120/208v 3phase mwbc

[liquidtite]

we had a job for a school were we feed a subpanel with a 120/208 feed. the bcs will be suppllying computor loads.so we ran emt to 2 sets of five receptacles.each daul plug is a dedicated circuit so each plug has its own hot the first 3 plugs share a nuetral and the last two on the line share a nuetral.the circuits ocp is 20I and is ran with 12awgthhn/thwn.the conductors have already been drated suddenly their are 5 cccs in each raseway all ungrounded conductors are terminated on seperate phases,but my question is suddenly the bc is feeding a nonleniar load wouldnt this interfer with the nuetral and their could be a possibility of overloading the nuetrla.I think my boss said each computor draws around seven amps

 

[jumper]

we had a job for a school were we feed a subpanel with a 120/208 feed. the bcs will be suppllying computor loads.so we ran emt to 2 sets of five receptacles.each daul plug is a dedicated circuit so each plug has its own hot the first 3 plugs share a nuetral and the last two on the line share a nuetral.the circuits ocp is 20I and is ran with 12awgthhn/thwn.the conductors have already been drated suddenly their are 5 cccs in each raseway all ungrounded conductors are terminated on seperate phases,but my question is suddenly the bc is feeding a nonleniar load wouldnt this interfer with the nuetral and their could be a possibility of overloading the nuetrla.I think my boss said each computor draws around seven amps

LT, honestly - I cannot understand your post. I am not trying to be mean, but the grammar and spelling errors are a bit much.

 

[liquidtite]

sorryI was in a rush and was not able to reread or use spell check what are you confused about

 

[Dennis Alwon]

Are you counting the neutrals in the raceway? If they are nonlinear loads then they must be counted but in reality it should not affect the 20 amp thhn wire until there are more than 9 CCC-- so I don't think there is a problem.

 

[liquidtite]

Are you counting the neutrals in the raceway? If they are nonlinear loads then they must be counted but in reality it should not affect the 20 amp thhn wire until there are more than 9 CCC-- so I don't think there is a problem.

thanks

 

[Smart $]

Look at it like this. 4-wire wye circuits cannot have a neutral current higher than the sum of all three line currents. It will likely never get this high, but will give you a number for frame of reference. In your case, 7 ? 3 = 21A.

3-wire wye circuits even add the fundamantal, and it can be higher than the fundamental, but as the distorted amount of current increases, the fundamental decreases. All the same, the neutral current cannot be greater than 200% of the line current value. 7 ? 2 = 14A

Now you know the likely reason why the circuits are only loaded to 7A.

 

[liquidtite]

yes the nuetrals I belive were counted bc I think he said he would be using the 70% chart so theirs 5hots and 2 nuetrals.I wasnt shure if you would have to increase the nuetral size

 

[liquidtite]

thanks for the information

 

[jeremysterling]

The nameplate may say 7 amps, but I seriously doubt that a plain-jane school-duty desk-top computer pulls more than a couple of amps. Plug them all in and measure the neutral current.

In my opinion, you are OK. In my opinion you are probably NEC compliant.

However, my supervisor firmly believes that the circuit and load you describe is the "NEC spec'd majority non-linear load." Therefore, at work, I count the neutral as a CCC and will even oversize it to #10.

 

[liquidtite]

you would only count the nuetral for computor loads right

 

[Dennis Alwon]

you would only count the nuetral for computor loads right

Nope-- nonlinear loads such as loads with harmonics-- fluorescent lighting , for instance

 

[jeremysterling]

you would only count the nuetral for computor loads right

The load analysis where I work is this:

Linear=
1. Toaster
2. Electric Heat
3. Incandescent Lamp

Non-linear=
All loads not named above!

 

[infinity]

The load analysis where I work is this:

Linear=
1. Toaster
2. Electric Heat
3. Incandescent Lamp

Non-linear=
All loads not named above!

You would call a motor a non linear load?

 

[jeremysterling]

You would call a motor a non linear load?

Rob, according to wikipedia, there are low power factor induction motors that are considered linear loads.

IMO, a couple of small fractional horse motors distributed across a MWBC would not require derating of the neutral (or counting as a CCC).

However, my supervisor insists that in my design, I count all loads that are not strictly 100% resistive (i.e. heating elements and incandescent lamps) as non-linear. Therefore, at work, I would count the neutral in the above scenario as a CCC.

 

[Strife]

LT, honestly - I cannot understand your post. I am not trying to be mean, but the grammar and spelling errors are a bit much.

Second that.
Sorry, I was getting a headache trying to read that.
And I never had headaches:-(

 

[Strife]

Nope-- nonlinear loads such as loads with harmonics-- fluorescent lighting , for instance

Unless we're talking a building chuckfull of T12's, aren't most ballasts in the last 10-15 years very little harmonics?

 

[fmtjfw]

The reason you limit the number of computers on a given circuit is not the steady state load current, it is the inrush current. Putting more than about 5 "standard" computers on a single 20 amp circuit can cause lots of inrush trips. These normally occur when power is restored after an outage. OR when someone puts a wall switch in to turn on and off all the outlets at once. The inrush current is enough to trip a standard 20 amp breaker but too fast to show up on most DVMs. I have an analog Amprobe in my "got to get serious" testing box for situations like this.

 

[liquidtite]

Second that.
Sorry, I was getting a headache trying to read that.
And I never had headaches:-(

aww poor baby do you need an asprin:weeping:

 

[John120/240]

aww poor baby do you need an asprin:weeping:

Second that.
Sorry, I was getting a headache trying to read that.
And I never had headaches:-(

LT, honestly - I cannot understand your post. I am not trying to be mean, but the grammar and spelling errors are a bit much.

liquidtite, A heads up. It would make your post easier to read if you had a paragraph break

every once a while, or double space. The members here at Mike Holt Code Forum are very

knowledgeable and eager to help when they understand the problem.

 

[liquidtite]

so whats the difference between nonlinear and linear loads ?and why when dealing with nonlinear loads you must count the nuetrals as ccc