Continuous Duty - Disconnect vs. Breaker

[mpcarnahan]

I have two inverters with max. output ea. of 41.6A. So, 83.2A, but 104A with continuous duty.
A non-fused AC disconnect is rated at 100% for continuous duty, so i could use a 100A disconnect. (as long as my wire is rated for the 104A)
But when i get to the interconnection breaker at the MDP, I need to be at 110A. right?
So, impso-facto... can i even use a 100A disconnect if the intertie breaker is 110A?

It seems to work, until you look at it in reverse feed.

~m

 

[ggunn]

I have two inverters with max. output ea. of 41.6A. So, 83.2A, but 104A with continuous duty.
A non-fused AC disconnect is rated at 100% for continuous duty, so i could use a 100A disconnect. (as long as my wire is rated for the 104A)
But when i get to the interconnection breaker at the MDP, I need to be at 110A. right?
So, impso-facto... can i even use a 100A disconnect if the intertie breaker is 110A?

Yes. The unfused disco is rated at 100%, so 83.2A fits. The breaker needs to be rated more than 125% of your AC Imax, so 110A. The ratings are derived independently, so there is no problem with the 110A breaker and the 100A disco.

 

[Carultch]

I have two inverters with max. output ea. of 41.6A. So, 83.2A, but 104A with continuous duty.
A non-fused AC disconnect is rated at 100% for continuous duty, so i could use a 100A disconnect. (as long as my wire is rated for the 104A)
But when i get to the interconnection breaker at the MDP, I need to be at 110A. right?
So, impso-facto... can i even use a 100A disconnect if the intertie breaker is 110A?

It seems to work, until you look at it in reverse feed.

~m

A 100A continuous duty rated disconnect can be used in any continuous load application where you'd use up to a 125A breaker. It is rated to be protected by the 125A breaker. Similar logic applies to all other continuous duty rated devices.

Most non-fused disconnects are continuous duty rated. It is also common for meters and meter bases. Few (if any) fused disconnects are, even though the blade of the fused disconnect probably is, given that it is likely the same blade assembly used in its unfused counterpart.

Whether or not you can use it for 125A of non-continuous current, that I do not know.

 

[electrofelon]

A 100A continuous duty rated disconnect can be used in any continuous load application where you'd use up to a 125A breaker. It is rated to be protected by the 125A breaker.
.

Hmmm I am not sure I follow that. Can you elaborate? I do not believe there is a requirement to protect a switch at its rating or 125% of its rating.....

 

[Carultch]

Hmmm I am not sure I follow that. Can you elaborate? I do not believe there is a requirement to protect a switch at its rating or 125% of its rating.....

What I mean is that if you place it in a circuit with a 125A breaker, the breaker will trip before the 100A disconnect is subject to the damage from overload. I don't anticipate that there is a requirement to "protect a switch", but the way I see it, is that the OCPD protects the entire series of components in the circuit that carry the current. Terminals, wires, splices, disconnect blades, meters, etc. All of these have an ampacity rating of some form or another.

I would expect that the ampere ratings of a disconnect have a lot more to do with load break arc extinguishing during the action of operating it under load, than it would with the ability to safely carry a load. As a thought experiment, suppose you were to somehow ramp the current in a 100A disconnect up to 200A and then back down to 100A. I'd expect that may still safely carry the 200A of current as long as you keep your hands off the switch during overload. Not that I'd recommend trying this experiment, but nevertheless, this is how I suspect disconnect ampacities are rated.

 

[ggunn]

I design systems configured the way the OP described it all the time, i.e., with a 100% rated unfused AC disco and a 125% rated interconnection breaker.

 

[Carultch]

I design systems configured the way the OP described it all the time, i.e., with a 100% rated unfused AC disco and a 125% rated interconnection breaker.

I've done it too.

One word of caution is that the factory terminals are selected with the nominal disconnect rating in mind. Usually, they are built for a couple sizes larger, in case you need to increase it in size for voltage drop. So if you do take advantage of the continuous duty rating, make sure you anticipate the terminal size limitations in your design.

The example I've used it for, is a 70A OCPD circuit with 53A of operating current in a 60A disconnect, where I needed to use #2 wire to curtail voltage drop. For the 60A disconnect, some datasheets indicated #3 was the limit of the 60A switch, and others indicated #2. The switch I ended up selecting by luck without thinking about this, had terminals marked for #2.

 

[ggunn]

I've done it too.

One word of caution is that the factory terminals are selected with the nominal disconnect rating in mind. Usually, they are built for a couple sizes larger, in case you need to increase it in size for voltage drop. So if you do take advantage of the continuous duty rating, make sure you anticipate the terminal size limitations in your design.

The example I've used it for, is a 70A OCPD circuit with 53A of operating current in a 60A disconnect, where I needed to use #2 wire to curtail voltage drop. For the 60A disconnect, some datasheets indicated #3 was the limit of the 60A switch, and others indicated #2. The switch I ended up selecting by luck without thinking about this, had terminals marked for #2.

Yes, the wire gauge capacity of the terminals is always a concern on every piece of equipment.

 

[electrofelon]

Sorry to be nitpicky, but I guess I dont like the use of the terms "100% rated safety switch" , "continuous duty safety switch", etc. Its an incorrect extension of the "80% rules" for branch circuits, feeders, service conductors, and OCPD's to all electrical equipment.

On a different note, another important thing to keep in mind with non fusible safety switches is the SCCR. The SCCR of most is only 10,000. They typically "series rate"with fuses, but not circuit breakers. Just something to be careful of.

 

[ggunn]

Sorry to be nitpicky, but I guess I dont like the use of the terms "100% rated safety switch" , "continuous duty safety switch", etc. Its an incorrect extension of the "80% rules" for branch circuits, feeders, service conductors, and OCPD's to all electrical equipment.

On a different note, another important thing to keep in mind with non fusible safety switches is the SCCR. The SCCR of most is only 10,000. They typically "series rate"with fuses, but not circuit breakers. Just something to be careful of.

What else would you call it?

The SCCR of the switch isn't an issue with DC PV circuits. PV modules are current limited devices and under most conditions cannot deliver more current than their Isc (short circuit current) rating. To account for cloud edge effects and other incident insolation over 1000W/m^2, a 25% safety factor is added to Isc to define the maximum available fault current, and the rating of the unfused disco must be higher than that.

 

[GoldDigger]

But the disco must be rated for DC loadbreak (non-inductive) if you are not going to break the circuit elsewhere first.

 

[ggunn]

But the disco must be rated for DC loadbreak (non-inductive) if you are not going to break the circuit elsewhere first.

Oh, sorry, brain fart; I misspoke. The unfused discos I use are usually on the AC side between the inverter(s) and the interconnection. The same thing holds, though; PV inverters are also current limited and cannot deliver more current than their rated Imax.

 

[electrofelon]

What else would you call it?

There is no NEC "80%" rule for non fused safety switches. Also as far as I know, no manufacturers require a derating for continuous currents. Thus there is no need to add a 100% qualifier.

 

[electrofelon]

Oh, sorry, brain fart; I misspoke. The unfused discos I use are usually on the AC side between the inverter(s) and the interconnection. The same thing holds, though; PV inverters are also current limited and cannot deliver more current than their rated Imax.

But you would still have high fault currents from the utility side resulting from an inverter failure or fault on the inverter output circuit. You can't just cosnider the pV side.

 

[Carultch]

But you would still have high fault currents from the utility side resulting from an inverter failure or fault on the inverter output circuit. You can't just cosnider the pV side.

For equipment rated to withstand up to 200 kA, I'm not worried about any realistic amount of fault current:
http://www.cesco.com/resources/785901/60922-AttachmentURL.pdf

 

[electrofelon]

For equipment rated to withstand up to 200 kA, I'm not worried about any realistic amount of fault current:
http://www.cesco.com/resources/785901/60922-AttachmentURL.pdf

That 200K figure is "max" If you look at the more detailed information in the square D catalog, the base SCCR is 10K. It is higher when there are certain classes of fuses ahead of it. The SCCR is only 10K with ANY circuit breaker ahead of it (they have not been evaluated when used in conjunction with circuit breakers - that is what square D says, I cant immediately comment on other brands). So this is an oft overlooked incorrect installation: say you have a supply or load side connection made with a circuit breaker in a switch or panel board. After that you hit a non fused disconnect on the outside of the building per utility requirement. You will likely have a problem with the 10K SCCR of the switch.

 

[Carultch]

That 200K figure is "max" If you look at the more detailed information in the square D catalog, the base SCCR is 10K. It is higher when there are certain classes of fuses ahead of it. The SCCR is only 10K with ANY circuit breaker ahead of it (they have not been evaluated when used in conjunction with circuit breakers - that is what square D says, I cant immediately comment on other brands). So this is an oft overlooked incorrect installation: say you have a supply or load side connection made with a circuit breaker in a switch or panel board. After that you hit a non fused disconnect on the outside of the building per utility requirement. You will likely have a problem with the 10K SCCR of the switch.

I would think that the default KA value indicated on a datasheet would be what it is, regardless of what other equipment is in series with it. And only when other specific equipment is in series with it, would it be overridden.

Thanks for letting me know what to expect. I never would've anticipated this being an issue, until now.

Other than just "that's what the documentation says", is there a reason why an unfused disconnect would be limited to the fault current it can withstand when breakers are present instead of fuses?

 

[electrofelon]

I would think that the default KA value indicated on a datasheet would be what it is, regardless of what other equipment is in series with it. And only when other specific equipment is in series with it, would it be overridden.

I agree. The default value is 10K. I think what they mean to say in the document you posted is, "you can get up to 200K when this switch is used in conjunction with certain fuses."

Here is the link to the square D catalog, pertinent information is on page 28.

http://static.schneider-electric.us/docs/Electrical Distribution/Safety Switches/3100CT0901.pdf

Note that it does say, "Consult the wiring diagram of the switch to verify the UL Listed short circuit current rating." But in the case I ran into that brought this to my attention, the label was not more generous than the chart in the catalog.

The base 10K values do seems surprisingly low.

Other than just "that's what the documentation says", is there a reason why an unfused disconnect would be limited to the fault current it can withstand when breakers are present instead of fuses?

I think it just comes down to tested combinations. I read somewhere, perhaps a square D bulletin, that simply stated that they had "not been evaluated with circuit breakers".

 

[ggunn]

But you would still have high fault currents from the utility side resulting from an inverter failure or fault on the inverter output circuit. You can't just cosnider the pV side.

That's what the OCPD at the interconnect is for. I guess I don't get what you are driving at; are you saying that an inverter that has a 25A maximum current and which is interconnected through a backfed breaker needs a 60A unfused switch instead of a 30A one? How would that be any different in the event of a fault fed from the utility side? I interconnect them through 30A switches virtually every day and I have never had an AHJ challenge it.

 

[electrofelon]

Ok consider a commercial rooftop system with 450 amps of inverter output current. I am using a larger system because it will have higher utility fault currents whereas a resi system may not have an issue. I come down off the roof with inverter output circuits to a combiner panelboard on the side of the building. Next to that I have a non fused disco. The panelboard and disco are 600 A. After the disco, the feeder goes through the exterior wall into the electrical room where it connects to the interconnect breaker (supply or load side, shouldn't matter for this discussion). The SCCR of the switch is 10k with this configuration. This is likely too low and a code violation. Do you agree with this or do you think my reasoning is incorrect?