Restaurant Sub Panel

Forry

125A sub panel serves a restaurant, feeds off of 200a main breaker in the electrical room. Cooks tend to push bake sheets off the back of the ovens onto the plugs for the gas oven igniters. The last time this happened, the main breaker in the elec. closet blew, and shut down the restaurant for the night. Of course the owner was po'd, as he can't get an access key. I told him to put a shield over the outlet in question, but he wants a solution, and after talking to the building manager, says the breaker in the restaurant sub panel should be the one to trip, and somethings wrong with the panel or wiring or something. Sounds right to me, but the electrician who wired the place (it's new work) is saying that it's normal, and that nothings wrong.
What's going on? I have a po's client and I mostly trust the electrician, but why would the main trip and not the one in the sub panel?

mdshunk

When you're dealing with typical breakers and a 'hard short', it's a horse race as to which one will trip first. The one that's closest (thermally or load wise) will be the one to trip. In this case, it was the "main-main". What you've described is completely normal (as if shorts are ever normal). The corrective action is to either replace the mains with high-dollar electronic versions, which are adjustable, or take steps to keep this type of short from happening again.

I'd suggest installing bubble covers on the receptacles in question, as a first step. If they're 120 volt, GFCI protect them as well.

Forry

So like I told him... Put a shield over the outlet, or stop dropping sheet pans on it...?

P.S. Thanks MD, Gotta get this client to quit whining... It's been constant from the start of the job. Jobs done now, but this guy thinks his life is ruined over a short curcuit!

mdshunk

Yup.

I know that 99% of regular people and 50% of electricians can't don't understand how a main can trip before the branch breaker. Just believe me that this isn't a desirable condition, and not even a normal condition, but it is not a hazard. Certain conditions, when they come into play, and all the stars align, it will happen every time. There's nothing "broken", per se, when this happens. Just luck of the draw.

Magnettica

I'll never forget the first time I accidentially tripped a branch circuit and the entire subpanel went out. I had no clue why it happened. Then I opened a book, read up on short circuit ratings, and became a better electrician for it. I was lucky because it was an office space that was doing some remodeling and was vacant except for construction crews.

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jbelectric777

I've seen it happen where the available AIC from the power company is higher than the AIC rating of the main, where that problem exists we install fuses, then it does'nt matter. Usually high in large load city areas (like for example Atlantic City) Down there I always use fusible mains or special ordered AIC equipment which you cant be competetive unless it's specified. But normally before any commercial bid, I call the utility and have them calculate the rating to that point of attachment, it has to do with how far their substation is and the load etc, it's not something that the contractor can do, and good to know inspectors now want (in NJ) on the UCC subcode tech application.

Magnettica

I don't fully understand fault current and the ratings of main breakers. If I have a 200 amp main with a 22k available interupt rating (AIC), does this mean that the breaker will only trip up to 22,000 amps of fault, or only less than. I'm not even sure if I'm asking the question correctly.

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jbelectric777

I checked via telephone with my continuing education guy, He's a PE, we do 34 hrs every triennial renewal and we covered this last year because believe it or not a lot of us did'nt understand it, basically the best way to explain it is available fault current is based on the amount of current the overcurrent protective device can handle before it destroys itself (thats a laymen term, i'm no engineer) but 22K is the old standard, I'm pretty sure the new standard is 25K, one job in ac i needed 60K and my supplier said to go with fuses. I had a home (in ac) and because of it's location the aic was like 88K, so i mean c'mon, i hadda use fuses. Its a hot topic with both inspectors and contractors. NJ is enforcing it, in other areas i work in they dont. In Penna it's a lot of third party inspection firms as well as NY with the exception of large cities where they enforce on a local level, I know Centre county has their own and they probably enforce it. The bottom line is I learned the hard way to check before the install, if your restaurant is located in a heavy load area like a city your 22K is prob too small.... call the utility and have them calculate it, compare it to what your main is rated and take it from there. like i said, most cases fuses are cheapest. and will never blow... also keep in mind that its for mains and feeders not branch circuits so your 22K branch circuit breakers are o.k.....

jbelectric777

actually the new standard (i heard) was going to be 60K but because of money issues who knows what it's gonna be , but it's not 25 as i mentioned before, but you can get them up to 100 but yer gonna pay bigtime....

jbwhite

sometimes the problem is that the main is adjustable, but has never been properly adjusted.

mdshunk

True, but probably not the case in this instance. He said it was a 200 amp main, so I'd bet it's nothing special.

Magnettica

What is the likelyhood that we have two JB's on a forum?

Anyway, we do.

JB Electric, I had a similiar situtation at a multi-family dwelling in Newark last year. We had four meters, and four main disconnects. One 200 amp, and three 100 amp. Each breaker had an AIC rating of 22k and the inspector had us bump them up to 40k.

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jbelectric777

Oh Yeah, they check now... And it's just habit now to pre-check it. And it's not just NJ , because the inspectors everywhere are being taught in their own continuing education courses the importance of it for the very reason this thread was started. The AIC as explained to me was the amount of fault current the OPD could safely handle without damage to the OPD (i.e the OPD having a complete failure and not opening at all) which as we all know is a terrible bad thing... two jb's what do ya know.

jbwhite

Nice to meet ya JB.

Is it just my immagination or does it see to ya'll that it is more likely for a breaker upstream of the branch breaker to trip on a fault to ground than on a fault phase to phase.

I seem to have noticed this, but then again faults to ground are probably more common than faults phase to phase.

Forry

Hey guys, thanks for hashing this one out... Like MD said, its a basic 200A 3PH feeder, and so I guess like JBWhite said, its a fault to ground short ( It must be right? It was a 115V 15A circuit that blew).

jbelectric777

good to meet u 2 jb , yeah it seems it's a to ground condition. I saw a 300A feeder that made fault to its metal raceway take out a 1200A 277/480 3PH ground fault main in a polymers plant years ago. I didnt think much of it then but would now..... back then we just cleared/repaired where it shorted and reset the 1200 and went on to the next job. when discussing adjustable trip circuit breakers i think thats more of a reaction time setting (not current) as far as time delay for motors etc. In CE class our instructor pretty much had a trick question for us, he asked " When should a 100A circuit breaker open? There was a host of answers from: 80% of rating to 125% of rating, lol it was a heck of a seminar but in the end they are set to hold rated current for a continuous period (3 hours) after 3 hrs at full load they are designed to trip, BUT because of dis similar voltages, power company phasing, voltage drop and other things that cant be controlled 100% they are within that range but i wouldnt put a stop watch on it if ya know what i mean.......

jbelectric777

thanks for the thread forry , its a very hot topic these days...

Magnettica

Great subject indeed.

So if I have a 200 amp main breaker rated at 22k AIC, the breaker will only trip if I have fault current above the 22,000 amps?

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mdshunk

No, it just means that if you have available fault current above that rating, your breaker may crash and burn. It's more like a "withstand rating", beyond which the breaker may fail.

Magnettica

So the breaker AIC rating always has to be above and beyond the impedence % the POCO is offering on their end.

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