Unusual specification.

[Klem531]

Hello, 25 year IBEW Journeyman here. Currently working in the office of a small contractor estimating jobs. Anywho, I came across a head scratcher today whilst doing a take off. There is a spec that states: "In order to reduce short curicuit, the electrical contractor shall route 30ft of #12awg wire for each power and lighting branch circuit connected to the lighting control panel. Run wire from panel, up to joist space, loop and route back down to the L. C. P. ".
Has anyone come across something like this? Thanks.

 

[infinity]

Yes I've seen this for lighting panels. It believe that it has to the with the 5000 amp SCCR rating of the panel. Someone will chime in with the details

 

[LarryFine]

Welcome to the forum.

No, not in branch-circuit wiring.

 

[MyCleveland]

Looks like the EE cannot provide any calc so he adds this ridiculous feature into the design.

If your source panel for the lighting ccts had a full 65k available & X/R = 3 ...
277/480 -> outbound #12 CU
10' - 6395A
15' - 4403A
20' - 3353A
120/208 -> outbound #12 CU
10' - 2922A
15' - 1972A
20' - 1488A

 

[Klem531]

Welcome to the forum.

No, not in branch-circuit wiring.

Thanks.

 

[suemarkp]

Perhaps you can clarify if the circuit has 30' of wiring before hitting the first fixture or switch, is that sufficient to meet the 30' requirement?

 

[petersonra]

I think typical light switches are only rated for 5 kA SCCR. So this probably makes some sense.

 

[topgone]

I think typical light switches are only rated for 5 kA SCCR. So this probably makes some sense.

Most switches are rated 5 kA (other than mercury tube type switches). I also guess that's a limitation of an AWG# 14 wire which is calculated to melt in 0.01 seconds (or thereabout) when subjected to 5 kA per Onderdonk formula. Switch lines are allowed to be #14, IIRC.

 

[kingpb]

Simply adding impedance to reduce short circuit current at the device. Gut tells me there a better way to do this, but maybe not.

 

[JoeStillman]

It might be easy to prove unnecessary by doing a quick calculation. For example, it the panel is fed from a transformer 75 kVA or smaller, with 5%IZ, the AFC at the secondary will be south of 5kA every time.

 

[jim dungar]

It might be easy to prove unnecessary by doing a quick calculation. For example, it the panel is fed from a transformer 75 kVA or smaller, with 5%IZ, the AFC at the secondary will be south of 5kA every time.

75kVA and smaller transformers rarely have impedances as high as 5%. Most utility transformers are often around 1.5%.

But yes, for small transformer the fault current can be extremely low on branch circuits which is one of the reasons many breakers are accused of not clearing when a hot wire 'is purposely' touched to ground.

 

[Klem531]

It might be easy to prove unnecessary by doing a quick calculation. For example, it the panel is fed from a transformer 75 kVA or smaller, with 5%IZ, the AFC at the secondary will be south of 5kA every time.

I'm not certain on the kva rating but the incoming utility is 800 amp 120/208 fed directly from the utility pad mount . So there is no 75kva or smaller transformer in the mix.

 

[Jraef]

I'm not certain on the kva rating but the incoming utility is 800 amp 120/208 fed directly from the utility pad mount . So there is no 75kva or smaller transformer in the mix.

That may be what's behind this.

 

[kingpb]

I'm not certain on the kva rating but the incoming utility is 800 amp 120/208 fed directly from the utility pad mount . So there is no 75kva or smaller transformer in the mix.

Then add one just for lighting circuits.

Question you say 120/208V, so are you intending a single phase service/panel fed from a three phase utility circuit, or do you have 208Y/120V, 3ph, 4W service?

 

[augie47]

Fairly common approach here due to the 5k rating as others have noted. Sounds like someone did their homework on design

 

[MyCleveland]

Fairly common approach here due to the 5k rating as others have noted. Sounds like someone did their homework on design

If this is becoming common...great googly moogly. I think you are being very generous on this.

When will NEC step in on methods for storing all this extra wiring.
I am visioning a rod suspended pipe with x number of 50' MC spools hanging from it, all with ends tied into a pullbox or wireway below.
20 cct relay panel with 20 - 50' spools. 1000' of wasted wire.

On the serious side, what are you seeing in the field?

 

[kingpb]

Contractor should issue an RFI requesting clarification and then offer a more reasonable alternative design, with cost difference.

 

[augie47]

If this is becoming common...great googly moogly. I think you are being very generous on this.

When will NEC step in on methods for storing all this extra wiring.
I am visioning a rod suspended pipe with x number of 50' MC spools hanging from it, all with ends tied into a pullbox or wireway below.
20 cct relay panel with 20 - 50' spools. 1000' of wasted wire.

On the serious side, what are you seeing in the field?

I see it most often on designs from some "chain" stores most often with lighting control panels located adjacent to MDPs with a SCA of 25k or higher. It doesn't take a long run but a 5 ft circuit from MDP to the control panel won't cut it.