three phase plug designations and protection

[projectmgr]

I have three questions to resolve a difference of opinion I am having with an outside consultant:
1) Me: The label or designation on a three phase plug or receptacle, i.e. 30a, calls out the
maximum current per leg.
Her: The 30a designation calls out a total amp load for the plug.

2) Me: The 30a labeled plug should be protected by three 30a breakers.
Her: Protecting the plug with 30a breakers would require a 90a plug. (I never heard of
one of those)

3) There is also the question if we protected a 30a rated plug with three 20a breakers we
would be basically overprotected the plug but will no be violating code.

Any guidance, particularily code references covering these issues, would be highly appreciated.

 

[Dennis Alwon]

I have three questions to resolve a difference of opinion I am having with an outside consultant:
1) Me: The label or designation on a three phase plug or receptacle, i.e. 30a, calls out the maximum current per leg.

This is correct

2) Me: The 30a labeled plug should be protected by three 30a breakers.

You are correct

3) There is also the question if we protected a 30a rated plug with three 20a breakers we
would be basically overprotected the plug but will no be violating code.

You cannot protect a 30 amp plug with a 20 amp circuit-- 210.21(B)(3)

 

[iceworm]

I have three questions to resolve a difference of opinion I am having with an outside consultant:
1) Me: The label or designation on a three phase plug or receptacle, i.e. 30a, calls out the
maximum current per leg.
Her: The 30a designation calls out a total amp load for the plug.

2) Me: The 30a labeled plug should be protected by three 30a breakers.
Her: Protecting the plug with 30a breakers would require a 90a plug. (I never heard of
one of those)
*********************

Any guidance, particularily code references covering these issues, would be highly appreciated.

Caution: Rant to follow - Delete before reading

Hummm .... A "consultant" charges money for their knowledge - Right? That is sad

Anybody that refers to the "total load" on a 3phase circuit and then adds the three currents togethers for the number needs to be taken out back and beat with a nerf bat until your attitude improves.

********** Or, better yet *********

Get the name of their instructor and take them out back for an appointment with the nerf bat.

Interesting note: I've found that anyone that uses the term "xxAmps per phase" is right there with those that would add the three phase currents togther.

As for a code reference, the NEC is neither a design nor instruction manual.

I have no idea what you could say to get your consultant to rethink this. I can't think of any gentle way to say, "Look at manufacturers' product lines:
A (3pole) 100A CB can feed (3) 100A conductors, which can feed a (3 phase) 100A panel. A 100A cb won't trip until there is more than 100A going through at least 1 phase. There can be 99A per phase and it won't trip. But with two phases at 99A and one phase at 101A - it might trip, This is not a 300A circuit."

AARRGGG ....
Take me out to the nerf bat. I used one of the dreaded phrases

So sayeth the worm

 

[charlie b]

I am going to disagree with the first answer that Dennis gave, but only because I agree with the worm's rant over the use of the phrase "amps per phase."

1) Me: The label or designation on a three phase plug or receptacle, i.e. 30a, calls out the maximum current per leg.
Her: The 30a designation calls out a total amp load for the plug.

The words that appear in these two claims do not contradict each other, and you are technically both right. But you both stated your opinions in unclear language. So I will reword both of these statements, to make them both clear, and both accurate.

• You: The 30a designation on the label tells you the maximum current that we are allowed to pass through any single leg.
• Her: The 30a designation on the label tells you the total load that the plug can handle, with "total load" being defined as the highest load on any one of the legs (or, as some would say, the average of the three phase currents).

Now, that might not be what either of you intended to say. But if you try to clarify your intent, and if in doing so you once again use the phrase "current per leg" or the equivalent "amps per phase," then I will be heading to the store to buy a nerf bat. :happyyes:

 

[infinity]

3) There is also the question if we protected a 30a rated plug with three 20a breakers we
would be basically overprotected the plug but will no be violating code.

Any guidance, particularily code references covering these issues, would be highly appreciated.

A 30 amp single receptacle is permitted on a 20 amp OCPD. Not sure if the circuit will properly serve the load but it is permitted.