Easy Question

[Mark P]

I know this is a simple question, but I'm having a brain fart.

What is the correcte term when a 200amp main disconnect is connected to a #3 AWG. Is it "over fused"???

[Scottpat]

Originally posted by AHIS

I know this is a simple question, but I'm having a brain fart.

What is the correcte term when a 200amp main disconnect is connected to a #3 AWG. Is it "over fused"???

I don't belive you have a problem.

[Mark P]

Well maybe I'm really haveing a brain melt down today but here is my thinking... #3 AWG is rated at 110 amps so since it is connected to a 200amp breaker the #3 is not protcted and the wire could melt....

Oh - I think I just saw the light. The 200 amp breaker is not protecting the #3. All this means is the home has 110amps of power not 200. But there is no safety concern.

I leave for Florida in the morning for a week at the beach. Guess I'm peroccupied.

[Eric B]

#3, as opposed to 3/0, and copper, right?

If the conductor from the meter is #3, then the 200 amp breaker will not protect it. It's a mismatch/safety concern. It would indeed be rated as 110 service.

Or am I the one with the brain fart?

[Mark P]

Originally posted by Eric B

#3, as opposed to 3/0, and copper, right?

Yes that is right

If the conductor from the meter is #3, then the 200 amp breaker will not protect it. It's a mismatch/safety concern. It would indeed be rated as 110 service.

The service conducter is live until the meter is pulled, so it does not matter what the breaker size is. I think where I was confused is that if it were a circuit with a 30amp breaker and 14 AWG, then the wire would not be protected in the event of an overload.

[hausdok]

Geez Louise!

I've read that all 4 times and I'm still confused as to what you're saying Mark. Can you sketch that out for me so I can see if I can understand it?

OT - OF!!!

M.

[Richard Moore]

The utility company has its own rules for overhead service drops and underground laterals, BUT from the weatherhead (or the meter in the case of an underground service) the service entrance conductors have to be rated for the service disconnect. If I'm reading Mark's first post correctly, he has #3 SEC's feeding a 200-amp disconnect and there's no way that is right. Over-fused would be a good term.

Yes, you might rate or describe it in a report as 110-amp service based on the weakest link, the SECs, but that doesn't mean that's the sum total load you could put on those conductors and just describing it as such doesn't mean you couldn't overload and burn those SEC's with 200-amps. The total load may be currently limited to 110-amps or less, depending on the existing circuits and the breakers in the panel, but as those can be changed or added to, the service disconnect should either be changed to 110 or the SECs to 2/0 Cu or 4/0 Al.

The 200 amp breaker is not protecting the #3. All this means is the home has 110amps of power not 200. But there is no safety concern.

All three of those statements are WRONG! One way of looking at it...Why would there be a table for Service Conductor size, describing the minimum allowable size, if it didn't actually matter what was "upstream" or the line side of the service disconnect controlling the possible load?

[hausdok]

Yeah,

That's what I was thinking when he made the intial post but I couldn't understand what / how they were coming to the conclusion that pulling power through #3 cables through a 200 amp breaker wasn't over-fusing the #3 cables. That's why I asked if Mark could sketch it out for me; sometimes it doesn't make any sense to me until I have something to actually look at.

There's no doubt about it now; my brain is definitely turning to oatmeal. I should probably buy a railroad car full of Depends now; given inflation, it will probably save me $50k over what I'll have to spend between the time I totally turn into a blithering idiot and I check out.

ONE TEAM - ONE FIGHT!!!

Mike

[Richard Moore]

LOL. Try suddenly jamming a foreign language into a 58 year old noggin.

C'est très difficile pour un vieux pet.

[hausdok]

Hah,

I know exactly what that's like. I spent a year at the Presidio of Monterey jamming German into my head and now I'm trying to learn Irish Gaelic.

ONE TEAM - ONE FIGHT!!!

Mike

[Mark P]

Sorry for the confusion. I guess I should never have doubted my first opinion. Sorry for the confusion Mike. It is 0340 in the morning and I'm trying to get the family up and in the car, so I can't draw anything now, but you've got it right. "I'm going down to Fl and I'm going to bowl a perfect game."

So "over-fusing" is the correct term in this case? See Ya'll in about 16 hours.

[StevenT]

That was easy!

[Phillip]

Let me see if I have it in my mind right.

#1 If the #3 is feeding the 200 amp breaker there is no problem and that would make it a 110 amp service.

#2 If the 200 amp breaker is feeding the #3 wire it is over-fused.

I go with #1.

[caryseidner]

I don't understand. I have seen main panels that have no main disconnect, and few enough circuit breakers that could allow someone to switch them all off with fewer than 5 hand movements. This is allowed...right? So then if an electrician were to install a 200 amp main disconnect, would you still consider it over-fused?

[hausdok]

Cary,

When you've got a split-bus panel the electrician installing it is supposed to have calculated the maximum potential load, given the number of branch circuits, permanent appliances, etc. in the home, and ensured that the service entrance cables are large enough to safely carry that load without overheating and melting down. If the panel is rated for 110 amps and there is a water heater, stove-oven, and clothes dryer that pull a combined load of 60 amps the breaker for the sub-main should be 50 amps. That ensures that all of those breakers combined are throttling the load imposed on that #3 cable.

When single main disconnects became the norm, the idea was that the main disconnect should be matched to the service entrance cables to protect them from being overloaded and should not be rated higher than the SEC's. In Mark's example, someone could load the home up with enough circuits and devices that it will pull a sustained load of up to 200 amps before that main breaker will trip. Since those #3 cables are only rated for 110 amps, they'll probably overheat and the insulation on them will melt and burn up and the panel will short out long before that breaker even comes close to tripping.

ONE TEAM - ONE FIGHT!!!

Mike

[StevenT]

The circuit breaker limits the amount of power that is allowed to pass through.

A 200 amp breaker will allow 200 amps of power to pass through.

This is more than #3 wire is rated for and can handle, so it is over fused.

The breaker on the downstream side of the SE cable, limits the amount of power being drawn through the cable. So, a 200 amp breaker protecting #3 SE cable is not good.

The breaker on the upstream side of the circuit wire limits the amount of power being drawn into the wire.

If a main panel has no main breaker. There should be a main breaker limiting the amount of power being drawn, upstream of the panel.

If a 200 amp breaker is installed before a main panel that can be shut off with up to 6 main disconnects, and does not require a single main disconnect, the individual disconnects limit the amount of power being drawn, and the 200 amp breaker is acting as nothing more than a shut off switch for everything.

I have trouble saying that a service with #3 SEC and a 200 amp breaker is rated at 110 amps.

I might say:

The 200 amp main breaker is being fed by a service entrance cable that is only rated for up to 110 amps. This is a fire hazard and should be corrected immediatly by a licensed electrician.

[caryseidner]

Thanks Mike & Steve. I understand now.

[Eric B]

You guys may have trouble with rating it as 110 service but in fact that is what it is. You need to consider the disconnect, the panel rating and the SEC from the meter - the less of the three is the rating. The conductors before the meter are not a consideration to the rating of the system.

The disconnect does in fact protect the cable from the meter. Two choices for the condition presented: replace the SEC or the disconnect - both are mis-sized for the other.

[Speedy Petey]

The conductors before the meter are not a consideration to the rating of the system.

In my area they certainly are since they are customer owned and supplied.

[hausdok]

Originally posted by Speedy Petey

The conductors before the meter are not a consideration to the rating of the system.

In my area they certainly are since they are customer owned and supplied.

I agree; not necessarily because they are supplied or owned by a customer, but because if the cables between the meter and utility pole are improperly sized for the distance from the transformer and the load they'll carry you can end up with less available voltage than you require. If you design a system that's supposed to be able to draw a 200 amp load and the drop can't handle a 200 amp load - say in 95°F summer heat - wouldn't the system fry the drop?

I know it's pretty unlikely, because I've heard that the drop can typically handle twice the rated load of a house's system, and I know that we usually don't consider it, but I should think it has to be a consideration when an electrician is trying to figure out what he's working with.

I dunno; maybe I should quit while I'm ahead, I know just about enough about electricity to get myself in trouble.

ONE TEAM - ONE FIGHT!!!

Mike

[StevenT]

Eric,

I agree that the service is rated at 110 amps, since in this case the SEC is the weakest link, and it is rated at 110.

If it were a 3/0 SEC with a 100 amp breaker, it would still be rated at 100 Amp, but it would not be dangerous.

Either way, the service is rated at 110amp (100)

So, for me to simply state that the service is rated @ 110 amps makes me uncomfortable. I would want to word whatever I inserted in a way that makes it perfectly clear that it is not only not right, but it is dangerous.

[Jim Katen]

Originally posted by AHIS

I know this is a simple question, but I'm having a brain fart.

What is the correcte term when a 200amp main disconnect is connected to a #3 AWG. Is it "over fused"???

That setup is wrong.

As a general rule, there's no such thing as a correct term for something that's done wrong. I think that it's better to simply describe the condition rather than to use a made-up name for it. I particulary dislike the term "over fused" in this case because it implies that the circuit is fused in the first place. It's not. A breaker at the end of a circuit might provide some overcurrent protection (if it's properly sized) but it will provide no protection against faults. The service conductors are unfused, so they can't really be over fused.

I'd also say that it's impossible to determine the size of this service as long as it's improperly configured. It's meaningless to even attempt it. Once someone has done a load calc for the building and corrected the improper service wiring, then you can determine the service size. In the meantime, calling it a 110-amp service or a 200-amp service is meaningless.

- Jim Katen, Oregon

[StevenT]

Jim,

Thank you for the clarification.

From the very beginning I was not comfortable with calling it "rated at 110" amps. I was not as confident and as good at putting it into words as you are, but I just knew that I could not make such a statment, and preferred to simply describe the fault.