaker under load. in other words he is saying
that the most amount of current on a 120v 20a breaker that has #12 feeding the load can only hold 16 amps....

i have not heard this pryor to him telling me this and would like very much to find it in code. HELP :blink:

thanks,
kort hawks

 

[augie47]

There are limitations on continuous loading/ See 210.19(A)(1)

 

[infinity]

An individual branch circuit with non-continuous loads you can have a load equal to the CB size. For continuous it drops down to 80% of the CB rating. Also for other applications take a look at 210.23.

 

[crtemp]

An individual branch circuit with non-continuous loads you can have a load equal to the CB size. For continuous it drops down to 80% of the CB rating. Also for other applications take a look at 210.23.

I though continuous loads were calculated at 125% (or 75% of the circuit breaker rating)
example would be a 20 amp 240 volt wall heater circuit could only have 3,600 watts (240x20=4,800. 4,800x.75=3,600)

 

[infinity]

The inverse of 125% is 80% so in your example a continuous load of 3840 watts @240 volts (4800 * 80%) is permitted on a 20 amp circuit.

 

[crtemp]

The inverse of 125% is 80% so in your example a continuous load of 3840 watts @240 volts (4800 * 80%) is permitted on a 20 amp circuit.

That's weird. I never knew that. I always just figured the since the breaker had to be 125% of the load the it would only make sense to calculate it at 75%. Maybe I'm completely wrong about the whole thing though. Lol. Learn something everyday!

 

[Smart $]

An individual branch circuit with non-continuous loads you can have a load equal to the CB size. ...

There are instances where derated conductor ampacity can be less than the ocpd rating (but not less than the next smaller standard ocpd rating). See 240.4(B). For example, with a 20A-rated circuit, the derated conductor ampacity can be as low as 16A. The continuous load is not factored by 125% for this determination. See 210.19(A)(1), first sentence

 

[GoldDigger]

That's weird. I never knew that. I always just figured the since the breaker had to be 125% of the load the it would only make sense to calculate it at 75%. Maybe I'm completely wrong about the whole thing though. Lol. Learn something everyday!

The way I prefer to convince myself of it is that if I increase something by 25% (multiply by 1.25) and then decrease the result by 25% (multiply by .75) I do not get 1.

This means, among other things, that if you mark the price of an item up 25% and then discount it by 25% the end price is lower than the original price.

The closer the percentage factor gets to zero, the closer the result of the combined operations is to 1.00.

1.2 times .8 equals .96, while 1.1 times .9 equals .99, and 1.01 times .99 equals .9999 exactly.

 

[crtemp]

The way I prefer to convince myself of it is that if I increase something by 25% (multiply by 1.25) and then decrease the result by 25% (multiply by .75) I do not get 1.

This means, among other things, that if you mark the price of an item up 25% and then discount it by 25% the end price is lower than the original price.

The closer the percentage factor gets to zero, the closer the result of the combined operations is to 1.00.

1.2 times .8 equals .96, while 1.1 times .9 equals .99, and 1.01 times .99 equals .9999 exactly.

Crazy. So I need to just multiply continuous load by 80%? (20 amp x 240 volts = 4800 watts)

(For wall heaters)
4800 watts x 80%=3840 watts total allowed on this circuit (at least for a heater load). Is this true with any continuous load? (just multiply by .8) (sorry I don't have my code book near by to confirm.)

 

[ActionDave]

. Is this true with any continuous load? (just multiply by .8) (sorry I don't have my code book near by to confirm.)

Pretty much.

210.19 Conductors ? Minimum Ampacity
and Size
(A) Branch Circuits Not More Than 600 Volts.

(1) General. Branch-circuit conductors shall have an am-
pacity 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 am-
pacity not less than the noncontinuous load plus 125 percent
of the continuous load.

210.20 Overcurrent Protection

Branch-circuit conductors and equipment shall be protected
by overcurrent protective devices that have a rating or set-
ting 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.

 

[templdl]

These discussions have always intrigued me. It's like getting the cart before the horse. Start with the load, the wire is sized to carry the load. Isn't the the wire sized based upon 125% of the continuous load plus 100% of the noncontinuous load. Based upon the 75degC column of table 310-16 the wire must carry that load. Then the breaker is selected to PROTECT the wire. As such the breaker would be naturaly sized based upon 80% of the continuous load. Why do people focus in on the 80% rating of the breaker when if the wire is sized correctly an the breaker rating based upon the wire size (not the load) the 80% rating of a breaker is a none issue.

 

[infinity]

These discussions have always intrigued me. It's like getting the cart before the horse. Start with the load, the wire is sized to carry the load. Isn't the the wire sized based upon 125% of the continuous load plus 100% of the noncontinuous load. Based upon the 75degC column of table 310-16 the wire must carry that load. Then the breaker is selected to PROTECT the wire. As such the breaker would be naturaly sized based upon 80% of the continuous load. Why do people focus in on the 80% rating of the breaker when if the wire is sized correctly an the breaker rating based upon the wire size (not the load) the 80% rating of a breaker is a none issue.

What if the load is only non-continuous?

 

[Smart $]

What if the load is only non-continuous?

That and breaker is required to be sized not less than non-continuous load plus 125% of continuous load... same as required for wire size (before the application of adjustment and correction factors).

 

[infinity]

I think that we're all saying the same thing. A 20 amp non-continuous load, on a 20 amp OCPD, with a 20 amp conductor is code compliant.

 

[suemarkp]

As long as it is hard wired or a single receptacle individual branch circuit. If you have a multi outlet circuit (one or more duplex receptacles or outlets), no one load can be more than 80% of the circuit rating {210.21(B)(2) and 210.23(A)(1)}.

 

[infinity]

As long as it is hard wired or a single receptacle individual branch circuit. If you have a multi outlet circuit (one or more duplex receptacles or outlets), no one load can be more than 80% of the circuit rating {210.21(B)(2) and 210.23(A)(1)}.

I agree and I was just responding to the OP in stating that not every instance requires a max of 80% load on an OCPD.

 

[Rick Christopherson]

Just to throw one more wrench into the mix, you are (I believe) permitted to load a circuit breaker to 100% even with continuous loads if the circuit breaker is rated for 100% load. I don't recall where this is written. Maybe someone can find it.

 

[Smart $]

As long as it is hard wired or a single receptacle individual branch circuit. If you have a multi outlet circuit (one or more duplex receptacles or outlets), no one load can be more than 80% of the circuit rating {210.21(B)(2) and 210.23(A)(1)}.

I too agree, but that doesn't prevent the total load from being more than 80% (any combination of non-continuous plus 125% continuous). Actually, there is really nothing in the NEC which prevents a multiple receptacle circuit from being overloaded in the real world. The OCPD is supposed to handle that. I should also note that for a multiple receptacle circuit, the conductor's derated ampacity cannot be less than the circuit breaker rating that I mentioned earlier.

 

[Smart $]

Just to throw one more wrench into the mix, you are (I believe) permitted to load a circuit breaker to 100% even with continuous loads if the circuit breaker is rated for 100% load. I don't recall where this is written. Maybe someone can find it.

For branch circuits it's 210.19(A)(1) Exception. However, it is not just the circuit breaker that has to be rated for 100% operation, it is the entire assembly: breaker, bussing, terminals, enclosure, etc.

 

[templdl]

I too agree, but that doesn't prevent the total load from being more than 80% (any combination of non-continuous plus 125% continuous). Actually, there is really nothing in the NEC which prevents a multiple receptacle circuit from being overloaded in the real world. The OCPD is supposed to handle that. I should also note that for a multiple receptacle circuit, the conductor's derated ampacity cannot be less than the circuit breaker rating that I mentioned earlier.

To add one more thing, remember that the breaker is calibrated at 40degC. Dealers are often located in 25 degC or even less ambient temperature which essentially moves the trip curve to the right. Thus a 20at breaker will end up being a 22at breaker, a 15at breaker 17at.
Often times it is thought that this is treated as an exact science when there are so many variables that one has to wonder how the wire is ever protected even when wire derating factors are taken into consideration.