Phase converter output conductors

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JWD

Member
Location
Idaho,usa
I have a couple static phase converters I am installing in a new residence to serve 3 phase heat pumps. Article 455 seems clear as to the size of my single phase supply conductors, however nothing is stated regarding 3phase output conductors. My local AHJ has been little to no help, being vague and comparing the phase converter to a transformer and separately derived system. So, should I be sizing these conductors at 125% of the equipment served or must they be the same as the single phase supply conductors? Also, am I required to provide a disconnecting means with over current protection on the immediate output side of the unit (as suggested by my AHJ)?

thanks much,
J
 
J

joeyde44

Member
Location
Chadds Ford, PA
Hey J, your static converter is not a transformer. It takes your single phase circuit and uses a starting capacitor to supply the 'B' phase of the 3 phase motor with a starting voltage that simulates a 3rd phase, and then after the motor comes up to speed, a running capacitor to keep things going. I would think that the normal 125% overcurrent on the supply would be Ok and as long as all of the conductors were sized for that you should not need a load side disconnect.
 
J

JWD

Member
Location
Idaho,usa
The phase converters are digital, spec'd by the heat pump manufacturer. My concern is only with the OUTPUT CONDUCTORS. The Phase converter input circuit has been established at 125A per article 455, the equipment served has a nameplate max fuse of 60 amps, do I need to size the output conductors larger than normally required due to the 125a over current device ahead of the phase converter? And why, per NEC...
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
JWD said:
The phase converters are digital, spec'd by the heat pump manufacturer. My concern is only with the OUTPUT CONDUCTORS. The Phase converter input circuit has been established at 125A per article 455, the equipment served has a nameplate max fuse of 60 amps, do I need to size the output conductors larger than normally required due to the 125a over current device ahead of the phase converter? And why, per NEC...
Click to expand...

Sounds like you will need a 60A fused disconnect per the nameplate and 60A worth of wire, since that what it will be protected at.
 
J

JWD

Member
Location
Idaho,usa
OK, I should elaborate a little. I will be locating the phase converters remote from the heat pumps, indoors nearly back to back. Would it be safe to consider this a feeder before it reaches the service disconnect and take 6awg tap conductors from the phase converter output as it will comply with a 25ft tap rule?
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
140318-2339 EDT

JWD:

You need to know more about your solid-state converter.

If this were a rotary converter that simply uses an induction motor as the converter, and there was no current limiting in series with the output wires, I would suggest that the output wires have to be the same rating as the input wires. The reason is that the two single phase input wires directly pass thru to two of the output wires. Thus, a short of one pass thru wire to the other will see the input side breaker or fuse and must be tolerant of that current limter's rating, 125 A in your case.

If you had a motor to alternator converter, then you would treat the output on its own. If your converter is basically a VFD or similar device with isolation, then it might be similar to the motor-alternator combination.

The easy solution is to use the same wire on the output side as the input.

.
 
H

hurk27

Senior Member
Location
Portage, Indiana NEC: 2008
I would go with a fused disconnect at the output side of the phase converter, only if it saved money on the cost of the run to the heat pumps, gar is correct on the two pass through conductors but nothing says you can't fuse at the rating of the equipment at a lower rating then just use a 60 amp non fused disco at the heat pump.

You also have to make sure the heat pump does not have any line to neutral loads on the B phase, as the phase converter will not reference the neutral on the derived phase, if there is a 120 volt to 24 volt transformer or any other type of 120 volt load it must be only on the A or C phase if they are the pas through phases.

As other said I very surprised that the manufacture doesn't have a problem with a static converter as these are not recommended for high torque loads such as a compressor like a air conditioner or heat pump, they do not have a very good stability on balanced current under heavy start up loads like this.

I have seen a digital phase converter that was nothing more then a VFD with a locked 60hz output, they did very well as far a keeping the voltage and current balanced , it was actually better then a roto-phase converter, but it wasn't cheap.
 
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StarCat

StarCat

Industrial Engineering Tech
Location
Moab, UT USA
Occupation
Imdustrial Engineering Technician - HVACR Electrical and Mechanical Systems
Fused Disconnects

Fused Disconnects

Fuses on any 3 Phase HVACR system are a bad idea. They always without fail lead to unnecessary single phase destruction that can in the case if using a circuit breaker instead, may be prevented.
Typical HP condensing units 5 tons and below will not have any type of control transformer on the outdoor unit. 24V control generally comes from the AHU.
When transformers are seen on condensing units they are 480V to 24V or 208-240V to 24V. I have never seen neutral conductors required on US made condensing equipment.
 
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