The last sentence of 422.10(A) allows the branch circuit of even a single household range to be sized according to table 220.19. I used to think that table (and section of the code) were only meant to be used for service load calculations. In essence, a single range rated at 75A (according to the marking on unit) can be hooked up with 50A wire and have 50A OCP. I realize that rarely is the unit going to be operated at full load but why is this allowed? It is like hooking up a group of motors and ignoring a couple dozen FLA’s just because "rarely" will all motors be used at once. Are the numbers listed on ranges overly optimistic on how much draw they have or what?

If you weren't designing like this, you would end up with huge wires and an 80A OCP. The range heats up pretty quick, and the load falls after that: Either the thermostats or the user turns the oven and rings down.

Sure, you could overload the circuit: Keep the oven open and boil water on all rings. You will have to replace the boiling water with cold water at regular intervals. The kitchen will be lika a sauna. To damage the wiring, not just tripping the breaker, you will have to keep doing this for quite some time.

The wire has absolutely no risk of being damaged. The OCP is sized for the wire and will protect it. That is not what my concern is. My point is, in most any other circumstance, if a piece of equipment has a nameplate current rating of "X" amps, how can the NEC dictate some lesser rating? Please refer to my motor example and explain the difference. I also have a more direct question. If a range has a tag, which says it will draw 75A at full load does it actually draw that much with all elements on? If so, how can a 50A breaker hold this load? It is very conceivable that a person cooking for a large group (perhaps a family get-together) will use all four burners and the oven at the same time. Will the OCP allow such an action dependably? If I just had my new range installed and properly wired up by a licensed electrician, I would be VERY upset if the code allowed my breaker to trip! I am led to believe that the VA's listed on ranges must be quite inaccurate or 220.19 would not be used for branch circuits. To be clear, I totally understand 220.19's use for service load calculations. Using it to size the wire and breaker for a single range in a household seems to go against everything else in the code book!

Triple
when refering to 220.19 from 422.10, there is a note # 4. Did you read this note? Maybe it will help clear this up for you.

Pierre

When I prepare guys for taking the test, I point out that in Part II of Article 220, Note 4 to Table 220.19 is for branch-circuit loads, the rest of Part II is for Feeders and Services.

Pierre

To get 75A you will have to turn on everything at the same time. Having everything turned on at the same time isn't enough, since the oven will have a low current draw once it is warm.

Most, if not all, types of overcurrent protection allows a current exceeding the rating for a limited period of time. The wires will take a significant time to become warm. Ordinary breakers have a thermal element that heats up, just like the wires. When it gets too hot, the breaker trips.

Without access to trip curves for American breakers, I can't give you any exact figures. But I would guess that you can take out about 125% of the capacity for half an hour or more. For a few minutes, I'd expect the breaker to allow 150%.

Remeber that you are starting with the breaker cold when you turn on the range to get the maximum load.


I on the other hand would be upset if the electrician oversized the wiring, since it costs me money. There are other things in life to spend money on than wire. I don't mind the breaker tripping sometimes. Even if it trips once a week, it will take a century or so before it wears out.

C-H

I can think of no time that the NEC allows us to intentionally use a breaker smaller than the load served.

Most times we must wire for 125% of the nameplate current.

I do agree the breakers will allow higher than the marked rating for a period of time, but the NEC does not.

As far as over sizing circuits I think this is common in the NEC.

When you do a service calculation per NEC you come up with a much larger service than the POCO knows you need.

Bob




[This message has been edited by iwire (edited 08-22-2003).]

Yes, I had read note 4 to the table before posting this topic. I understood WHAT the code was saying but I didn't understand WHY. Where else in the code is this type of potential overloading allowed?

triple

The heating elements in ranges are constantly turning on and off. For the cooktop portion this is controlled by the hi-low control. If you turn the control to low it turns the element on for a short time and then off for a long time then back on for short time to maintain the low temperature. If you set the control to hi it will turn the element on for a longer period of time then off for a short time then back on. The over element runs the longest time during the initial heat up of the oven but after that it normally is only on for very short periods of time. If you were to measure the load on a range circuit the only time you would ever see it draw the nameplate rating is if the oven and all burner elements were turned on from a cold start but this would only last a very short time before the elements started cycling on and off.

A perfect example of where the NEC permits loads in excess of the branch circuit conductors ratings would be for welders (Article 630)

Curt

OK, you got me with the welders I had forgotten about those.

But welders are one of the items listed in Table 240.4(G) "Specific Conductor Applications", I see nothing about ranges or appliances.

You are right that the load is not continuous but other than welders I can not think of a place you can undersize conductors and breakers.

Motors can have a larger breaker than the conductor is rated for but the conductors still must exceed the load.

I will keep an open mind here as I have been wrong before and I can always learn something new.

Bob

Hi Bob

The reason ranges are not listed in 240.4 is because you are still require to protect the branch circuit conductors at their ampacity rating. If you run #8 60C conductors to a range you must protect them with a 40 amp overcurrent device. The conductors listed in 240.4(G) are generally allowed to be protected by branch circuit overcurrent devices exceeding their rating.

I don't know how the demand factor for ranges ever came about or any of the demand factors for dwelling units for that matter. I do know that the seem to work. I have wired many ranges to 40 amp branch circuits and have never had a call back because of a tripping breaker. There are also probably over a million apartments in this country supplied with 50 or 60 amp feeders that have 12kw ranges in them.

My guess is the NEC looks at ranges similarly to the way it does welders. The full load nameplate rating will never be on the branch circuit very long so running full sized branch circuit is unnecessary. The reduced branch circuit can easily handle carrying more than its rating for short periods of time.

Curt

Curt, OK you have converted me, I have learned something, thanks.

When I think of demand factors I think of more than 1 unit and the chances of them all running at the same time.

This really blows me away for two reasons.

1)It goes against most of the NEC rules for OCP, conductor and receptacle selection

2)It is a blatant use of common sense by the NEC.

Bob