What size OCD?

[websparky]

Hi Guys,

There is a disscussion at another forum site about the code requirements for this circuit;

This is an appliance circuit with one receptacle.This appliance has a nameplate that says the load is 18.75A at 120V. The load is NOT continuous. This is not a trick question.

According to the code, what is the required OCD for this appliance?

 

[templdl]

Re: What size OCD?

Did you size the wire? You should size the wire per 2002 NEC art 220. Then protect the wire size applied as included in 2002 NEC art 240.

 

[websparky]

Re: What size OCD?

Well, the wire size is based on how you will compute the OCD size. Sure, 12AWG will carry the load but, will a 20A OCD carry the appliance? If so, what section in the code tells us what is the minimum required circuit size based on the 18.75 non-continuous load? If 20A will not carry the load, then what size will?

I'm looking for code references to validate an answer. Really, I'm not kidding, I just can not find a reference myself.

 

[don_resqcapt19]

Re: What size OCD?

Dave,
Both 210.19(A) and 422.10(A) say that the minimum ampacity of the circuit is 18.75 amps. The OCPD would be 20 amps.
Don

 

[websparky]

Re: What size OCD?

Hi Don,

That's kind of what I thought.
It seems odd though, because I would have picked a 30A OCD and a 30A receptacle to ensure that the 20A ocd would not trip from time to time or during a time when there may be a voltage drop on the entire system. It just seems like 18.75A is too close to 20A.

Am I right to figure it at 30A if it were a continuous load?

Which way would you size it if the nameplate said "minimum 20A - maximum 30A"?

 

[booman]

Re: What size OCD?

I believe 422.11(E)(3) would be the correct code rule to apply here. I assume that since you did not mention it, that the overcurrent protection rating is not listed on the appliance or in the installation instructions. Since the appliance is rated over 13.3 amperes, the overcurrent device should not exceed 150 percent of the appliance rated current. Since 150 percent of the 18.75 amp appliance does not correspond to a standard overcurrent device ampere rating, you are allowed to go to the next higher standard rating. Which I believe a 30 amp overcurrent protection device would be appropriate.

 

[gwz2]

Re: What size OCD?

The appliance is probably ' LISTED '. If so, what does the instructions tell?

If the appliance is provided with a plug and cord, I would use that to determine the receptacle and OCPD.

Otherwise, contact the Listing organization. They came be helpful.

gwz2

 

[templdl]

Re: What size OCD?

I agree Don. My intent was to keep the answer simple. If the 18.75a load did fall under the definition of a noncontinuous load in sizing the wire under art. 220 then #12 wire protected with a 20a breaker per art. 240 would apply. However, one could add belts and suspenders to it be applying #10 wire with a 30at breaker.
As far as a concern for a 20at breaker being to close to 18.75a and causing nuisance tripping as mentioned by websparky I wouldn't be concerned about it. If the breaker were to trip thermally it would be protecting the wire and as such both the wire and the breaker would have been sized incorrectly in the first place which they haven't been. If the breaker trips instantaneously, it would be tripping because of a load inrush requirement which really is related not related to the wire size but the breaker's magnetic calibration.
Knowing the characteristic of a breaker is a great help in the continuing attempt to stay out of an application problem.
As a bench mark for comparison the NEMA AB4 values for inverse time trip test for 0-30at breakers require that the maximum trip time for a 0-30at breaker equal to or less than 250v trip in 50 seconds or less. If the breaker did trip thermally one could get a pretty good idea of the current timing the trip and comparing it the breaker trip curve.
Would the overcurrent condition last long enough to trip the breaker?
As far as instantaneous (magnetic) triping, a benchmark value would be 10x the rating of the breaker. However, small frame breakers trip calibrations can be all over the map and it is not uncommon that a given family of breakers with different rating to have the same instantaneous calibration for the entire group, that is breaker from 15-30at are the same, 40-70at trip the same, etc. Also, some manufacturers may call a breaker "high magnetic" where that same breaker magnetic rating is standard with another manufacture who has a "low magnetic breaker" which may be a standard for the other manufacturer.

 

[charlie b]

Re: What size OCD?

Let me try to directly answer the question that I think you are really asking. You only ?code answer? is going to be 20 amps, as Don has already said. The rest of your concerns go beyond the code, and are design issues. I would probably go with the 30 amp circuit and the #10, for the same reasons you have named. But that would be a design decision on my part, and not an issue of code compliance.

 

[tom baker]

Re: What size OCD?

"you are allowed to go to the next higher standard rating. Which I believe a 30 amp overcurrent protection device would be appropriate"
The next standard OCPD is 25 amperes. See section 240.6(A)

 

[iwire]

Re: What size OCD?

OK guys lots of great answers, the appliance is on my job now.

It is a commercial coffee maker.

The plans showed four locations for coffee maker outlets.

The panel schedule showed 30 amp 2 pole breakers for these, no cut sheet or specs on the equipment.

I pulled 2 hots and a ground 10 awg THHN from panel to outlet boxes.

The units show up and have labels that say 240-208/120 18.75 amps.

The instruction book says 20 amp minimum, 30 amp, maximum circuit size.

I open one up and has a sticker in the way of the terminal strip that says use 12 awg 20 amp circuit, copper conductors only.

Well after adding the neutrals I need, I had to decide on a cord, plug and outlet for these units.

I have 2 pole 20s on the job so I could have switched but stayed with the 30s. (I never think of 25 amp breakers)

I used L14-30 plugs and outlets with 10/4 cord,

I thought I could only pull 16 amps off a 20 amp outlet, Table 210.21(B)(2)

But as these are single outlets it turns out I could use them to 20 amps.

[ November 12, 2003, 03:48 AM: Message edited by: iwire ]

 

[websparky]

Re: What size OCD?

My thanks to all that replied!

Charlie's answer is the one I was going for as to the code requirements. I thought that there was a section that I was overlooking.
When I saw the 18.75A, I figured a 30A OCD and etc. as a correct sizing. Then I wondered where the "code" was in my decision!

Thanks again,

 

[iwire]

Re: What size OCD?

OK guys I think I found a requirement that forces this circuit (18.75 amps) to have at least a 25 amp breaker and a 30 amp outlet and plug.

What do you all think?

210.23 Permissible Loads.

(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. The rating of any one cord-and-plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating.

 

[bill addiss]

Re: What size OCD?

(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.

Bob,

I have a question to toss in here ... How does an 1875 watt Hairdryer get used on a 15 amp circuit?

Bill

 

[gwz2]

Re: What size OCD?

Bill,

Quickly. ie, a short period of time so the OCPD does not see the small over-load.

gwz2

 

[iwire]

Re: What size OCD?

Bill I asked the same question before and never really got an answer.

 

[websparky]

Re: What size OCD?

Hi Bob,
I think the key here is that this reference applies to only "two or more".

(bold added)

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.

 

[iwire]

Re: What size OCD?

Yeah I see that now Dave.

That still leaves us with the 1875 watts on a bathroom duplex.

Bob

 

[gwz2]

Re: What size OCD?

I was just trying to answer Bill's question.

Probably a better answer would be " because the plug fits the receptacle ".

I do not want to imply that my last two post's are NEC compliant, just comments about what happens in-the-field.

A violation is always a violation until corrected.

A driver running a red-light and there was not any one to see the incident, that is just what it was, and that violation is over-with and the driver cannot go back and correct the violation.

An NEC violation is always a violation until corrected. Hopefully a catastrophe will not happen in the mean-time.

gwz2

 

[roger]

Re: What size OCD?

I'm still living in the past and don't use hairdryers. (yes the wife does but is not an 1875 watt)

Shouldn't the NEMA,UL,ETL,CSA, etc... listing or standard mandate this type load be equipped with a 20 amp cord cap?

In reality it is probably like my 120 volt 20 amp served table saw, that boasts a 3HP peak capacity. (it is based on OCPD trip averages at a set period of seconds)


Roger

[ November 13, 2003, 06:24 PM: Message edited by: roger ]