Delta MWBC code

[mbrooke]

Ok, not sure what its technically called:ashamed1:, but can you take for example 3 208 volt hand dryers, run 3#12s or #10s fed via a 3 pole breaker and connect one A&B, another B&C and the last C&A instead of running 6#14s with 3 double pole breakers?

 

[charlie b]

Yes. It is not a MWBC, it is just a branch circuit.

 

[mbrooke]

Yes. It is not a MWBC, it is just a branch circuit.

:lol: Didn't know the name.:ashamed1: But nothing in code prohibits, this correct?

Dryer specs are 208/230 volts 10 amps 2300 watts. Im thinking 2x 10= 17.3 amps so #12 per phase? Is my math correct?

 

[GoldDigger]

:lol: Didn't know the name.:ashamed1: But nothing in code prohibits, this correct?

Dryer specs are 208/230 volts 10 amps 2300 watts. Im thinking 2x 10= 17.3 amps so #12 per phase? Is my math correct?

Your math is conservative. If you run one dryer from A to B, the current in both A and B will be 10A. But when you add another dryer from B to C, the current in B will be less than 20A because of partial cancelation of the vectors.
I would give you the actual number but it is too early in the morning for me to deal with sqrt(3) properly.

 

[charlie b]

Last time I checked, 2 times 10 was 20. I don't know how 17.3 come into this discussion. So no, the math is not correct.

If the dryer is rated for 2300 watts, then the current at 230 volts would be 2300/230, or 10 amps. So that is what I think the rating of 10 amps is based upon. But this would be a resistive load, so the power it will deliver at a lower voltage will not be 2300 watts. At 208 volts, the current would be (10) times (208/230), or about 9 amps. So #12 wires should be fine. Just to complete the story, at 208 volts the power delivered will be (2300 watts) times (208/230)-squared, or about 1880 watts. Are you sure the owner will be happy with that? Perhaps a different model of hand dryer might be a better choice?

 

[mbrooke]

Your math is conservative. If you run one dryer from A to B, the current in both A and B will be 10A. But when you add another dryer from B to C, the current in B will be less than 20A because of partial cancelation of the vectors.
I would give you the actual number but it is too early in the morning for me to deal with sqrt(3) properly.

Vector wise I take the highest of the 2, (in this case both are the same) and get around 17.3 amps, so its well worth a 20? Reason why I ask is because most job specs look for individual runs rather than sharing phases.

 

[charlie b]

To be clear, my previous post only addressed a single dryer. You have a total load of 1880 watts times three dryers, or 5640 watts. However, to be conservative, I would ignore the impact on the power delivered of using the lower voltage, and say the total load is 2300 times 3, or 6900 watts. Divide 6900 by 208 volts, and divide again by the square root of 3, and the current will be 19.1 amps. Please note that these hand dryers will not run continuously for 3 hours. So you don't need to include a 125% factor for the wires or the breaker. A 20 amp, 3-pole breaker with #12 wires would be an acceptable design choice.

 

[mbrooke]

Last time I checked, 2 times 10 was 20. I don't know how 17.3 come into this discussion. So no, the math is not correct.

If the dryer is rated for 2300 watts, then the current at 230 volts would be 2300/230, or 10 amps. So that is what I think the rating of 10 amps is based upon. But this would be a resistive load, so the power it will deliver at a lower voltage will not be 2300 watts. At 208 volts, the current would be (10) times (208/230), or about 9 amps. So #12 wires should be fine. Just to complete the story, at 208 volts the power delivered will be (2300 watts) times (208/230)-squared, or about 1880 watts. Are you sure the owner will be happy with that? Perhaps a different model of hand dryer might be a better choice?

I ended up with 17.3 by taking 10 and multiplying it by square root of 3, which I believe is correct when adding equal phase currents. Remember 2 dryers will share 1 phase.


As for the wattage the dryer is slash rated 208/230 volts, 10 amps 2300 watts. I would imagine the number current wise is based on 230 volts instead of 208. you are correct ohms law puts this at 1800 watts, the same as a 15 amp 115 volt model.

 

[mbrooke]

To be clear, my previous post only addressed a single dryer. You have a total load of 1880 watts times three dryers, or 5640 watts. However, to be conservative, I would ignore the impact on the power delivered of using the lower voltage, and say the total load is 2300 times 3, or 6900 watts. Divide 6900 by 208 volts, and divide again by the square root of 3, and the current will be 19.1 amps. Please note that these hand dryers will not run continuously for 3 hours. So you don't need to include a 125% factor for the wires or the breaker. A 20 amp, 3-pole breaker with #12 wires would be an acceptable design choice.

The way I do it is when connecting loads phase to phase I take the largest number and multiply by 1.73, which gives a good approximation of current per phase. Thanks for brining up the 3 hour rule, almost was about to do it automatically since Im thinking of commercial lighting loads:slaphead: