Slash rated breakers

Status
Not open for further replies.
Let's say I have a breaker rated 120/240. If I wanted to use that to feed a 240v single phase load out of a 120/240 3 phase panel would it be allowed to go on phase A and B (high leg). OK now on a slash rated breaker if I recall correctly the NOMINAL voltage to ground can not exceed the lower of the two values (120). IMO the 208 to ground on the high leg is not a nominal voltage. So I think this would be fine. Am I understanding this correctly? I believe code article was 240.87
 

edward

Senior Member
Location
CA
Occupation
Electronologist
The 120/240 are OK on a 3 phase 240V.

You are correct the line to ground voltage can not exceed the lower of the rated value.

A 120/240 rated breaker can not be used on the system with 208 to ground.
 
The 120/240 are OK on a 3 phase 240V.

You are correct the line to ground voltage can not exceed the lower of the rated value.

A 120/240 rated breaker can not be used on the system with 208 to ground.
But my argument is that 208 is not a NOMINAL VOLTAGE according to the definition. I know that the intent was for that breaker but to be used in that situation but that's not what the code says.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I think that you are correct that the word "nominal" does not belong in the section, however based on 110.3(B) and the UL Guide Information for "Circuit breakers, molded case and circuit breaker enclosures" (DIVQ), I see the use of that breaker on that system as a code violation. The following is from the UL Guide Information (White Book).
Multi-pole common trip circuit breakers rated 120/240 V ac are suitable for use in a single-phase multi-wire circuit, with or without the neutral connected to the load, where the voltage to ground does not exceed 120 V.
 

edward

Senior Member
Location
CA
Occupation
Electronologist
Let's say I have a breaker rated 120/240. If I wanted to use that to feed a 240v single phase load out of a 120/240 3 phase panel would it be allowed to go on phase A and B (high leg). OK now on a slash rated breaker if I recall correctly the NOMINAL voltage to ground can not exceed the lower of the two values (120). IMO the 208 to ground on the high leg is not a nominal voltage. So I think this would be fine. Am I understanding this correctly? I believe code article was 240.87

What do you mean it is not a nominal voltage? Then what do you see it as?

IMO, It is not the actual voltage so I see it as nominal voltage.
 
The nominal voltage of that system is 120/240. Not 120/208/240. That 208 is an unusable voltage so there for not a nominal voltage. The definition says nominal voltage is a value assigned for ease of designating a voltage class of a system.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I see the use of nominal as a way of avoiding any implication that slash rated breakers cannot be used on 120V designated circuits if the actual delivered RMS voltage occasionally goes to 125 volts, as allowed by POCO tolerances.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I think that you are correct that the word "nominal" does not belong in the section, however based on 110.3(B) and the UL Guide Information for "Circuit breakers, molded case and circuit breaker enclosures" (DIVQ), I see the use of that breaker on that system as a code violation. The following is from the UL Guide Information (White Book).

Don, from that wording do you think it would be permissible to use a slash rated breaker as long as you connected to Phase A & C ?
 
I see the use of nominal as a way of avoiding any implication that slash rated breakers cannot be used on 120V designated circuits if the actual delivered RMS voltage occasionally goes to 125 volts, as allowed by POCO tolerances.
I completely agree that that was the intent of the definition but for dinner reason I don't see it saying that
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
The definition says nominal voltage is a value assigned for ease of designating a voltage class of a system.

In this case nominal means effectively means 'rounded' value, not the exact number. Another meaning would be industry standard values rather than measured values

Given a 240/120V system, the calculated high leg voltage is 207.84V, its nominal voltage is 208V.
So with a measured 236/118V, the high leg voltage would be 204.38V, but we would still call it nominal 208V.
 

kwired

Electron manager
Location
NE Nebraska
In this area anything that is nominal 208 volts generally will read around 215 with no load.


I believe the 120/240 rated breaker is not listed for use where applied to more then 120 (nominal) to ground. On a high leg system it could be used on both legs that are 120 to ground but can not be used on the high leg even if only supplying line to line loads. 3 pole breakers are not an issue because they are always rated 240 volts, but two pole 240 volt breakers are not so easy to come by. Most all mfgrs have a catalog number for them but not a lot of demand so they usually are not stocked at supply houses.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
In this case nominal means effectively means 'rounded' value, not the exact number. Another meaning would be industry standard values rather than measured values

Given a 240/120V system, the calculated high leg voltage is 207.84V, its nominal voltage is 208V.
So with a measured 236/118V, the high leg voltage would be 204.38V, but we would still call it nominal 208V.
Jim,
I agree that was the intent, but the term "voltage, nominal" is defined in Article 100 and that is the only definition that we can use for the application of code rules.
Voltage, Nominal. A nominal value assigned to a circuit or system for the purpose of conveniently designating its voltage class (e.g., 120/240 volts, 480Y/277 volts, 600 volts).
It appears to me the nominal voltage of a 3 phase high leg delta system is 240/120.

Jim,
On a closer reading of 240.85. The wording says "nominal voltage of any conductor", so I guess the definition in Article 100 does not actually apply, and you are correct.
 
Last edited:
Status
Not open for further replies.
Top