i was just reading another topic from another forum about having 2 30a 220v receptacles on one circuit (welder and compressor) on one 30 amp breaker.(not being used at the same time0).Is this a safe practice? To my knowledge this shouldnt be done regardless if you dont operate at the same time....what do you think?

Why not?
I don't know of anything that says you can't have 2 receptacle outlets on a 30a circuit.

Depends on the load.... 210.21+210.23

And no, over-all I don't think it a safe or wise practice. There is no guarantee of only one in use at a time. One device cycles automaticaly, the other is a welder. See 630... Although a welder is intermitant, and it is allowed to have multiple welders on the same circuit the OCP and wire size rules are expanded, and differ slightly from a normal branch circuit.

Without knowing the data plates and circuit particulars, I would say, no.

Double pole - double throw switch?

All of us install multiple receptacle outlets on 15 and 20 amp circuits all the time with no idea at all what will be plugged into them.

Why would this practice suddenly become unsafe once we move to 30 amp circuits?

Article 630 gives us some options for saving money when supplying welders but we do not have to use those options, we can supply a welder with a 30 amp cord cap with a 'standard' 30 amp branch circuit.

I see no NEC reason not to do this, it in my opinion is a design choice.

If the person paying the bill tells me they are fine with one circuit I would have no problem supplying them with one circuit once I explained the circuit may trip if both units operate at the same time.

Bob

I agree with Bob.
If both units are used at the same time... the circuit breaker will open...exactly what the NEC wants to happen... but the customer, if not warned will not be too happy.

Assuming the nameplates require similar circuits, I see no problem.

BUT- that's a big assumption. The two appliances are treated quite differently as to wire sizing and over-current protection; Just because they have the same plug is no guarantee. Nevertheless, in Harry Homowners' garage, where Harry is going to putz around solo, it ought to work out OK.
Change that to a guy trying to run a business in his garage....and my answer will change.

I am fixin' to do exactly the same thing with my welder and a new pressure cleaner I just ordered. In a practical sense I know I could unplug one and plug in the other but I am going to install another receptacle.

I did the same thing at a big honkin' laboratory complex a few years ago. Each lab had a piece of equipment that the staff would roll to different places in the lab based on their particular research project. There were 3 possible locations, but each lab only had 1 machine...1 circuit, 3 receptacles.
Worked great.
No problems...Code or otherwise.

Please tell an innocent European why two appliances with the same voltage and amp draw would need different wire? I assume with some loads you could downsize some for cycling, but generally 30 amps are 30 amps, right?
Trip characteristics are a different piece of cake of course.

I think mainly it's a design thing - we usually put our CEE receptacles on dedicated circuits, so they can be loaded to their full ampacity.

To play devil's advocate here:

In a commercial, or as stated above, lab environment, I would have no problem with this setup.

But in a home, I picture the frustrated homeowner driving over to the big box store and having the following conversation with the "expert":

Harry Homeowner: "Can you help me out? I have a welder and compressor in the garage and they keep tripping the circuit breaker."

Eddie Expert: "What size is the breaker?"

HH: "30 amps."

EE: "No problem. All you need to do is change it to 50 amps. We have those right here."

HH: "I'll take it."

One $25 breaker and possibly a house fire later, problem solved in both counts.

Admittedly, one would have to have a big welder and big compressor running full-tilt at the same time to cause significant heating of the {assumed} #10 feeder. But the repeated stress of overloading can trigger insulation failure down the road.

So, I would prefer to run two circuits just to be sure. If you have to lay pipe or fish NM to the location anyway, upsized pipe or two runs is not a lot more expense or labor than one run or pipe size.

Good question and the simple answer is they do not.

The NEC has a section devoted to welders that can be used if you want to use smaller conductors than would normally be used with a specific overcurrent device.

Basically they allow us to protect a conductor above its rated ampacity based on the duty cycle of the welder.

That does not mean we have to do it that way.

Bob

"All of us install multiple receptacle outlets on 15 and 20 amp circuits all the time with no idea at all what will be plugged into them."

Speak for yourself Bob... Doing Commercial Kitchen right now and asking the usual questions about equipment schedule, and use of certain areas. Knowing full well when the job gets trimmed out, I'll get the question about those "round single outlets", and why they can't be duplexes or quads. ('Cause the Inspector will want all of the equipment in place at trim out, and will look at all data plates...)

MX I could make that argument for every circuit in the panel. They also sell 30a single pole breakers that could get put on that pesky 14 ga circuit "that keeps tripping when we hook up the xmas lights".

If the wire was properly protected when it was installed and inspected I don't think we have any control over what another installer *might* do.

You do not install any branch circuits with more than one receptacle outlet?

Or do you just go back to each job and keep tabs on what gets plugged in after you leave?



Bob

iwire: If, rather than being cord and plug connected loads, the welder and compressor were both _hard wired_ onto the same circuit, would your answer be different?

My opinion is that if it can be reasonably anticipated that 210.19(A)(1) will be violated, then the installation is not up to code. This may not be enforceable, because as you note you can't determine ahead of time what will be plugged into the receptacles, but having 50A of load on a 30A circuit _is_ a code violation.

This does require a judgement: Do you as the installer think that the customer will be serious about not operating the two loads at the same time.

I know many people for whom the standard of circuit overload is 'does the breaker trip'. In the case of a welder and compressor on the same circuit, both very intermittent loads, I would not be surprised if you could have both devices 'on' and work with both effectively at the same time (welding, then using a grinder, then going back to welding, etc) without tripping the breaker too frequently......

-Jon

Bob, I ask the questions up front: (You've said you do the same...)
Use of space? (Electricaly)
Types of equipment, voltage, wattage, amperage...
And have the conversation up front, a little customer edjucation, about circuit limitations. Over-loads, nuisense tripping, loss of income due to outage because of those things... (The latter usually sells a circuit or two.) And when I come back, for that circuit thats tripped, they get the 'I told you so' when I find the extra commercial toaster, 3 20A printers, or the space heater under every other desk. Or the 20 extra track heads.... If it doesn't trip.... They were forewarned, conscience clean...

You are right, its a design choice, not a very good one IMO.

On 15-20a circuits its easy to assume that most types of general use items will only be a few amps. Could take 10-20 items to even get to 15A. On 30 amp equipment, you already know it is at least approaching 20a as a load. So with 2 items like that you know you are already approaching 40A right? Easy to assume without seeing a data plate> See 210.21(B)(2).

Jon, did you mean 210.20(A)(OCP-and a pretty clear violation for the reason stated above), on 210.19 (Conductors) you could just over-size the wire...


[This message has been edited by e57 (edited 03-25-2006).]

"Small conductors" get significant additional protection from 240.4(D). That is precisely because the installer has no control over what the user will plug in.
Bear in mind the OP said "receptacles".

e57: I actually meant 210.19, but was being sloppy; thinking 'why would anyone install 50A of conductors on a 30A breaker?' Being a couple of hours more awake, I realize that there are reasonable situations where one would do so

I think that you made a very good point which should impact the understanding of this sort of installation as a design issue: loads plugged into 30A receptacles tend to be 16A or larger; if the load required less than this it would be on a 15 or 20A cord. One could be virtually certain that 2 loads, each with a 30A cord cap, would total to more than 30A.

But when it comes down to it, what lots of us feel in our guts about this installation is embodied in 210.23(A)(2) which basically says that the _total_ rating of utilization equipment fastened in place on a circuit should not be more than 50% of the circuit capacity. A welder and an air compressor are both large loads that feel 'fastened in place' at least as much as a plug in range or dishwasher. However 210.23(A)(2) does not apply for several reasons; 1) It only applies when 'portable' equipment is supplied as well as the fastened in place loads, 2) It only applies to 15 and 20A branch circuits, 3) neither of these loads is likely really fastened in place.

As an aside, it seems to me that lots of aspects of circuit ratings are more about remaining consistent with the capabilities of old technology than what would be idea given new technology. Using current installation practices, it is entirely possible for a circuit to be overloaded without tripping the breaker; simply run the circuit at >80% capacity on a continuous basis. But it is entirely possible to build systems where it would be correct to say 'if the breaker doesn't trip, then the circuit is not overloaded.'

-Jon

gfretwell wrote:


True, but if you opened a panel door and saw 30 amp single pole breakers in a resi panel, that would trigger a red flag, right? Now look at the same panel, with 30, 40 or 50a double pole breakers, maybe labeled "garage", "welder", "compressor", etc. and I don't think your first thought would be to look for oversizing.

There is no preventative for what some fools will try to do, especially to their own service equipment. The main issue is the horrid advice given out at the big box stores. I have yet to hear of anyone who has gotten the right advice from one of those "experts."



Very true. That's the kind of situation that helps to keep the fire departments and fire restoration people (as well as EC's, of course) in business.

This subject touches on "design" issues...which is a good thing! Too many folks would consider any monkey with a pair of Kleins to be an "electrician," but only a true journeyman has the training, and experience, to make the sort of judgement calls required in design work.

As for the "Box" store: I have seen their clerks give decent advice; more common (in my experience) is the customer who doesn't want to accept what the clerk says, and just has to keep trying to make a mess of things.

Depending on the actual ratings of the equipment most likely yes.

However that was not the question, the question was about receptacles.

Quite frankly NEC rules that try to control the load that will be plugged into an outlet are ineffective at best.

The end users do not read the NEC, they often do not know how much power the equipment they plug in consumes.

There is no rule in the NEC that says I can not take a customers word as truth if they tell me they only want to operate one appliance at a time.


Consider 220.21. (I know it does not apply to branch circuits)


Notice it does not say the loads need to be interlocked only that they will not operate at the same time.

If the customer says I will not run these two widgets at the same time that is enough to satisfy 220.21

My point here is simply that this is strictly a design issue.

Of course two dedicated circuits will be more reliable.

Will two dedicated circuits make it safer?

I don't see how.

I am a firm believer in letting the person paying the bill decide what is best for them as long as it does not violate the code or result in an unsafe condition.

I will give them the benefit of my knowledge and make suggestions but ultimately it is their decision and if it happens they call me back in a month to add the things I recommended to start with I get the last laugh.

Bob

By the way, I don't buy into the '15 and 20 amp circuits are hard to overload' line of reasoning.

A hair dryer and a vacuum cleaner.

A coffee pot and a microwave. (Just because there are two kitchen circuits does not mean the load will always be dived between them.)

A portable heater and a large TV.

I tripped a 15 amp circuit at the house the other day for the first time in a while.

My wife was using a vacuum plugged into a circuit that a TV and 1,500 watt heater I was using was also on. After about 5 minutes it tripped no harm no foul, moved the vacuum to another outlet and I went back to watching TV.

Bob

Assuming that it's not a FPE breaker....

Bob,
"By the way, I don't buy into the '15 and 20 amp circuits are hard to overload' line of reasoning.'

I hope you dont think I was trying to imply that... What the customer does with a receptical is one thing if he says "I want an L6-30 right here, and a 6-30 there." But if he points to two 16A items.... "I want to use these on one 30A circuit." I'd say NO! That is not a design issue. 16X16=32, you now know that the intent is to exceed the OCP, and a violation of 210.20(A).



[This message has been edited by e57 (edited 03-26-2006).]

We can agree to disagree, I certainly respect your opinion and clearly you are driven by a desire to do the right thing.

From my point of view.

If the customer says;


I would ask if they mean at the same time or not.

If the customer tells me "No not at the same time" than there is no violation and I can proceed with the one circuit installation.

If the customer tells me "Yes at the same time" then I am obligated to refuse one circuit.

IMO what you are suggesting would be same as walking into someones home kitchen and adding up all the ratings of every plug in kitchen appliance they own and telling them that they must have enough circuits in the kitchen to power them all simultaneously.

Lets use another example.

You need to run a feeder to a garage that the customer is setting up as a well equipped wood working shop. This shop is strictly for hobby woodworking, one person working alone.

Planers, shapers, saws, drills lathes, lighting, HVAC etc.

Would you say the feeder must be large enough to supply all the loads at the same time?

After all it will all be connected and the customer could turn on everything at the same time.

Is the service to a typical house large enough to power all the electrical items the customer owns at the same time?

I guess it comes down to listening to and believing the customer.

If they tell "No not at the same time" and later they end up tripping the circuit because they did use them at the same time, I have not violated the NEC, they have.

Bob



[This message has been edited by iwire (edited 03-26-2006).]

Double pole - double throw switch?

I don't think these should be compared to a dwelling's kitchen appliances. This is utilization equipment. Personally, I read the NEC differently, and would not place these two items on one circuit sized for only one of them.
210.23 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 was rated. A branch circuit supplying 2 or more outlets or receptacles shall supply only the loads specified according to it's size as specified in 210.23(A) through (D).
210.23(B) A 30 amp branch circuit shall be permitted to supply... or utilization equipment in any occupancy. A rating of ANY ONE cord and plug connected utilization equipment shall not exceed 80% of the branch circuit ampere rating.
430.24 Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity of not less than 125% of the full load current rating of the highest rated motor plus the sum of the full load current ratings of all the other motors in the group..., PLUS the ampacity required for the other loads.
220.18 The total load shall not exceed the rating of the branch circuit and it shall not exceded the maximum loads specified in 220.18(A).
220.18(A) ... For circuits supplying loads consisting of motor operated utilization equipment that is fastened in place and has a motor larger than 1/8 HP in combination with other loads, the total calculated load shall be based on 125% of the largest motor load PLUS the sum of the other loads.
I can assume that the compressor has a motor, and will be fastened in place, and the welder is the "other load". Also, the NEC 2005 does not have 220.21 in it that I can see.

If we get back to reality for a second, lets look at my 20a circuits in the garage. One has a drop light, drill press, grinder, table saw and fan, plugged in all the time. There are also several open receptacles and a cabinet full of power tools. Certainly it is possible to turn them all on at once. More certainly it would trip the breaker.
Until I staff up this "shop" to run all of the tools, I won't need the extra circuits.
The same would be true with the pressure cleaner and the welder. Would you even want to turn on your welder if everything was wet?
I think that is safely a "noncoincidental load.

OK Bob and I will agree to disagree....

The items in say a garage or kitchen if connected would also be a violation, just as much as an 30A air compressor and 30A welder on the same circuit.

Like I said before, I think we can safely assume they are approching 20A ea (>15A continuous, or non continuous), or the would have been 20A ea, right?


It says load shall not exceed the continuous and noncontinuous loads, not intermitant, or single user loads. Right?


From Table 210.21(B)(2)

• 15A Circuit = 12A Max total cord-and-plug-connected load
• 20A Circuit = 16A Max total cord-and-plug-connected load
• 30A Circuit = 24A Max total cord-and-plug-connected load

Where does it say 'used at the same time'? It says "total cord-and-plug-connected load"

I think it is pretty clear....

Neither a welder or a compressor is a continuous load and I doubt either of them are fastened in place.
I know most of the stuff in my shop is on wheels.

It says load shall not exceed any combination of the continuous and noncontinuous loads.

And the words "fastened in place" don't show up in the code until 210.23. And is not an exception to 210.20, or 210.21 It simply says that the item fastened in place can't be >50% of the circuit rating when you have other outlets, or lighting on the circuit. That might only further limit this type of use.

This is not a feeder, we are not dealing with load diversity....

Am I missing something? Could anyone please point to a code(s) that would allow it?

If someone said to me that they only want to use one at a time, I would only install (1) receptical with matching cord caps, or a DPDT switch. The only way I can see it being allowed...

Rant off....



[This message has been edited by e57 (edited 03-27-2006).]

e57 Than your only choice is to install only a single receptacle on every branch circuit you run.

Any other method could possibly lead to an NEC violation.

Its that simple, the same rules that you are applying to the 30 amp circuit apply to 15 and 20 amp circuits as well.

Bob

This


Has little to do with load diversity.

220.21 is straight forward common sense.

If an AC and heating system will not run at the same time so we do not count them together.

If you where figuring the feeder to this 30 amp circuit and the owner said the welder and the compressor will not operate at the same time you would only be obligated to count the larger of the two.

However you are right and I also pointed out this particular section does not apply to branch circuits.

What this section does point out is the NEC does not require two loads to be physically interlocked as to not run at the same time.

These suggestions of SPDT switches would work but are not required.

Wow I thought this was an easy straight foreword question I guess I was mistaken.

Bob

Sometimes it hurts us to read between the lines of the NEC.

210.20(D) says the branch circuit shall have a rating not less than specified in 210.21.

210.21 says the outlet device shall not be less than the load served. It does not address the circuit.

210.21(B)(2) says the total load of a single receptacle shall not exceed Table 210.21(B)(2).

No where in these sections does the NEC talk about the possible load of cord and plug connected equipment. It only talks about the actual connected load.

If the device is turned off, its connected load is 0. If the device is not plugged in then it's possible load is also 0.

Texas Ranger, I'm sorry I didn't get around to your question earlier.
The main reason you can't say they're both "30 amp" appliances is because the nameplates don't carry the same information.

The compressor will be marked with amp information. The welder, however, wil be marked with terms like "Imax", Ieff", and "duty cycle."

Let's assume, for the moment, a 30 amp breaker on a circuit run with #10 THHN. (This wire has table ampacity of 35 amps).

Using the NEC, we can infer that this circuit can handle a compressor with a "full load amp" rating of between 13 and 17 amps.

Now, let's look t the welder. Again, looking at the appropriate section of the code, we can infer that the welder can have an "Imax" as large as 15 amps. Assuming a duty cycle of 20% (common on small welders), and using the code-provided multiplier of .55, we get a figure of 8 amps to work with.

Compressor (17 Amps) + Welder (8 amps) = 25 amps; this is well within the loads allowed for either a 30 amp breaker, or #10 wire.

BUT- and here is the kicker...what if the compressor neds a larger breaker to handle starting current? This same circuit could be inadequate.
AND- 8 amps is a pretty small welder; assuming a 20% duty cycle is also assuming that it will be used for little more than tack-welds on light sheet metal. Change those numbers, and everything else changes.

So, in theory the circuit proposed just might be OK; without seeing the nameplates of the specific equipment, we can't really be sure.

Here is where the circuit differs from a household convenience circuit: household appliances don't get treated differently for each appliance. You can turn everthing on, put your amp clamp on the wire, and know right away if you need another circuit.
For the circuit we're discussing, the rules are different for each appliance.

Some might say "look at the plugs." Not really a workable plan; welders, in particular, are likely to have all sorts of plugs on them. The air compressor may not even have a plug, being intended to be 'hard-wired.'

An application like this is what separates the "cook book" electricians from the true journeymen.

You highlighted the wrong word. Read the sentence before that to get the context. They are talking about "continuous loads".
You can't cherry pick words out of context.

220.21 does not exist in the NEC 2005

John

Again I point out that we do not have to use article 630.

Most of what Article 630 does for us is allow us to save money on the conductor sizes supplying a welder.

If I go and buy a welder that has a factory installed 30 amp cord cap it is perfectly legal and NEC compliant to plug that welder into a 30 amp general purpose circuit.

If I go and buy a compressor that has a factory installed 30 amp cord cap it is perfectly legal and NEC compliant to plug that compressor into a 30 amp general purpose circuit.


Quite frankly I find this insulting.

You obviously feel your view is the only 'right' view and that those that feel otherwise are not 'true journeymen'

There are many ways to do our job, none of us has a corner on what the right way is.

Bob




[This message has been edited by iwire (edited 03-27-2006).]

Scenario:

Someone installs a receptacle for a "buzz box" welder in their garage.
There's a 50A 250V cord cap on the welder, so, by 630, they use #10 conductors to wire a 50A receptacle for it.

5 yrs later, they sell the house. The new owner has a 40 Amp Kiln that they use for firing pottery.
New owner is delighted to find the 50A recptacle (and circuit breaker) already installed.

Iwire...I am truly sorry- I did not mean to insult you, or anyone else!!

What I did mean was to contrast the hard-working guy, who learned the trade 100% in the field, doing the same sorts of jobs for the same employer, and by virtue of his time in trade calls himself a 'journeyman,'....and someone with a braod range of experience, backed by a systematic education in all facets of the trade.

I was also referring to the idea that 'journeymen,' by virtue of their training and experience, are paid for their knowledge and judgement. Contrast this with the attitudes of some, who consider even the most qualified tradesmen to be little more than apes with tool belts!

Disagreements? You bet- especially when a question takes us off the beaten track. And, yes, it is possible that actual experience at an installation will confirm some of these design calls...and discredit others!

Again....I didn't mean to insult you, or anyone else.

OK points noted.

Now....
Here's where I think I have figured out where I differ from everyone else on this topic... "30 amp general purpose circuit" is not in my vocabulary. In my mind, general purpose circuits are 15-20, depending on what they are intended for. Especially, when I know what they are intended for. And yes, I do ask what they are intended for. 30A circuits, are individual branch circuits - in my mind, something I do not think will change for me.... And maybe thats a design issue.... My apologies.

Now a question, and I am not asking this to be cheeky....
For those of you who do install 30 amp general purpose circuits, how many outlets do you generally put on one? Would they to be counted as 180VA per strap?

Thanks Reno.

Mark? (It is Mark right? sorry I stink with names even though I can remember code sections

I don't think your question is bad at all.

I can not think of a time where I have installed 30 amp GP circuits.

But they are not prohibited by the NEC.

I have seen 50 amp welder outlets installed with many outlets on one breaker, there is one welding machine rolled around to different locations.

Interesting point, there would be no outward signs to a layman that they are doing anything wrong.

Bob

I would have a lot more trouble with a 50a receptacle on 10ga wire than ten 30a receptacles. To start with I would see no relief from 240.4(D) if this was a receptacle.

Greg

"Unless specifically permitted in 240.4(E) through (G)"

'G' would allow the 10s on a 50 suppling 50 amp receptacles.