I have a main breaker in the house, 60 amp. we ran an underground cable to the shop, that also has a 60 amp main breaker. When the ambient temperature in the shop is high (80 +) if my cnc plasma is running, and the air compressor starts, some time it will trip the shop breaker. My question is, could I install larger breaker in the shop to stop this from happening. My concern is having a larger breaker in the shop that feeds from the 60 amp in the house. Would it become a problem.

No issues with upsizing the subpanel main breaker. It's basically acting like a disconnect and the upstream breaker is still providing protection.

Sounds like this would be a 60A main in a panel, and then a bigger-than-60A breaker feeding the shop. If so, the limiting factor is the main 60.

BenTheGoodAg said:

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No issues with upsizing the subpanel main breaker. It's basically acting like a disconnect and the upstream breaker is still providing protection.

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The question is if it's going to trip the main breaker in the house. If so then it's easier to leave it as is because it a lot longer to walk to the house than the sub panel in the shop.

UnderoosAg said:

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Sounds like this would be a 60A main in a panel, and then a bigger-than-60A breaker feeding the shop. If so, the limiting factor is the main 60.

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Since it only happens when it's hot in the shop. I'm thinking that it's possible that the 60 amp in the main panel wouldn't get as hot and would hold up better. Also, being the cable to the sub panel is pretty long, the sub may have some voltage drop, that the main does not

What amps is the CNC plasma pulling by itself?

Rattler12 said:

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What amps is the CNC plasma pulling by itself?

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it varies, but full power 45

BenTheGoodAg said:

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Gotcha. Theoretically, the two breakers will perform the same. But it's possible that they:

• A - have slightly different trip curves (Typical for mains and feeder breakers)
• B - the mechanisms in the shop breaker are a little weaker and it's just a bad breaker.
• C - the warmer ambient temperature causes the element in the shop breaker to trip faster in the summer.

It may be worth trying it in the summer just to troubleshoot it. But then you can return the breaker if it doesn't kick it. Voltage drop isn't a factor in this case. It may raise the system current, but both breakers see the same current draw. Would be interesting to see what your nameplate loads are.

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With the CNC machine pulling 45 amps, maybe the A/C compressor kicking in momentarily kicks the total amps above 60 causing the trip? When both are running, the CNC machine at 45 amps and the A/C @ 10-15, the 60 holds up but as the A/C comp cycles the amps go to 65/70?

Put a soft start on the compressor

Rattler12 said:

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BenTheGoodAg said:

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Gotcha. Theoretically, the two breakers will perform the same. But it's possible that they:

• A - have slightly different trip curves (Typical for mains and feeder breakers)
• B - the mechanisms in the shop breaker are a little weaker and it's just a bad breaker.
• C - the warmer ambient temperature causes the element in the shop breaker to trip faster in the summer.

It may be worth trying it in the summer just to troubleshoot it. But then you can return the breaker if it doesn't kick it. Voltage drop isn't a factor in this case. It may raise the system current, but both breakers see the same current draw. Would be interesting to see what your nameplate loads are.

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With the CNC machine pulling 45 amps, maybe the A/C compressor kicking in momentarily kicks the total amps above 60 causing the trip? When both are running, the CNC machine at 45 amps and the A/C @ 10-15, the 60 holds up but as the A/C comp cycles the amps go to 65/70?



that's what I'm thinking, thus maybe going to a 70 amp will solve the problem

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Rattler12 said:

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With the CNC machine pulling 45 amps, maybe the A/C compressor kicking in momentarily kicks the total amps above 60 causing the trip? When both are running, the CNC machine at 45 amps and the A/C @ 10-15, the 60 holds up but as the A/C comp cycles the amps go to 65/70?

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It might even be worse than that with other loads in the shop (ie lights). Generally speaking, breakers have some ability to handle the inrush of the both the CNC (current spike when initially generating arc), and the compressor (high current when starting motor), so just because temporary load hits 70A doesn't always mean a breaker will trip.

But agree with your point, seems like OP is at the limit of what the 60A main can handle. I doubt the 45A on the CNC is the 'continuous load' that you would size the system for, but it still seems like a big load with little margin for a 60A main breaker. The manual for the CNC should give some guidance on what the supply needs to be. The part that is interesting to me is the winter non-trip vs summer trip.

the pit man said:

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that's what I'm thinking, thus maybe going to a 70 amp will solve the problem

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You could go to a 70A breaker if the feeder cable is #4 Copper and the panel is rated for larger than 60A (next size up is typically 100A). I figure you'll have a hard time finding a 70A main for the shop panel, but easier for the feeder breaker back in the house. So you could end up with 70A in the house and 100A in the shop and you'd be good.

BenTheGoodAg said:

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Rattler12 said:

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With the CNC machine pulling 45 amps, maybe the A/C compressor kicking in momentarily kicks the total amps above 60 causing the trip? When both are running, the CNC machine at 45 amps and the A/C @ 10-15, the 60 holds up but as the A/C comp cycles the amps go to 65/70?

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It might even be worse than that with other loads in the shop (ie lights). Generally speaking, breakers have some ability to handle the inrush of the both the CNC (current spike when initially generating arc), and the compressor (high current when starting motor), so just because temporary load hits 70A doesn't always mean a breaker will trip.

But agree with your point, seems like OP is at the limit of what the 60A main can handle. I doubt the 45A on the CNC is the 'continuous load' that you would size the system for, but it still seems like a big load with little margin for a 60A main breaker. The manual for the CNC should give some guidance on what the supply needs to be. The part that is interesting to me is the winter non-trip vs summer trip.

the pit man said:

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that's what I'm thinking, thus maybe going to a 70 amp will solve the problem

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You could go to a 70A breaker if the feeder cable is #4 Copper and the panel is rated for larger than 60A (next size up is typically 100A). I figure you'll have a hard time finding a 70A main for the shop panel, but easier for the feeder breaker back in the house. So you could end up with 70A in the house and 100A in the shop and you'd be good.



I appreciate all the information. I'll check the shop panel to see if it will accommodate the 70 amp. If not I'll go the other way.

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the pit man said:

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BenTheGoodAg said:

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Rattler12 said:

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With the CNC machine pulling 45 amps, maybe the A/C compressor kicking in momentarily kicks the total amps above 60 causing the trip? When both are running, the CNC machine at 45 amps and the A/C @ 10-15, the 60 holds up but as the A/C comp cycles the amps go to 65/70?

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It might even be worse than that with other loads in the shop (ie lights). Generally speaking, breakers have some ability to handle the inrush of the both the CNC (current spike when initially generating arc), and the compressor (high current when starting motor), so just because temporary load hits 70A doesn't always mean a breaker will trip.

But agree with your point, seems like OP is at the limit of what the 60A main can handle. I doubt the 45A on the CNC is the 'continuous load' that you would size the system for, but it still seems like a big load with little margin for a 60A main breaker. The manual for the CNC should give some guidance on what the supply needs to be. The part that is interesting to me is the winter non-trip vs summer trip.

the pit man said:

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that's what I'm thinking, thus maybe going to a 70 amp will solve the problem

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You could go to a 70A breaker if the feeder cable is #4 Copper and the panel is rated for larger than 60A (next size up is typically 100A). I figure you'll have a hard time finding a 70A main for the shop panel, but easier for the feeder breaker back in the house. So you could end up with 70A in the house and 100A in the shop and you'd be good.



I appreciate all the information. I'll check the shop panel to see if it will accommodate the 70 amp. If not I'll go the other way.

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Would this be safe or even solve OP's problem? A 100 amp breaker in the shop trying to pull 70 or larger amp load from the shop equipment is just going to result in the 60 amp house breaker tripping. He'd need 100 amp breaker in each box and is the current wiring from the house 60 amp rated at handling 100 amps ?????

I prevented this kind of delima by having our electric coop run a separate pole, line and meter to my woodshop and metal shop and put in a 200 amp box to feed everything else and didn't mess with the house setup. Of course if OP is in the city that might not be possible and if I were doing it today I don't know if I'd go that route again because PEC back then didn't charge for running a pole to the shops. Now there's a charge of $4k or so.

Rattler12 said:

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Would this be safe or even solve OP's problem? A 100 amp breaker in the shop trying to pull 70 or larger amp load from the shop equipment is just going to result in the 60 amp house breaker tripping. He'd need 100 amp breaker in each box and is the current wiring from the house 60 amp rated at handling 100 amps ?????

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I could have written more clearly, but I don't think you and I are talking about the same breaker.

Current set-up: A 60A Feeder breaker in house main panel supplies power to a 60A Main breaker in a subpanel in shop. The subpanel in the shop has other feeder breakers, but I'm not considering those.

Proposed set-up: A 70A Feeder breaker in the house main panel supplies power to a 100A Main breaker in the subpanel in the shop. Subpanel itself needs to be rated for >60A and cable between the house and subpanel needs to be #4 AWG or greater

If the panel rating and cable size allow for it, then you can upgrade both breakers to allow for 10A more capacity, which definitely could solve OP' s problem. Only one of the two breakers needs to be sized to protect the cable between the house and shop. It depends on the brand of the subpanel, but it's unlikely that the subpanel main can be upgraded to 70A (I'm guessing 100A is more readily available). But for the feeder breaker in the main panel in the house, a 70A breaker is likely to be more readily available. If the subpanel itself isn't rated for >60A, or the cable is smaller than #4, then this is moot and other options will need to be considered

BenTheGoodAg said:

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Rattler12 said:

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Would this be safe or even solve OP's problem? A 100 amp breaker in the shop trying to pull 70 or larger amp load from the shop equipment is just going to result in the 60 amp house breaker tripping. He'd need 100 amp breaker in each box and is the current wiring from the house 60 amp rated at handling 100 amps ?????

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I could have written more clearly, but I don't think you and I are talking about the same breaker.

Current set-up: A 60A Feeder breaker in house main panel supplies power to a 60A Main breaker in a subpanel in shop. The subpanel in the shop has other feeder breakers, but I'm not considering those.

Proposed set-up: A 70A Feeder breaker in the house main panel supplies power to a 100A Main breaker in the subpanel in the shop. Subpanel itself needs to be rated for >60A and cable between the house and subpanel needs to be #4 AWG or greater

If the panel rating and cable size allow for it, then you can upgrade both breakers to allow for 10A more capacity, which definitely could solve OP' s problem. Only one of the two breakers needs to be sized to protect the cable between the house and shop. It depends on the brand of the subpanel, but it's unlikely that the subpanel main can be upgraded to 70A (I'm guessing 100A is more readily available). But for the feeder breaker in the main panel in the house, a 70A breaker is likely to be more readily available. If the subpanel itself isn't rated for >60A, or the cable is smaller than #4, then this is moot and other options will need to be considered

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OK. My mistake. I thought you were talking about leaving the 60 amp breaker in the house panel.