Sub-panel service cable

**Richard Roshak** · 09-10-2010, 04:03 PM

I was told years ago that sub-panel service wires must be of the four conductor type. I'm not sure if this is true for sub-panels with a mix of 120 and 240v circuits, panels with just 240v circuits or both. The sub-panel from todays inspection just had a three wire service cable (one entering box on right side).

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Roger Frazee · 09-10-2010, 04:38 PM

Is this a non-metallic panel?

**Richard Roshak** · 09-10-2010, 05:18 PM

A 40-amp breaker services this sub-panel and the panel is for the a/c air handler and compressor. The panel is plastic.

Roger Frazee · 09-10-2010, 06:12 PM

Originally Posted by Richard Roshak

A 40-amp breaker services this sub-panel and the panel is for the a/c air handler and compressor. The panel is plastic.

The 40 amp breaker protecting the feeder to that panel would be determined from the calculated load of the compressor and air handler. So no way to tell if that is correct or if the feeder conductors are correct without the information for the electrical supply for those a/c units. I'm not sure that would be in requirements of a home inspection .

There are two metallic cable clamps .. these should be changed to nonmetallic type. Other wise if one of those clamps is too tight damaging the insulation of a hot conductor ...the metal clamp could come to line voltage and pose a shock hazard. There is no fault path to the feeder grounding conductor thru plastic. A circuit breaker will not trip.

Joseph Stevens · 09-10-2010, 07:59 PM

Originally Posted by Richard Roshak

A 40-amp breaker services this sub-panel and the panel is for the a/c air handler and compressor. The panel is plastic.

Isn't anyone going to point out that the panel is fed by a 40 amp breaker and wire that appears to be no larger than 12 ga.

**Matt Fellman** · 09-10-2010, 08:23 PM

Originally Posted by Joseph Stevens

Isn't anyone going to point out that the panel is fed by a 40 amp breaker and wire that appears to be no larger than 12 ga.

Hard to say from the picture... it could be 10 gauge from days before romex was color coded. If I had to guess I'd agree that it looks like 12 gauge.

Roger Frazee · 09-10-2010, 10:19 PM

Originally Posted by Joseph Stevens

Isn't anyone going to point out that the panel is fed by a 40 amp breaker and wire that appears to be no larger than 12 ga.

It is not uncommon for protection to be sized much larger than the ampacity of the branch circuit conductor for air condtioning equipment. Same for feeders. It is possible for a 12 awg nm-b with 25 amp ampacity serving a compressor and fan motor having a combined rla of 22 amps to be protected by a 40 amp breaker.

In the pictured panel a feeder with #12 awg is going to require some very small equipment for the air handler and compressor as for running load amps. As an example the air handler could be 10 amps rla and the compressor 12 amps rla on the nameplates. The feeder conductors would then be (1.25 x 12) + 10 = 25 amps which could be served by a #12 awg nm-b cable as a feeder. If the maximum breaker allowed for the compressor was 30 amps (not likely) then the feeder protection would be 30 + 10 amps (rla) for air handler = 40 amps protection on 12 awg.

We need more information to be positive.

**Matt Fellman** · 09-10-2010, 10:43 PM

Originally Posted by Roger Frazee

It is not uncommon for protection to be sized much larger than the ampacity of the branch circuit conductor for air condtioning equipment. Same for feeders. It is possible for a 12 awg nm-b with 25 amp ampacity serving a compressor and fan motor having a combined rla of 21 amps to be protected by a 40 amp breaker.

In the pictured panel a feeder with #12 awg is going to require some very small equipment for the air handler and compressor as for running load amps. As an example the air handler could be 10 amps rla and the compressor 12 amps rla on the nameplates. The feeder conductors would then be (1.25 x 12) + 10 = 25 amps which could be served by a #12 awg nm-b cable as a feeder. If the maximum breaker allowed for the compressor was 30 amps then the feeder protection would be 30 + 10 amps (rla) for air handler = 40 amps.

This is interesting... I'm aware of the undersizing of wire being okay for an A/C compressor but didn't know (or never really considered) it being okay for an entire panel serving A/C equipment. I'm just shooting from the hip here but I'd think a panel's feeder would have to be sized to match the breaker at the service equipment that protects it. If for no other reason, because a panel could be used for something different than what it was initially installed for.

Roger Frazee · 09-11-2010, 07:28 AM

Originally Posted by Matt Fellman

This is interesting... I'm aware of the undersizing of wire being okay for an A/C compressor but didn't know (or never really considered) it being okay for an entire panel serving A/C equipment. I'm just shooting from the hip here but I'd think a panel's feeder would have to be sized to match the breaker at the service equipment that protects it. If for no other reason, because a panel could be used for something different than what it was initially installed for.

Matt

I was giving an example where this could be true to have #12 protected by a 40 amp breaker. If this feeder was labeled A/C equipment and that was considered its only use and probably would be then it would be ok ... but I do not think the loads are going to allow that #12 wire in Richards post. In a residential setting you would calculate the loads and come up with your minimum size feeder for the installed equipment. That panel being 4 spaces is only going to handle 2 double pole breakers so you would likely maintain the panel as a/c only.
On the other hand if I was running general purpose branch circuits out of that panel .. say one double pole for base board heat and 2 120 volt circuits for lights or receptacles I would likely supply a feeder equal to the rating of the panel. There would also be no reason to not supply the maximum size feeder if you wanted, regardless of what the panel served as long as it meets the load requirements.
If I were wiring that panel in a residential setting to serve A/C equipment I would give it the maximum feeder ampacity possible for the panel rating to allow for upgrades in the future assuming the panel rating allowed enough amperage to make that possible.
Frankly I'm not familiar with this particular non-metallic panel .. the ones I am are for spa panels which are generally rated 60 amps.

BTW that panel is in the USA so it is incorrectly mounted with the handles in the down position being 'on'....

240.81