Ring Main / Ring Wiring Method

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Edit: looking at your illustration I'm seeing two separate sources feeding the OCPDs, the output from the UPS and a direct "maintenance" source that you would use when the UPS is taken off line for service. As shown, that would not be considered ring circuit wiring, if that's what you are talking about.

-Hal
 

romex jockey

Senior Member
Location
Vermont
Question is what advantage will this will give you?

-Hal
Well the history of ring circuits would be the place to start Hal

Originally, left with few resources after WW2 , the Britts ingenuity created what was essentially a 14G ring on a 30A RCD, with point of use fusing

One could almost copy paste typical poco line configuration here

Of course, i could be a tad off.......is there an EE in the house? :) ~RJ~
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
It is a ring. You have two sets of 50mm conductors fed from the ups, going to opposite ends of the infrastructure block. Note this is roughly 1/0 awg.

Jon
 

mpillow

Member
Location
Ohio
Occupation
Business Owner
So it's just a physical space limitation. Couldn't you just parallel the conductors? Seems to me it's just a matter of how you connect them.

-Hal
If the infrastructure block tied both sets of wire together at the first terminal block, then we would have to double the conductors (count or size) between each of the subsequent boxes.
 

mpillow

Member
Location
Ohio
Occupation
Business Owner
The hand sketch above is really the claim that we need to compare against the NEC. IE, is 310H the relevant section, and does that sketch comply with the requirements of 310H.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The hand sketch would not be compliant. If you think about the structure of a ring circuit, _each_ load could be considered to be supplied in parallel from the source, so at one location you would have two equal lengths of supply conductors, but at every other load you would have unequal lengths.

Under the NEC, if you want to combine two 150A conductors to act as a 300A conductor, you are pretty much limited to following the same path for the same distance, and a ring explicitly does not do that.

This sure looks like something being designed to comply with European standards but then being evaluated using NEC standards. You might want to look at the NEC 'tap rules' to see if any apply in your case. You _might_ be able to have a 300A breaker supply 150A conductors which then individually supply 150A breakers if suitable length limits are met.

-Jon
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Both ends are attached at the same point on the UPS, as Jon mentioned.
I did not see the little dot.

IMO, this is not a NEC legal wiring means.

My personal opinion is that it might be a good idea, and it certainly reduces voltage drop and may have other benefits, but is not NEC legal.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
I did not see the little dot.

IMO, this is not a NEC legal wiring means.

My personal opinion is that it might be a good idea, and it certainly reduces voltage drop and may have other benefits, but is not NEC legal.
A ring circuit seems very much like a Class A circuit in fire alarms. For system reliability, they have a lot going for them.

So, let's say there was a requirement to wire a ring circuit to handle the full current. Setting aside labor and wire costs, would there be any safety reason to not install the circuit, aside from not being allowed by the NEC?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
So, let's say there was a requirement to wire a ring circuit to handle the full current. Setting aside labor and wire costs, would there be any safety reason to not install the circuit, aside from not being allowed by the NEC?
Sorry, under the conditions you specified, what part of the NEC would prohibit the ring circuit?

Cheers, Wayne
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Under the NEC, if you want to combine two 150A conductors to act as a 300A conductor, you are pretty much limited to following the same path for the same distance, and a ring explicitly does not do that.
Suppose this ring circuit was on a 150a breaker. What's your opinion then?
 

paulengr

Senior Member
Rings have two advantages. The impedance is half, and there is a redundant path. They are used with two separate breakers sometimes in substations where a main-tie-main is operated with the tie closed. So you get lower voltage drop and higher ampacity.

The downside is fault current is doubled, and you get uneven loading since every point on the ring except the center has different impedance. So you don’t get double the ampacity. This is illegal under NEC because of the requirement that parallel conductors are the same length because you would need to protect the conductors independently. It can be done if you treat it as a distribution system under NESC and treat it as the same ampacity as a radial feed. So that would make it practically possible.

UK systems have a separate fused disconnect on each port of the RMU. There are 3 ports. Two are “ring” ports. The third is either a source or load. Multiple sources quickly becomes a “network” which requires lot of computer simulations to determine how it all works.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
Sorry, under the conditions you specified, what part of the NEC would prohibit the ring circuit?

Cheers, Wayne
Apparently the parallel conductor rule. Two hots under the same breaker forming a loop? You can do this now?
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As the OP is drawing things, you have a ring composed of 150A conductors, but protected by a 300A breaker.

It is debatable if this is a parallel conductor arrangement by the specific NEC definition (lengths of conductor are joined at both ends to form a single conductor). However if you make the argument that this isn't a prohibited parallel installation, then you need to make the additional argument that 150A conductors are properly protected by the 300A breaker.

-Jon
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Apparently the parallel conductor rule. Two hots under the same breaker forming a loop? You can do this now?
I don't see 310.10(I) as prohibiting a ring, when both supply ends are protected by the same breaker, sized for each individual conductor.

310.10(I) uses the phrase "electrically joined" which only occurs twice in the 2014 NEC, rather than the more much common "electrically connected." So I take "electrically joined" to mean a physical connection, rather than a connection through any electrically conductive path. That means the ring conductors aren't "electrically joined at both ends," they are only electrically joined at the supply end (assuming more than one load on the ring). So 310.10(I) doesn't apply.

Cheers, Wayne
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
I don't see 310.10(I) as prohibiting a ring, when both supply ends are protected by the same breaker, sized for each individual conductor.

310.10(I) uses the phrase "electrically joined" which only occurs twice in the 2014 NEC, rather than the more much common "electrically connected." So I take "electrically joined" to mean a physical connection, rather than a connection through any electrically conductive path. That means the ring conductors aren't "electrically joined at both ends," they are only electrically joined at the supply end (assuming more than one load on the ring). So 310.10(I) doesn't apply.

Cheers, Wayne
I'm tempted to pay money to watch you argue that with an AHJ.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
The downside is fault current is doubled, and you get uneven loading since every point on the ring except the center has different impedance.
Doesn't the shorter half compensate for the longer half? Yes, that creates the uneven loading, but again, what if you protect at the conductors' rating
 

myspark

Senior Member
Location
SCV Ca, USA
Occupation
Retired EE
I don't see 310.10(I) as prohibiting a ring, when both supply ends are protected by the same breaker, sized for each individual conductor.

310.10(I) uses the phrase "electrically joined" which only occurs twice in the 2014 NEC, rather than the more much common "electrically connected." So I take "electrically joined" to mean a physical connection, rather than a connection through any electrically conductive path. That means the ring conductors aren't "electrically joined at both ends," they are only electrically joined at the supply end (assuming more than one load on the ring). So 310.10(I) doesn't apply.

Cheers, Wayne
If the two wires are not "electrically" joined at the end or at any portion of the loop--then we are defeating the purpose of the ring wiring scheme.
We could simply call it radial wiring scheme that we are all familiar with.
 
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