3 phase dimmers on 1 phase service??
- Thread starterlightingguy1
- Start date
In the US there are typically two types of multi-legged power. First is single phase, which is two legs of power(120v to ground), a neutral and a ground. Second is three phase Wye, which is three legs of power( 120v to ground), sometimes a neutral and always a ground. If you are using it for something like lighting then a neutral is needed. If you are running motors where it is always balanced then no neutral. Delta is the third type. It has three legs of power, two 120v to ground, one 208v to ground.
If you are faking 3 phase off of single phase you do not need to overrate the neutral. The neutral cannot exceed the capacity of any single leg from the single phase system. For example I have a single phase panel feeding a three phase dimmer rack. The panel has 2/0 HHN cables coming in and the main breaker has a rating of 200 amps. I have 3 100 amp breakers feeding off of the panel to the dimmer rack. Line 1 of the panel has two of the 3 legs of the dimmer rack connected. I turn on the first set of the dimmers that are all on leg 1 of the dimmer. Let's say I pull 100 amps. I show pulling 100 amps on the neutral and L1 of the panel. I now turn on the second leg of the dimmer rack also pulling 100 amps. So now line 1 and 2 on the dimmer rack is showing 100 amps each but now the neutral is showing 0 amps. Line 1 and 2 since they are each 180 degrees out of phase they are canceling themselves out on the neutral leg. Ok I now turn on all the dimmers on leg 3 of the dimmer rack. I am now pulling 200 amps on L1 of the panel and 100 amps on L2. I meter the neutral and it shows I am pulling 100 amps due to the 100 amps on L2 is canceling out 100 of the 200 amps on L1. Ok lastly I now turn off leg 2 of the dimmer rack. I now have 200 amps on L1 and 0 amps on L2. My neutral is now metering at 200 amps. I cannot exceed the rating of the neutral at 200 amps without popping the main breaker.
The numbers are for demonstration purposes only. Remember not to exceed 80 percent of the breaker or you may be sitting in the dark.
Tim, let me see if I understand your scenario:
Feed:..........................DimmerRack:
L1 (200A capacity)........ØA (100A load)
L2 (200A capacity)........ØB (100A load)
L1 (200A capacity)........ØC (100A load)
Neutral current cannot exceed 200A. For argument's sake, let's say that all wire is 2/0, which in this application is rated for 200A.
Your example appears to negate the reasoning for a dimmer rack's double neutral [and NEC 520.53 (O)(1) ]:
Feed:..........................DimmerRack:
L1 (200A capacity)........ØA (100A load)
L2 (200A capacity)........ØB (100A load)
L1 (200A capacity)........ØC (100A load)
Neutral current cannot exceed 200A. For argument's sake, let's say that all wire is 2/0, which in this application is rated for 200A.
Your example appears to negate the reasoning for a dimmer rack's double neutral [and NEC 520.53 (O)(1) ]:
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STEVETERRY
Well-Known Member
This example actually illustrates the problem well. When a three phase rack (without internal conversion) is connected to a large enough service to handle 2x the phase current of the rack (let's say a 200A single phase service to acommodate a three phase rack of 100A per phase), the neutral feeder cable and the rack neutral bus must both be large enough to handle a 200A imbalance.
ST
I need to make some assumptions since I have never seen how it works in reality...
You have your 3 loads of 100A each.
You connect one to "phase" A
You connect two to "phase" B.
This connection may be made at the local switchboard when tying in via lugs.
In this instance you are still running 5 cores of feeder to the dimmer rack.
Each active will never see more than 100A load.
The neutral may see a 200A load.
Substitute 200A loads for 100A loads and I can see very quickly why duplicating the neutral would be neccessary...
You have your 3 loads of 100A each.
You connect one to "phase" A
You connect two to "phase" B.
This connection may be made at the local switchboard when tying in via lugs.
In this instance you are still running 5 cores of feeder to the dimmer rack.
Each active will never see more than 100A load.
The neutral may see a 200A load.
Substitute 200A loads for 100A loads and I can see very quickly why duplicating the neutral would be neccessary...
JD
Well-Known Member
Not sure what you are saying here. You can never assume a cancel-out as this varies with dimmer settings. You have to look at worst case scenario. So, if each of your hots is gauged for 100 amps, (assuming a 100 amp per leg OCPD) your neutral would have to be gauged for 200 amps, therefore twice the size. If you gauge everything for 200 amp you do not have a problem, but you have effectively over-gauged all of the other conductors to achieve this.
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I have always made sure that I was sufficiently below the main limit to allow the neutral to be oversized. I was talking to an electrician yesterday and he claims that because you have to account for 200 amps twice that it is already taken care of. I couldn't of a counterargument. So if your main is single phase and 200 amps, the neutral is already gauged to carry 400 amps. I have never installed a panel from scratch, I have done panel swithouts, I don't know that to be true.
lightingguy1
Active Member
epimetheus
Well-Known Member
I don't think this is what the OP was asking. I know of no common reason that four poles worth of circuit breakers would be ganged together. It looks like those are 2 double pole breakers with a handle link between them.
You can get 4 pole breakers, but they are generally intended for high DC voltages (2 poles per leg) or 3 phase AC systems with a switched neutral.
I-T-E Electrical Products EQ9685
It appears even the experienced can't agree: 4 Breakers all tied together for 120/240 single phase? .
lightingguy1
Active Member
If there is space available install a breaker then run the cable from the breaker to your gear. Or go to the tub of tricks and pull out the TWEECOS and send everyone away.
Also I now see where y'all are coming from on essentially doubling the neutral. In writing up my example I left out a key component. I forgot to put in the feeders coming off of the panel to the dimmers. I stated that there was 2/0 feeding the panel. Ok for our feeders we can use 2 awg for our hots but then you will have to use 2/0 for the neutral. Due to there being 100 amps max on each hot but there is a potential for 200 amps on the neutral. I now feel like an idiot for not calculating in the feeder from the panel to the dimmers. Also you do not want to double your legs unless it is unavoidable. Just because two wires can carry 100 amps a piece does not mean they can carry 200 amps together. There is a formula to go by that I will need to look up. I have a book that mainly covers the calculations for doubling conductors. Also when using double conductors they MUST both be the exact same length. You need to lay both side by side to make sure of this. If one is longer by even a foot or two electricity will take the path of least resistance and begin to overload the shorter conductor. Thus is the reason you always meter both conductors to make sure the load is balanced.
David Ashton
Well-Known Member
"If one is longer by even a foot or two electricity will take the path of least resistance and begin to overload the shorter conductor. "
Not that critical, a slight imbalance will warm the cable with the higher current and increase its resistance which then reduces the current, a basic feedback loop.You're going to need a pretty good meter to measure the diiference of 1' in a decent length cable.
Not that critical, a slight imbalance will warm the cable with the higher current and increase its resistance which then reduces the current, a basic feedback loop.You're going to need a pretty good meter to measure the diiference of 1' in a decent length cable.
That is true though there are plenty of systems out there that use doubled hots due to the excessive current (more than what a single set of 400a cams can handle) I have a cd80 rack that has a 600 amp main so when connecting it for full capacity it is a beast when it comes to pulling cam. We use a matched set of 4/0 cams.
JD
Well-Known Member
Well, "path of least resistance" does not really work that way. Example: If you take three resistors and put them in parallel, and one is 4 ohms, the second and third are each 8 ohms, everything would travel through the 4 ohm resistor as it has the least resistance. Instead, about half the current would travel through the 4 ohm resistor, and 1/4 the power through each of the 8 ohm resistors, giving the circuit a value of 2 ohms. (1 / R = 1 / R1 + 1 / R2 + 1 / R3) If you had two feeders that were the same gauge and were in parallel, one being 100 feet and the other being 101 feet, there would be about a 1% variance in the current between them. Still good practice to keep them as close as possible.
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