Six Hand/Throw Rule

[conmgt]

Hello,

I'm going to ask about the Six Hand or Throw rule and how the service conductor is protected from a current overload. I've yet to install a service without a main breaker in the service panel but can see how the Six Hand/Throw rule can come in handy.

I met with an inspector earlier today and asked him "How is the service conductor protected from an overload?" He replied that it was protected by the fuses on the transformer. That may be true for a transformer dedicated to that one service but what if the transformer supplies the whole block ? I'm sure that it's fuses exceed the service conductors of any building on the block.

I'm installing a 200A 3PH service and the client wants a 200A panel on each floor. I don't think that there's enough wire bending room for 200A feeder breakers in something like a SqD NQ panel so a trough under the meter socket and some protected disconnects came to mind but I know that the transformer on the pole serves the building across the street and that it will be the transformer used for my project.

 

[infinity]

Many large services are installed without a main and use the six throw rule. We typically install 4000 amp services with up to 6 service disconnects and no main. Customers are cheap and don't want to spend the extra dollars for a 4000 amp switch. It's fine until you need to shut the service down and need to pay the POCO to come in and pull the links.

 

[GoldDigger]

It is the nature of service conductors, both under the NEC and the NESC that the wires are not protected against simple overload, and to some extent not even against shorts. If there is an overload on the service conductors you hope (with some justification) that the conductors themselves will burn out to interrupt the circuit but will not, in the process, set anything else on fire. Because of the "nearest the point of entry" rule for service disconnects mounted inside the building this is usually true.
If anything, POCO fuses are there to protect the transformer(s) and the integrity of the distribution wiring.
Fuses requiring replacement to protect the transformers, and reclosers which have a good chance of burning away the short to protect lines.

 

[charlie b]

The question of whether the transformer will be overloaded is one that only the utility can answer. The issue of protecting the service conductors from the transformer to your meter is, again, one of interest only to the utility. We are only responsible for protecting conductors and equipment downstream of the point of common coupling (i.e., where the utility's responsibility ends and the owner's responsibility begins). Having a main breaker versus six throws changes none of that.

 

[Jraef]

The question of whether the transformer will be overloaded is one that only the utility can answer. The issue of protecting the service conductors from the transformer to your meter is, again, one of interest only to the utility. We are only responsible for protecting conductors and equipment downstream of the point of common coupling (i.e., where the utility's responsibility ends and the owner's responsibility begins). Having a main breaker versus six throws changes none of that.

To this point, the NEC rules that we must follow are decidedly conservative because from the standpoint of safety of the consumer, there are too many unknowns. But on the utility side they don't have to play by our rules, partly because (in theory anyway) there are almost NO unknowns. They are in control of (or assuming risk of) all aspects ahead of the service connection point. So if those conductors melt, it's their problem, not only from a replacement cost standpoint, but from a consequential standpoint as well, meaning if you are the consumer and their connection fails, you can sue them. That's a risk/reward equation they play with a lot behind the scenes.

 

[infinity]

I know that ConEd has fuse protection within their network compartment for 480/277 services. Typically they're 5000 amp fuses just before the bus leaves the network section to feed the customer switchgear. Photos show both sides of the stabs and the fuses.

 

[conmgt]

So the utility company is responsible for the service conductors that I install between the meter and the weatherhead ?
Even so, my client will be looking to me to replace them, not the utility company.

Isn't a MCB in the service panel like a breaker at the end of a tap ? A tap breaker needs to be sized to protect the conductor. The only thing that protects the service cables (installed by the contractor, not the utility) on a Six Hand install would be the contractor's calculations and those calculations are only good on the day the job is finished. We can't control additional loads added after the install.

 

[GoldDigger]

You are, IMHO, correct that although a single main needs to be sized to no more than the service size it may not really protect the service drop if POCO thinks the load will not be that high.
And there is no such limitation on the sum of the up to six breakers making one service disconnect.


Sent from my XT1585 using Tapatalk

 

[jaggedben]

If there's a single main breaker then the service conductors will be protected from overload, but not short circuit, by that breaker. However if there are two to six breakers then, interestingly enough, the NEC doesn't require the service conductors to be sized for the sum of the breakers; they just have to be sized for tue calculated load, and the breakers can add up to quite a bit more than that.

 

[texie]

If there's a single main breaker then the service conductors will be protected from overload, but not short circuit, by that breaker. However if there are two to six breakers then, interestingly enough, the NEC doesn't require the service conductors to be sized for the sum of the breakers; they just have to be sized for tue calculated load, and the breakers can add up to quite a bit more than that.

That about sums it up. I might add that this has been the rule for many decades. I know of a number of NEC adoptions that prohibit this though.
Also, you often see this with 480 volt services to get around the GFPE requirement.

 

[electrofelon]

If there's a single main breaker then the service conductors will be protected from overload, but not short circuit, by that breaker. However if there are two to six breakers then, interestingly enough, the NEC doesn't require the service conductors to be sized for the sum of the breakers; they just have to be sized for tue calculated load, and the breakers can add up to quite a bit more than that.

It is somewhat surprising to me that the NEC allows such a "loophole" considering how conservative it is on most other things. But, there are a number of reasons why it is extremely unlikely that the SEC serving a 2-6 disconnect rule service would burn up. First they would be sized to the load of course, which is very conservative. Ok lets say someone adds load down the road. Assume they do it renegade without load calcs, inspection, etc. The way most 2-6 services are set up, the individual mains dont add up to way over the conductor rating. One I did that comes to mind had 1025 amps worth of breakers, and 750 amps worth of conductor feeding them. It s very unlikely that all those breakers would be fully loaded. Most likely one or two would be overloaded and start tripping, while several others would be far below their rating. Lets take another leap and say all the breakers are fully loaded. Ill bet those 750 amps of conductors can handle 1025 amps for quite a while. The NEC ampacity is conservative and based on the 75 degree rating, and the conductor will be rated 90 degree. The utility will, almost every time, be feeding that service with about 1/2 of the conductor of the SEC on the customer side of the service point, so they will burn up first, and they are usually underground or in the air where it wont damage anything. Finally, there is a good chance a utility OCPD will open.

 

[conmgt]

Thanks for all of the replies.

I certainly more comfortable with going forward on a Six Throw service.

 

[kwired]

Hello,

I'm going to ask about the Six Hand or Throw rule and how the service conductor is protected from a current overload. I've yet to install a service without a main breaker in the service panel but can see how the Six Hand/Throw rule can come in handy.

I met with an inspector earlier today and asked him "How is the service conductor protected from an overload?" He replied that it was protected by the fuses on the transformer. That may be true for a transformer dedicated to that one service but what if the transformer supplies the whole block ? I'm sure that it's fuses exceed the service conductors of any building on the block.

I'm installing a 200A 3PH service and the client wants a 200A panel on each floor. I don't think that there's enough wire bending room for 200A feeder breakers in something like a SqD NQ panel so a trough under the meter socket and some protected disconnects came to mind but I know that the transformer on the pole serves the building across the street and that it will be the transformer used for my project.

For 200 amp service supplying a 200 amp panel on each floor - why not put in a single 200 amp main breaker panel at the service with subfeed lugs, and main lug panels with subfeed lugs on each floor? This basically leaves you with a 200 amp feeder running through all the floors instead of 5 feeders from main floor to the second floor, 4 to the third....

If 200 amp service were initially enough then obviously the load isn't there to require more then one feeder, but if you want to future load proof it to some extent maybe you run a 200 amp feeder for lower half the building and a 200 amp feeder for upper half - that way all one needs to do if load increases is change the service to 400 amps when that time comes.

If you want individual feeders anyway then maybe opt for an I line main panel. Get a 600 or 800 amp main lug panel and if the need ever comes up for more service capacity you can leave most things as is and just increase service conductors.

 

[conmgt]

kwired

That is an excellent solution but here are a couple of points which may or may not make your suggestion viable:

-The service will be a 240/120 Delta with a high leg. 1/3 of each 3ph panel will not be usable for 120V loads, correct ?
-First floor will be a wood shop that needs 3ph. I'll put a 3ph and a 1ph panel on that floor.
-Second floor is office/warehouse/man cave which will need only 1ph panel. 200A is ridiculous but so many folks have in their head that 200A is better than 100A despite a 50A load.
-There is a quite small basement that will only need 1ph panel for a 1/2hp sprinkler compressor, lights, a few receptacles and that's it.
-There is a traction style elevator. A disconnect for that.
-There is a chance that power will be needed for a chiller/AC compressor on the roof. A disconnect for that.
-The expected panel locations don't stack one over the other. On one jobsite they did and your suggestion would have been an excellent layout.


I am considering an I-Line but it's a pricey way to go. The smallest is the HCN14522N and has enough room exactly (6) 3 pole Qx breakers. Those breakers are expensive if I'll need anything higher than a 10kA IR.

 

[kwired]

kwired

That is an excellent solution but here are a couple of points which may or may not make your suggestion viable:

-The service will be a 240/120 Delta with a high leg. 1/3 of each 3ph panel will not be usable for 120V loads, correct ?
-First floor will be a wood shop that needs 3ph. I'll put a 3ph and a 1ph panel on that floor.
-Second floor is office/warehouse/man cave which will need only 1ph panel. 200A is ridiculous but so many folks have in their head that 200A is better than 100A despite a 50A load.
-There is a quite small basement that will only need 1ph panel for a 1/2hp sprinkler compressor, lights, a few receptacles and that's it.
-There is a traction style elevator. A disconnect for that.
-There is a chance that power will be needed for a chiller/AC compressor on the roof. A disconnect for that.
-The expected panel locations don't stack one over the other. On one jobsite they did and your suggestion would have been an excellent layout.


I am considering an I-Line but it's a pricey way to go. The smallest is the HCN14522N and has enough room exactly (6) 3 pole Qx breakers. Those breakers are expensive if I'll need anything higher than a 10kA IR.

Maybe a feeder running to all floors, a splice/tap box at each floor (except the top floor which would be end of the line and land in whatever panel you place there), and only bring single phase out of the splice box on floors where you only need a single phase panel?

Main breaker panels do have some advantage of being able to shut down an individual panel (other then the supply conductors to it) without shutting down the entire feeder/entire building.