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

[fsu-eng]

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.

 

[Dennis Alwon]

There should not be an issue with a straight single phase 240V load

 

[GoldDigger]

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]

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

 

[Dennis Alwon]

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

Good point-- I wonder why that is an issue

 

[roger]

Good point-- I wonder why that is an issue

I have wondered that as well. I have never torn any apart to see what the difference is.

Roger

 

[ggunn]

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]

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]

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]

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]

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.