Intrepid
Member
- Location
- Newburgh, Indiana
Could use some help on this one. I was installing Type 2 Surge Protection Devices after a local fire department took a direct hit. In the course of doing that job, I came upon an ugly electrical situation.
An air compressor had been added by "someone" in recent years. The building is fed with 2- 150a panels, and in '86 an addition was made and fed by an 80 amp breaker by "someone", leaving panel # 1 with the following loads:
(1) clothes dryer
(1) hvac blower (240v 10 a)
(1) outdoor a/c unit (240v 40 a)
(1) 80a feed to a subpanel (new addition)
(2) 20a circuits for general lighting and receptacles for a main office area
and the compressor addition - (1) 60 amp breaker tapped off of the main lugs upstream of the service disconnect and feeding a small 60a 2-pole encased breaker mounted on the wall adjacent to the #1 panel. This circuit then fed, via a very expensive #2 awg, 3 conductor w/ gnd armored cable, a 60 amp 2-pole disconnect.
This disconnect was fused and fed the air compressor with #6 awg wire via liquid-tight metal flex conduit. "Someone" later came along and tapped off the upstream fuse side the following loads:
(1) 50 amp 240v welding outlet
(1) 6 awg 240v circuit (nm cable) that goes somewhere way up in a high attic
(1) flourescent light fixture over the compressor via a wall-mounted switch
The overload condition, instead of tripping a breaker, actually was melting the plastic casing that held the feeder breaker in place. I assume the breaker was faulty...
Now the question.
I have been asked to fix the problem and my best solution is to add a new service panel straight from the utility transformer, which requires an entire rework of the service (have to go to CT metering now since the addition will exceed the utility company's max allowed by in-line metering).
The upgrade will require installation of outdoor disconnects for all three panels (2 existing and the new one).
How do I size the new panel? I understand that the compressor load must be sized at 125% of motor loading, but what about the 50a welding receptacle? and the mystery circuit - which I know will have to be traced to find its loading.
Normally, service conductors are sized at 125% of the continuous load and 100% of the non-continuous load.
In this case, do I size the service conductors at 100% of the compressor motor rating (since it does not run continously), and calculate the 50a welding receptacle per 220.44 at 100% (All others), and then add the mystery circuit at 100%, assuming it is non-continous? This would give me 39amps (compressor) + 50 amps (weld recept) + 40 amps (assume for now for mystery circuit) = 129amps. Another 150a panel would suffice to replace the illegally-tapped 60a disconnect switch, if this calculation is correct.
If this is right, I would base my conductor size on the 129 amp calculation for the service conductors, right?
I could use some guidance here since the Fire Department situation does fit into the normal commercial calculation scheme.
An air compressor had been added by "someone" in recent years. The building is fed with 2- 150a panels, and in '86 an addition was made and fed by an 80 amp breaker by "someone", leaving panel # 1 with the following loads:
(1) clothes dryer
(1) hvac blower (240v 10 a)
(1) outdoor a/c unit (240v 40 a)
(1) 80a feed to a subpanel (new addition)
(2) 20a circuits for general lighting and receptacles for a main office area
and the compressor addition - (1) 60 amp breaker tapped off of the main lugs upstream of the service disconnect and feeding a small 60a 2-pole encased breaker mounted on the wall adjacent to the #1 panel. This circuit then fed, via a very expensive #2 awg, 3 conductor w/ gnd armored cable, a 60 amp 2-pole disconnect.
This disconnect was fused and fed the air compressor with #6 awg wire via liquid-tight metal flex conduit. "Someone" later came along and tapped off the upstream fuse side the following loads:
(1) 50 amp 240v welding outlet
(1) 6 awg 240v circuit (nm cable) that goes somewhere way up in a high attic
(1) flourescent light fixture over the compressor via a wall-mounted switch
The overload condition, instead of tripping a breaker, actually was melting the plastic casing that held the feeder breaker in place. I assume the breaker was faulty...
Now the question.
I have been asked to fix the problem and my best solution is to add a new service panel straight from the utility transformer, which requires an entire rework of the service (have to go to CT metering now since the addition will exceed the utility company's max allowed by in-line metering).
The upgrade will require installation of outdoor disconnects for all three panels (2 existing and the new one).
How do I size the new panel? I understand that the compressor load must be sized at 125% of motor loading, but what about the 50a welding receptacle? and the mystery circuit - which I know will have to be traced to find its loading.
Normally, service conductors are sized at 125% of the continuous load and 100% of the non-continuous load.
In this case, do I size the service conductors at 100% of the compressor motor rating (since it does not run continously), and calculate the 50a welding receptacle per 220.44 at 100% (All others), and then add the mystery circuit at 100%, assuming it is non-continous? This would give me 39amps (compressor) + 50 amps (weld recept) + 40 amps (assume for now for mystery circuit) = 129amps. Another 150a panel would suffice to replace the illegally-tapped 60a disconnect switch, if this calculation is correct.
If this is right, I would base my conductor size on the 129 amp calculation for the service conductors, right?
I could use some guidance here since the Fire Department situation does fit into the normal commercial calculation scheme.