CB for MWBC on 120/208 three phase Eaton PRL1a panel

[oceanobob]

Time to run some electrical outlets at 120VAC: would like to run as a Multi Wire Branch Circuit (MWBC).

Having difficulty with the circuit breaker (CB) selection since the two pole breaker has the slash rating (120/240) but the three pole breaker does not. This has been confirmed by the salesrep at the supply house in contact w mfg and available literature.

Original idea was to have two sets of 'three hots and one neutral' for ea string w a #10 copper stranded conductor THHN in 3/4" EMT (thinwall) with one 20a CB Circuit Breaker with three poles for each set. Total thinking is/was six hots and two neutrals and two three pole circuit breakers.

But with the limitation of the 120/240 label for the CB looks like I need to go with 6 hots, three neutrals and three two pole circuit breakers. EMT limit getting close with 9 number #10s but OK.

Derating at 70% of the 90C 10's value is 28 amps (fyi number 12's at 21 amps but I'll prolly use the 10's) - this checks out.


Otherwise this plan now seems valid, yet am unable to understand why the three pole breaker is not available slash rated?

 

[infinity]

Your system is 208Y/120 you can use regular single pole circuit breakers with a handle tie or a 3 pole CB.

 

[LarryFine]

Having difficulty with the circuit breaker (CB) selection since the two pole breaker has the slash rating (120/240) but the three pole breaker does not.

Are you saying it's rated only 120v or 240v?

 

[infinity]

3 pole CB's (straight 240 rated) can be used on a 3Ø, 4 wire delta on all three phases. A 2 pole slash rated CB (120/240) cannot be used on the high leg phase because the voltage is 208 and exceeds the 120 volt rating. The Wye system in the OP does not have a high leg.

 

[texie]

Time to run some electrical outlets at 120VAC: would like to run as a Multi Wire Branch Circuit (MWBC).

Having difficulty with the circuit breaker (CB) selection since the two pole breaker has the slash rating (120/240) but the three pole breaker does not. This has been confirmed by the salesrep at the supply house in contact w mfg and available literature.

Original idea was to have two sets of 'three hots and one neutral' for ea string w a #10 copper stranded conductor THHN in 3/4" EMT (thinwall) with one 20a CB Circuit Breaker with three poles for each set. Total thinking is/was six hots and two neutrals and two three pole circuit breakers.

But with the limitation of the 120/240 label for the CB looks like I need to go with 6 hots, three neutrals and three two pole circuit breakers. EMT limit getting close with 9 number #10s but OK.

Derating at 70% of the 90C 10's value is 28 amps (fyi number 12's at 21 amps but I'll prolly use the 10's) - this checks out.


Otherwise this plan now seems valid, yet am unable to understand why the three pole breaker is not available slash rated?

3 pole breakers are not slash rated. Does not matter for your application. You can use a 3 pole common trip or, as mentioned, 3 single pole breakers with handle ties are compliant as well.
Are we clear the you have a 208/120 Y system?

 

[winnie]

Just to be clear: the 240/120V slash rating is a 'weaker' rating than the straight 240V rating. You can use a straight 240V rating anywhere the slash rating would be acceptable plus a few more places.

A 240/120 slash rated breaker is acceptable for systems with up to 240V L-L but only 120V L-G.

A straight 240 rated breaker is acceptable for 240V L-L as well as 240V L-G.

Jon

 

[oceanobob]

To clarify: The center tap delta and its characteristic high leg nomenclature is not the system being described here. Indeed the transformer for this is/was installed in the building primarily for these types of loads and has a wye secondary and it is 120/208.


Thank ye - the solution is the handle tie for three singles: because all outlets will be 120vac. There is no plan nor intention to utilize the 208.
The neutrals will be marked to allow identification, and if there are two, then one can be gray and one can be white.

As to me wondering why the three pole breaker does not have the slash rating, I realize it doesn't need to.

Many Thanks!

 

[infinity]

As to me wondering why the three pole breaker does not have the slash rating, I realize it doesn't need to.

Many Thanks!

Basically the 3 pole is designed to work on either 208 volt or high leg systems. I really wonder what the internal difference is between that and a slash rated breaker.

 

[LarryFine]

A 120/240v slash rating is of a "lower" rating than a straight-rated 240v breaker is.

 

[texie]

I really wonder what the internal difference is between that and a slash rated breaker.

I've often wondered about this as well. Reminds of the days when we had different part numbers for breakers rated HACR before the standard was changed to include the HACR rating on all breakers. This seems similar to me.
And before this was combined many electricians didn't even know what a HACR breaker was. There were an awful lot of non HACR breakers installed where there should have been as a result just as I am sure, as I've seen many, there are a lot of slash rated breakers installed line to high leg line for the same reason.