Control/Dimming Requirement for fusing an onstage switch?

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I am attempting to explain to a licensed electrician why an onstage switch would need to be fused.

In our example, there is a 100A 3-phase (4 wire + ground) feed to the stage, which is directly wired from a 100A 3-pole breaker in a separate panel. That panel is in another room.

The box they have installed (which I am questioning) is a simple safety switch with no internal fusing. For budgetary reasons, the client needed to go this route instead of installing a pre-manufactured theatrical company switch, which would of course have its own breakers. The switch is located onstage where it would only be operable by qualified personnel. It is likely that the first uses of this power source will use temporary Cam-Lok tails (as would be installed by a touring company) and when budget permits the theatre can install a wall panel with Cam receptacles so there won't be a need to open the switch to tie in.

The electrician claims the wiring comes from a breaker, so the box need not be fused. He says, "they cannot be fused twice."

I quoted NEC 520.51:
IV. Portable Switchboards on Stage
520.51 Supply.
Portable switchboards shall be supplied only from power outlets of sufficient voltage and ampere rating. Such power outlets shall include only externally operable, enclosed fused switches or circuit breakers mounted on stage or at the permanent switchboard in locations readily accessible from the stage floor. Provisions for connection of an equipment grounding conductor shall be provided. For the purposes of conductor derating, the requirements of 520.27(B) shall apply.

I think the underlined areas in the code quoted above make it clear but I want to get my argument organized. So I'm wondering what the compelling reasons are to fuse the box--of course it seems safer--but what are the specific issues that can be addressed in this conversation?

Many thanks to the group.
 

STEVETERRY

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Aug 12, 2007
Location
New York
I am attempting to explain to a licensed electrician why an onstage switch would need to be fused.

In our example, there is a 100A 3-phase (4 wire + ground) feed to the stage, which is directly wired from a 100A 3-pole breaker in a separate panel. That panel is in another room.

The box they have installed (which I am questioning) is a simple safety switch with no internal fusing. For budgetary reasons, the client needed to go this route instead of installing a pre-manufactured theatrical company switch, which would of course have its own breakers. The switch is located onstage where it would only be operable by qualified personnel. It is likely that the first uses of this power source will use temporary Cam-Lok tails (as would be installed by a touring company) and when budget permits the theatre can install a wall panel with Cam receptacles so there won't be a need to open the switch to tie in.

The electrician claims the wiring comes from a breaker, so the box need not be fused. He says, "they cannot be fused twice."

I quoted NEC 520.51:
IV. Portable Switchboards on Stage
520.51 Supply.
Portable switchboards shall be supplied only from power outlets of sufficient voltage and ampere rating. Such power outlets shall include only externally operable, enclosed fused switches or circuit breakers mounted on stage or at the permanent switchboard in locations readily accessible from the stage floor. Provisions for connection of an equipment grounding conductor shall be provided. For the purposes of conductor derating, the requirements of 520.27(B) shall apply.

I think the underlined areas in the code quoted above make it clear but I want to get my argument organized. So I'm wondering what the compelling reasons are to fuse the box--of course it seems safer--but what are the specific issues that can be addressed in this conversation?

Many thanks to the group.
The electrician is correct that one cannot have two overcurrent protective devices of the same rating in series. So, if the existing upstream OCPD is 100A, you cannot have another set of 100A fuses in the disconnect. 520.51 requires a fused disconnect under the assumption that the fuses need to be readily accessible at the connection point. If the upstream fuses are readily accessible but just in another location easily reachable from the stage, I don't think any inspector would argue with you. In any event, you are covered from an OCPD point of view.

ST
 

click

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Joined
Mar 1, 2011
Location
NYC
The electrician is correct that one cannot have two overcurrent protective devices of the same rating in series. So, if the existing upstream OCPD is 100A, you cannot have another set of 100A fuses in the disconnect. 520.51 requires a fused disconnect under the assumption that the fuses need to be readily accessible at the connection point. If the upstream fuses are readily accessible but just in another location easily reachable from the stage, I don't think any inspector would argue with you. In any event, you are covered from an OCPD point of view.

ST
Thanks, this makes sense. The upstream breaker panel is in the basement, certainly accessible but not convenient to the stage. Given that one cannot have two overcurrent protective devices of the same rating in series, how could one put fuses at the connection point?
 

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Thanks, this makes sense. The upstream breaker panel is in the basement, certainly accessible but not convenient to the stage. Given that one cannot have two overcurrent protective devices of the same rating in series, how could one put fuses at the connection point?
Come off the main of that panel without a branch circuit.

Whats your object here? Why would you want fuses over a breaker? There is really no reason to install a fused disconnect these days... and one could say the same thing about a bare end tie in.
 

sk8rsdad

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I defer to Steve Terry and others who live and breath the NEC but I believe there were papers published in the 70s that discuss the behaviour of series OCPDs and how at the moment of interruption the OCPD that is opening can cause unpredictable currents to be observed by the the other OCPD which cause it to fail to perform as intended.
 

SteveB

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Brooklyn, NY
Come off the main of that panel without a branch circuit.

Whats your object here? Why would you want fuses over a breaker? There is really no reason to install a fused disconnect these days... and one could say the same thing about a bare end tie in.
I'm visualizing a fused panel with empty lugs on the bottom, live feeders coming on top. You go kill the breaker in the basement and tie in bare ends, then go energize the breaker.

Cheap gear.

As opposed to a panel with a breaker in it, which is against code, which if you did away with the basement OCPD, would need to supply a separate non-fused tie-in box with lugs for tails.

This is the reason the Lex and other company switches are now standard.
 

robartsd

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I defer to Steve Terry and others who live and breath the NEC but I believe there were papers published in the 70s that discuss the behaviour of series OCPDs and how at the moment of interruption the OCPD that is opening can cause unpredictable currents to be observed by the the other OCPD which cause it to fail to perform as intended.
I could see it being annoying to have both OCPD trip when high current is detected requiring resetting of both. It would be quite a nuisance if the inrush of resetting one OCPD caused one in series to trip. I certainly can see how it could be poor system design, but the only safety argument I can see is that the nuisance might cause people to seek ways to bypass the protection of one or both OCPDs.
 

sk8rsdad

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I think it's more complicated than that. The "fail to perform as intended" covers more than both OCPDs that protect against the same type of fault. Throwing out some hypotheticals:
  • What happens when one of the devices is a GFCI?
  • What happens when one of the devices is 3-phase and the other isn't?
  • What happens when one protects against surge and the other protects against continuous overcurrent?
 

tjrobb

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Cedar Rapids, Iowa
I don't have my NEC in front of me, but I seem to recall a "no more than two OCPD" statement in there.
Also, our theatre was remodeled in 2010. Our stage switch has a 200A breaker in the main gear, and another 200A breaker at the unit on the stage. Several inspections later, no one's complained.
 

BillConnerFASTC

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Clayton NY 13624
I don't have my NEC in front of me, but I seem to recall a "no more than two OCPD" statement in there.
Also, our theatre was remodeled in 2010. Our stage switch has a 200A breaker in the main gear, and another 200A breaker at the unit on the stage. Several inspections later, no one's complained.
I always love the "several inspections later" view. I see so many clear non-compliant with code conditions. I simply don't expect any building official or authority having jurisdiction to really know all of the requirements especially for stages. ADA violations are rampant - and I'm always hearing something justified because they saw it elsewhere and erroneously assume it was up to code. I was surprised when an architect on a code committee used that as rationale. Just be careful assuming that what you see "inspected" and allowed to remain actually meets code.
 

tjrobb

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I always love the "several inspections later" view. I see so many clear non-compliant with code conditions. I simply don't expect any building official or authority having jurisdiction to really know all of the requirements especially for stages. ADA violations are rampant - and I'm always hearing something justified because they saw it elsewhere and erroneously assume it was up to code. I was surprised when an architect on a code committee used that as rationale. Just be careful assuming that what you see "inspected" and allowed to remain actually meets code.
Yeah, I have another work site that, pending further legalities, I can't go into detail. Suffice to say, the building shouldn't be open with the violations I found.
I more wanted to state that it might not be an issue, legal or no.
 

BillConnerFASTC

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More simply, I do not believe an inspection with not items requiring correction is much of a defense. I suppose it could lower an award for negligence a few points.
 

robartsd

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I don't have my NEC in front of me, but I seem to recall a "no more than two OCPD" statement in there.
Also, our theatre was remodeled in 2010. Our stage switch has a 200A breaker in the main gear, and another 200A breaker at the unit on the stage. Several inspections later, no one's complained.
I'm certainly not an expert, but I'm not convinced that equal rated OCPD in series poses a critical safety risk - I can certainly see an AHJ being OK with it as installed even if it's not up to code. I would have hoped that they would catch the code violation in plan check and verified that construction matched the plans.
 

STEVETERRY

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New York
The electrician is correct that one cannot have two overcurrent protective devices of the same rating in series. So, if the existing upstream OCPD is 100A, you cannot have another set of 100A fuses in the disconnect. 520.51 requires a fused disconnect under the assumption that the fuses need to be readily accessible at the connection point. If the upstream fuses are readily accessible but just in another location easily reachable from the stage, I don't think any inspector would argue with you. In any event, you are covered from an OCPD point of view.

ST
So, this doesn't happen too often :) , but.....

I can find no prohibition in the NEC, and specifically in Article 240 (Overcurrent Protection), against having two overcurrent protective devices of the same rating in series. I really, really,really tried to find one, but as far as I can see, it ain't there. Sorry for my previous misstatement! Or, can someone else find it?

ST
 

TJCornish

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St. Paul, MN
I can't say if it's in the NEC or not, but series OCPD is pretty commonly done in the portable equipment world - most distros have a main breaker and these are connected to company switches with OCPDs.

While we're talking about goofy code things, can someone explain why #2 SC is rated for 190A, and several places in the code allow rounding up to the next standard OCPD size, but #2 is not considered acceptable for 200A feeder use? I understand voltage drop concerns - looking for a code reason. Or where can I buy #1.75 ga SC? :)
 

STEVETERRY

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New York
I can't say if it's in the NEC or not, but series OCPD is pretty commonly done in the portable equipment world - most distros have a main breaker and these are connected to company switches with OCPDs.

While we're talking about goofy code things, can someone explain why #2 SC is rated for 190A, and several places in the code allow rounding up to the next standard OCPD size, but #2 is not considered acceptable for 200A feeder use? I understand voltage drop concerns - looking for a code reason. Or where can I buy #1.75 ga SC? :)

When you consider cable ampacity--at 200A you have options. 2/0 AWG seems like a prudent choice. At 400A, you have less options, and are forced to use 4/0 at a 405A ampacity. This is a tussle between industry conventions (100A=#2, 200A =2/0, 400A=4/0) and NEC allowable ampacities. The conservative route wins, since it builds in leeway for voltage drop.

ST
 

TJCornish

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St. Paul, MN
The cost and weight of #2 SC vs 2/0 is significantly different, and the purported "190 amps" of #2 cable is attractive to the uneducated. It would be nice if a couple numbers like this - the other one being the ampacity of 6/4 cord - 45 amps as currently defined for linear loads - be more closely aligned to standard breaker sizes, or at least their use for entertainment purposes be more clearly defined - i.e. that 50A California/NEMA 14-50 require #4 wire.

Voltage drop typically becomes the limiting factor sooner than the fire hazard, but most people don't see past the cost and weight of the wire, and are left with under-built assemblies and poor gear performance. This is not helped by some 50A connectors topping out at #6 wire for their terminal capacity.
 

JD

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Jan 1, 2005
Location
North Wales PA
The same piece of wire can have a lot of different ratings! For example, in a Prius, the batteries which are fused at 160 amps, are wired to the inverter with #4. (220 volts DC) Local power companies will often feed a 200 amp service with #6 pole loop. The amount of current that is needed to blow open a given conductor size is astounding!
So, the wire itself doesn't really have a clear rating until you factor in the use. What is the duration of high current? In the Prius example, it is very low. Conversely, a constant current draw (24/7) enclosed in a conduit can be damaged quite easily.
Is the wire in open air? Is it a single conductor laying on the ground? Is it part of a multi-pair cable buried in the middle of other conductors that are generating heat?
This is why we have the NEC tables. Each application is different. The same conductor will have several different ratings dependent on the factors above. The safest route is to follow the ampacity / size guides in the chart.
... and then there are those engineered assemblies like Socapex ;)