120/240V load question. (No Brainer for most)

[bendomaniac]

Lets say I have a 100A 120/240V service. Is it feasible to max out each one the poles at a constant 80A load?, or is the 100A meant to be split between each of the poles (50A each)? Load would be continuous lighting for a commercial building. In no way do I claim to be master electrician, I am however trying to learn proper code adherence and implementation.

 

[Smart $]

Loading a 100A service to 80A continuous is practicable and compliant... and that is 100A measured on each line. However, your question appears to be hypothetical. A commercial building typically has loads other than just lighting.

 

[brantmacga]

its 100A symmetrical . . . .

does that help?

don't forget about continuous load ratings. . . . you'd be dead on the threshold.

 

[bendomaniac]

80A continuous on each pole(80A each pole), or 80A continuous between the two(40A each pole)? Thanks for the reply.

 

[bendomaniac]

Thank you. I did find in the code that 80% is the max draw for continuous loads. Two branch circuits with 80A max continuous load. One on pole A and one on pole B on a 100A 120/240V service. Is this overloading since there will be a constant 160A draw? or, is each pole individually rated for 100A independent of the other? Thank you for your reply.

 

[suemarkp]

Each pole is rated for 100A. If you have 120V loads, both poles can supply 100A (or 80A continuous) to neutral. If you have a 240V load, you can only have 100A because what comes out one pole returns via the other (neutral load is 0 amps for 240V loads).

With a mix of loads (120V, 240V), then answer is still the same -- 100A from or to each pole. You just have to keep track of whether the load is line to neutral or line to line.

 

[mivey]

Thank you. I did find in the code that 80% is the max draw for continuous loads. Two branch circuits with 80A max continuous load. One on pole A and one on pole B on a 100A 120/240V service. Is this overloading since there will be a constant 160A draw? or, is each pole individually rated for 100A independent of the other? Thank you for your reply.

100 amps each pole, independently. The currents do not sum in the way that you might be thinking. If they were on the same pole, then they would add.

Add: what Mark said

 

[Smart $]

Thank you. I did find in the code that 80% is the max draw for continuous loads. Two branch circuits with 80A max continuous load. One on pole A and one on pole B on a 100A 120/240V service. Is this overloading since there will be a constant 160A draw? or, is each pole individually rated for 100A independent of the other? Thank you for your reply.

It's not 160A draw. The 80A on one pole is the same 80A on the other pole. The neutral only carries the imbalance. If you had 80A of 120V loads connected L1-N and 60A of 120V loads connected L2-N, 60A of L1 current would be the same 60A on L2. The imbalance of 20A would be carried by the neutral. Where each Line is load to 80A of 120V loads, the Neutral current would be 0.

 

[infinity]

You could have 2-100 amp 120 volt loads or one 100 amp 240 volt load. This is before your 80% continuous factor.

 

[gar]

101106-1442 EDT

bendomaniac:

Consider this approach.

The power dissipated in a resistance is I^2*R. Therefore wire temperature is a function of the current thru the wire.

The power dissipated in a transformer or a wire determines the temperature rise. Wire insulation fails based its absolute temperature. Temperature rise is the temperature above the ambient temperature. So a 105 deg C rated wire can handle less current at a high ambient temperature compared to a low ambient.

Now consider the pole transformer. Its secondary may be considered as one continuous coil of wire with a tap at the center. Neglecting some other considerations a rating of 100 A thru the secondary applies to anywhere in the secondary. So if you use only one half of the secondary by applying a 100 A load on one phase, the maximum based on the wire, then your maximum load is 120*100 = 12,000 VA. However, the transformer is rated for 24,000 VA if you fully load both sides of the secondary, 240*100 = 24,000 VA. If you put a 24,000 VA load on one half of the transformer secondary, then that secondary half would burn up. But no problem with a 24,000 VA load across the 240 V output.

You can use any distribution of loads on this transformer so long as you do not load either side more than 12,000 VA.

In actuality there is a little slack in the transformer when using only half of the secondary. However, if the above discussion is applied to the service lines, then it is a precise description.

Insulation does not just abruptly fail at its temperature rating. So operating at 104 deg C does not mean life time is infinite, nor does operation at 106 deg C mean immediate failure. Life of insulation is a function of its prior history, and its current stress level.
.

 

[LarryFine]

Is it feasible to max out each one the poles at a constant 80A load?, or is the 100A meant to be split between each of the poles (50A each)?

To answer in my usual, simple, plain-English way:

The answer is the former. If it was the latter, there would be no advantage to using the higher voltage. Ultimately, we care about how much power we can receive.

For a given current, you can get twice as much power at 240v as you can at 120v. Or, at 240v, we only need half as much current to deliver the same amount of power.

If you need to supply 200a worth of 120v loads, you could either supply them with 200a at 120v, or with 100a at 120/240v. Plus, you can serve 240v loads.

Those 240v loads could be built as 120v loads, but the supply circuits would have to have twice the ampacity, and the equipment itself would require beefier components.

It's basically a toss-up between safety and economics. Insulation is cheaper than conductor, so higher voltage can be more economical, but it's also more dangerous.

 

[readydave8]

You could have 2-100 amp 120 volt loads or one 100 amp 240 volt load. This is before your 80% continuous factor.

And if I was upstream with my amprobe it would read the same either way:

balanced load--no current on neutral--100 amp on each of the hots.

 

[Venamil]

To answer in my usual, simple, plain-English way:

The answer is the former. If it was the latter, there would be no advantage to using the higher voltage. Ultimately, we care about how much power we can receive.

For a given current, you can get twice as much power at 240v as you can at 120v. Or, at 240v, we only need half as much current to deliver the same amount of power.

If you need to supply 200a worth of 120v loads, you could either supply them with 200a at 120v, or with 100a at 120/240v. Plus, you can serve 240v loads.

Those 240v loads could be built as 120v loads, but the supply circuits would have to have twice the ampacity, and the equipment itself would require beefier components.

It's basically a toss-up between safety and economics. Insulation is cheaper than conductor, so higher voltage can be more economical, but it's also more dangerous.

Maybe Safety, but economics has nothing to do with it.
A 60 watt bulb will use 60 watts of power, does'nt matter if you use 120, 208,240,480v

 

[PetrosA]

Maybe Safety, but economics has nothing to do with it.
A 60 watt bulb will use 60 watts of power, does'nt matter if you use 120, 208,240,480v

Economics is realized in the wiring and equipment. You would need about half the copper and much smaller switchgear if you wired a house for 230/380V.

The safety part only comes in to play when there is a high probability of shock hazard. In the UK, many construction sites have temp services with 120/240V service to reduce the shock hazard. 100V less per leg is that much safer...

 

[infinity]

Maybe Safety, but economics has nothing to do with it.
A 60 watt bulb will use 60 watts of power, does'nt matter if you use 120, 208,240,480v

Economics is realized in the wiring and equipment. You would need about half the copper and much smaller switchgear if you wired a house for 230/380V.

I agree, higher voltage equals smaller conductors, less circuits, smaller panels etc.

 

[LarryFine]

Maybe Safety, but economics has nothing to do with it.
A 60 watt bulb will use 60 watts of power, does'nt matter if you use 120, 208,240,480v

I didn't mean economics of power use, but of installation.


Added: Or, what the other guys said.

 

[Rick Mack]

It's not 160A draw. The 80A on one pole is the same 80A on the other pole. The neutral only carries the imbalance. If you had 80A of 120V loads connected L1-N and 60A of 120V loads connected L2-N, 60A of L1 current would be the same 60A on L2. The imbalance of 20A would be carried by the neutral. Where each Line is load to 80A of 120V loads, the Neutral current would be 0.

Good explanation!
I agree.