Single-phase 240V Loads on 240V High-Leg Delta Systems

Status
Not open for further replies.
Are there any code restrictions (or common sense restrictions) for using the high-leg on a high-leg system for single-phase 240V loads? For example, assuming B-phase is the high-leg, putting a 240V water heater across A/B phase? Certainly wouldn't work for 120/240V loads (e.g., a range, etc.), but I haven't seen any restriction for 240V loads.
Thanks in advance for your help.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
There should not be an issue with a straight single phase 240V load
True, but with attention paid to the size of the load compared to the capacity of the POCO pot(s) feeding the high leg and the size of the B phase service conductor.
Some older residential high leg, especially using a delta breaker, had limited capacity on the B leg.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
I agree with Dennis but, be sure to use a straight rated 240 volt breaker, a slash rated breaker (a 120/240 volt breaker) cannot be used for these circuits.

Roger
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
True, but with attention paid to the size of the load compared to the capacity of the POCO pot(s) feeding the high leg and the size of the B phase service conductor.
Some older residential high leg, especially using a delta breaker, had limited capacity on the B leg.

I have seen that on commercial buildings as well.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I have wondered that as well. I have never torn any apart to see what the difference is.

Roger
Slash rated breakers are not built to withstand the same mechanical forces as those without the slash rating, because on a slash rated breaker the maximum asymmetrical fault (line to ground) is at 120V, so less total ENERGY in the fault flow. In other words 10,000A at 120V is a lot less energy than 10,000A at 240V, so therefore the mechanical forces are lower and the parts that must stop that from turning the breaker into shrapnel are going to be smaller / lighter. You can't necessarily see anything different from the outside, but going with the slash rating allows them to build them slightly cheaper
 

kwired

Electron manager
Location
NE Nebraska
Slash rated breakers are not built to withstand the same mechanical forces as those without the slash rating, because on a slash rated breaker the maximum asymmetrical fault (line to ground) is at 120V, so less total ENERGY in the fault flow. In other words 10,000A at 120V is a lot less energy than 10,000A at 240V, so therefore the mechanical forces are lower and the parts that must stop that from turning the breaker into shrapnel are going to be smaller / lighter. You can't necessarily see anything different from the outside, but going with the slash rating allows them to build them slightly cheaper

Makes sense - until you have a 120/240 volt rated breaker being asked to interrupt a line to line fault with 10kA available - then what?
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
Slash rated breakers are not built to withstand the same mechanical forces as those without the slash rating, because on a slash rated breaker the maximum asymmetrical fault (line to ground) is at 120V, so less total ENERGY in the fault flow. In other words 10,000A at 120V is a lot less energy than 10,000A at 240V, so therefore the mechanical forces are lower and the parts that must stop that from turning the breaker into shrapnel are going to be smaller / lighter. You can't necessarily see anything different from the outside, but going with the slash rating allows them to build them slightly cheaper
Or at least that's what they claim. :happyyes:

Roger
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Makes sense - until you have a 120/240 volt rated breaker being asked to interrupt a line to line fault with 10kA available - then what?
If it's a Line-to-Line (symmetrical) fault on a 120/240V breaker, there will be 2 poles sharing the burden, at least mechanically, which is the issue.
 
Status
Not open for further replies.
Top