Breaker size on a 240V circuit with a 40A time-delay fuse?

[Rick Singer]

I am adding a 240V circuit to a 100A sub-panel in an attached garage. The circuit will feed a 5hp, 20A table saw. The saw manufacturer, Delta, specifies the use of 10 AWG wire and 40A dual-element time-delay fuses. The fuses are in a disconnect box between the sub-panel and saw. What is the proper size branch circuit breaker to use for this circuit?

An electrician suggested 30A, which corresponds to 2011 NEC Article 240.4(D)(7) and also corresponds to the 30A rating in conduit of 10 AWG wire. However, I don’t follow the logic of this choice. If the purpose of the time-delay fuse is to allow a maximum startup current of 40A, why not use a 40A breaker? And if that were done, the wiring would of course need to be increased to 8 AWG.

Any thoughts are greatly appreciated!!

[Sam Layton]

Hi Rick,

I will start out by saying, that I an not an electrician. That being said, I have several 5 hp motors in my shop. They are all 10 gage wire, and 30 amp breakers. 10 gage wire is rated at 30 amps. I like the idea of having a disconnect between the saw and panel. Works out good for changing blades. I would use one rated for 5 hp motor, and install a 30 amp fuse. I don't understand how Delta is saying to use a 10 gage wire and a 40 amp fuse. The 10 gage wire is good for 30 amps. In addition, why would you use a 40 amp fuse on a 20 amp motor.

Sam

[Rick Christopherson]

You don't need the secondary fuses that Delta suggested. Just run a standard 30 amp circuit with #10.

[ian maybury]

I'm likewise no expert, and what's more am in Europe. Over here though it's fairly typical to fit a type C or slow release breaker (don't know if this designation applies in the US) on a dedicated circuit running machine like a saw that can pull quite a significant over current on start up - this type of breaker can tolerate a current significantly over its nominal rating for a bit longer than a standard breaker.

On the basis that it's a better solution than a std breaker of the same size (which is likely to cause nusiance trips as a result of start ups etc), or an oversized one fitted to avoid the nusiance which may not give such good protection.

I'm not sure what the considerations are in the relative sizing of machine fuses relative to breakers, but the chances are that the saw manufacturer will specify a recommended breaker size and type in the manual. Felder/Hammer certainly do, and despite their recommendation seeming to sail close to the wind (a 16A type C breaker on a machine plated as drawing around 13A+ full load) it's so far working just fine.

ian

[Rick Singer]

Thanks for the comments so far, guys. I should have been more clear about the fuses: I installed them in order to be in compliance with their installation manual in the event of a warranty problem. Again, they are in a 60A disconnect box (rated for 5hp) between the sub-panel and the saw (the new Unisaw, 5hp variety).

And unfortunately, Ian, Delta did NOT specify a breaker size or type.

It just seems to me that a starting current of more than 30A would cause a 30A breaker to pop, although the 40A time-delay fuse element would be happy. So....... guess I'll put in a 30A breaker and "see what happens." Wondering what other Delta 5 hp owners have experienced. Probably I should have labelled this thread "New Unisaw 5 hp owners."

[Brian Brightwell]

Motor overload protection is provided by the thermal overloads in the motor starter. It should be attached to your machine.
The circuit breaker is to protect the wiring to the machine and should be sized accordingly. And the wiring has to have a rating greater than or equal to the current draw of the machine. That is the way industry does it. Use circuit breakers not fuses.
There are general rules of thumb that work well for wiring ampacities but there are charts in the National Electric Code written by the National Fire Prevention Agencie that takes wire type, insulation type, run in free air or conduit and a bunch of other stuff I can't think of into account to determine wire ampacity.

[Ron Natalie]

Actually, the NEC has a WHOLE section on motors which specify both what wire ampacities and breakers to use for the motor circuits.
There's not enough information here to tell you what the official NEC answer is going to be, but 30A breaker on #10 is certainly going to be the conservative answer.

[Curt Harms]

Here is my experience with breakers. I was running a circuit years ago. Checked Home Depot and Lowes for breakers, same amperage rating, Home Depot was cheaper. Bought it, installed it, turned to device on and 'pop' it tripped. Took that one back and bought the more expensive one from Lowes, installed it and it's still there. I expect the more expensive one had better tolerance for brief amperage exceedances so didn't trip on startup. It seems to me like a 14 amp wire would carry 40 amps without getting hot if that 40 amps last less than 1 second and quickly tails off to less than 15 amps. And yes my Rikon 10-325 would show around 35-40 amps momentarily on startup when wired for 120 volts but would then settle down to around 8 amps idling, 10-12 under load.

[Thomas Bank]

You have to remember that the 30amp breaker is protecting the 10ga wire running TO the disconnect. If you increase the size of the breaker without increasing the size of the 10ga wire you have the potential for the wire to fail - and possibly start a fire inside your wall - rather than popping the breaker. As such, it is against code to use more than a 30amp breaker on 10ga wiring. You'd need to upgrade to 8ga wire if you put a 40amp breaker in the panel.

I cannot speak to why Delta is suggesting the 40amp fuses. Downstream of the 30amp breaker - and protecting a 20amp motor - they are going to be the last thing to "pop." But since everything else is going to be limited by the 30amp breaker, there is no problem putting them in - just useless.

[Bud Millis]

All wall outlets (110 volt and higher) are 10 gauge. All lights are 12 gauge. For (220 volt toys) the 5 HP planer I use a 30 AMP, the 5 HP table saw (its been a long time, I'm going to say 30 amp) but I think its 20 amp, the 5 hp DC 30 amp. DJ-20 planer 20 amp. Its a 1 man shop and I have no "disconnects". No matter what power tool I use, portable or stationary, the plug is ALWAYS removed from the outlet before I work on the tool (ir changing blade or bit). That is a habit that I started when I got into woodworking 35+ yrs ago.

This is in New York State and things may be different where you live. My cousin is a retired life long electrician and have followed his advise and have exceeded code when it comes to building.

[Rick Singer]

Ron, agreed—and I have been burrowing deep into the NEC.
Thomas, we are definitely on the same wavelength.
Bud, I’m with you 100% on pulling plugs. Code allows a disconnect as an option, so with or without the fuses in it I figure it will be a bit more convenient than yanking the plug.

Sounds like we’re pretty much in agreement that 40 A "slow-blow" fuses on a circuit with a 30 A breaker is kinda wacky. I just returned from Lowes with a two-pole 30 A breaker, so will post my results in the next day or so after I pull the wire. Thanks again, everyone!

[Chris Friesen]

For a dedicated motor circuit, you can actually run significantly higher breaker sizes than would normally be allowed for a given wire size--the idea is that the motor has it's own overload protection so the breaker really only needs to deal with short-to-ground type events, which will typically be very high current.

I believe the rationale for the oversized breaker is to deal with high inrush current in some motors. That said, I suspect you're probably fine with a 30A breaker--try it and see, the worst thing that will happen is that you get nuisance pops.

[Rick Singer]

Finished pulling wire today and slipped a 30 A breaker into the sub panel. Pushed up the lever on the disconnect and hit the blue GO button on the saw. Wow --- Thought for a minute there it was gonna take off!! Big change from my 1.5 hp Jet. Don't know what it will take to give this thing the full-load test; probably a 3 1/2 inch solid oak table leg. But it looks like we're good for now.
Thanks to all who wrote !

[Nick Gless]

Rick,
I'm thinking about buying the same saw and have been researching how to wire it. Now that you've been running yours for a few months how's it working for you?
You said you put 40 amp breakers into a 60 amp box, did you put in two 40 amp breakers in the box?
And what are your thoughts on the saw after using it for awhile?


Thanks,
Nick

[Rollie Meyers]

Rules for motors in NEC article 430 are a bit different then for normal branch circuits if certain requirements are met, there can be wire & fuse/circuit breaker combinations that may not seem OK to unqualified people but is quite code compliant, when the motor has overload protection the fuse or circuit breaker is just for short circuit protection. If a piece of equipment is cord & plug connected though, then overcurrent protection is limited to the rating of the device. The worst that can happen to the OP is a tripped circuit breaker because of the 30A breaker,so there is no harm in the way it was done, if the saw was hard wired the 40A fuses w/ 10 AWG would have been fine since Delta does supply a mag starter w/ their Unisaws.