Home wiring question

[Mears]

I have a subpanel directly off my main breaker box outside that has a single double-pole 50 amp breaker. Currently, the wires from the breaker are sitting idle under my deck. I believe a previous owner had a hot tub out there. I just purchased an old school air compressor that requires 220V, so I rerouted the wire into my garage. Instead of hard wiring the air compressor, I?d like to add a 220V receptacle since I?m planning on purchasing a table saw down the road. My questions are as follows:
1. I don?t know how much current my air compressor requires since the motor isn?t marked and I don?t have a manual (it is over 30 years old). However, I am willing to bet that it is less than 50a at 220V. Is it really that important to exactly match the current draw of the motor? I would assume that ensuring the breaker will trip before exceeding the wire?s current limits will prevent an electrical fire, and I would just have to live with the risk of burning up the motor.
2. Is there any reason I should add a breaker box (other than to correctly match the amperage requirements of the compressor) in my garage? The current run to my desired receptacle location is about 10?. Previously, the wire was stretched 20?+ to the deck.

 

[hellokeith]

The instantaneous load of the air compressor is probably higher than the hot tub motor startup, but the run load of the compressor likely less than the run load of the hot tub heater, so probably balances out ok. It'd be pretty tough to trip a 50a breaker. My only concern of what you describe is the gauge and condition of the wire you rerouted.

 

[skyking]

1.) if the wire is properly sized to the outlet, it is not a concern to have the motor with a few too many amps available.
2.) If you want to run multiple 220V items at the same time, it might be a good idea to put a subpanel in the garage. Don't put it behind that existing subpanel, run it fresh from the main panel.
I wired our new garage that way, with a 100 amp subpanel and a handful of breakers out in the garage for easy access, all with a permit and properly inspected.

 

[alkemyst]

Hopefully the run is first in proper conduit.

Second you don't really match the breaker in a panel with what's running on it. You want it to be able to prevent whatever wiring you have going from it from causing a fire if overloaded.

You will need the proper gauge for the breaker you want (sum up all the amps of what you think you will run at any one time) and determine the wire size.

That's it.

There was someone in another thread that felt since they could screw in a 50amp fuse to their apartment panel to keep the old fuse (15-20amp?) from blowing they fixed their problem.

 

[Mears]

Well, the original run is 8awg romex. I think I am going to replace it with 6awg, since 8awg probably isn't protected on the 50amp breaker.

 

[Evadman]

A basic residential wiring gauge list is below. If that cable is 8 gauge, the breaker is too large, and needs to be reduced to a 40 amp breaker.

Gauge Amps
14 - 15
12 - 20
10 - 30
08 - 40
06 - 65


40 amp at 220v is insane for a compressor. 746 watts is one HP. At 40 amps and 220v, you are looking at a maximum of 440*220= 96800 watts or about 130 HP. Your compressor is not 130 HP.

even with startup and bad capacitors, you are fine. But please change the wire gauge to 6 or change the breaker to a 40.

 

[alkemyst]

Originally posted by: Evadman
40 amp at 220v is insane for a compressor. 746 watts is one HP. At 40 amps and 220v, you are looking at a maximum of 440*220= 96800 watts or about 130 HP. Your compressor is not 130 HP.

5HP compressors are usually rated at 20-30 amps for 220v

 

[Mears]

I decided to replace the breaker in the subpanel with a 30 amp breaker. When I throw the (original) breaker in the subpanel, I get a reading of 0VAC on the breaker's outputs. However, when I throw the breaker in the main panel that feeds it and read the subpanels input voltage, it drops from 240VAC to just over 1VAC. Is the breaker in my main panel defective? I was hoping to work on the subpanel without completely disconnecting power, but now I'm not sure I'll be able to.

 

[Evadman]

Originally posted by: alkemyst
5HP compressors are usually rated at 20-30 amps for 220v

What the hell are you talking about? A motor is not rated in amps. A motor is rated in HP or watts. 20 or 30 amps is a huge range at 240v; a manufacturer doesn't say 'this may draw 20 amps. Wait, it could draw 30 too'.

The only possible things I can think of are when you are reading the owners manual, it will say to use a 20 amp circuit or a 30 amp circuit. The other is that a motor will draw more amps at stall then running. But if either is what you mean, you have serious problems explaining what is going on in your head.

By the way, there are quite a few manufacturers that give a 5 HP rating to a regular 120v appliance, like a shop vac. the problem with this is that at 120v, the max you are going to get out of a regular outlet is 20 amps or slightly over 3 HP. It is flat out impossible to get more by the standard definition.

Originally posted by: Mears
I decided to replace the breaker in the subpanel with a 30 amp breaker. When I throw the (original) breaker in the subpanel, I get a reading of 0VAC on the breaker's outputs. However, when I throw the breaker in the main panel that feeds it and read the subpanels input voltage, it drops from 240VAC to just over 1VAC. Is the breaker in my main panel defective? I was hoping to work on the subpanel without completely disconnecting power, but now I'm not sure I'll be able to.

Sounds like internal corrosion if you are still getting a voltage difference.

 

[AlienCraft]

Originally posted by: alkemyst
Hopefully the run is first in proper conduit.

Second you don't really match the breaker in a panel with what's running on it. You want it to be able to prevent whatever wiring you have going from it from causing a fire if overloaded.

You will need the proper gauge for the breaker you want (sum up all the amps of what you think you will run at any one time) and determine the wire size.

That's it.

There was someone in another thread that felt since they could screw in a 50amp fuse to their apartment panel to keep the old fuse (15-20amp?) from blowing they fixed their problem.

breakers and fuses are there to protect the wire.There's only hope for people that screw larger fuses into old cartridge sockets.

 

[Elstupido]

Oh boy, the correct way to wire anything is to determine the load being served, and that is by taking the nameplate rating of that load. If say for example, the rating of the air compressor as shown on the motor is 23 amps, then a 30 amp circuit breaker should suffice. As a general rule, you can not exceed 80% of breaker rating. If it draws 25 amps then the minimum breaker rating will be 40 amps.

When you determine the draw and breaker rating, the next step is to size the wire for that rating. #10 copper is good for 30 amps, #8 wire is good for 40 amps, depending if it is romex, or if the conductors are in conduit, then the wire is good for up to 50 amps. This gets into the latest NEC code requirements, and if you are unsure have a qualified electrical contractor look at it for you

 

[alkemyst]

Originally posted by: Evadman

Originally posted by: alkemyst
5HP compressors are usually rated at 20-30 amps for 220v

What the hell are you talking about? A motor is not rated in amps. A motor is rated in HP or watts. 20 or 30 amps is a huge range at 240v; a manufacturer doesn't say 'this may draw 20 amps. Wait, it could draw 30 too'.

The only possible things I can think of are when you are reading the owners manual, it will say to use a 20 amp circuit or a 30 amp circuit. The other is that a motor will draw more amps at stall then running. But if either is what you mean, you have serious problems explaining what is going on in your head.

By the way, there are quite a few manufacturers that give a 5 HP rating to a regular 120v appliance, like a shop vac. the problem with this is that at 120v, the max you are going to get out of a regular outlet is 20 amps or slightly over 3 HP. It is flat out impossible to get more by the standard definition.

why do people have to go down these roads?

You can go ahead and search 5HP 220V air compressors from top tier manufacturers and see that they will draw between 20-30amps running. Startup can be about double for some.

This may help you: http://www.redhillsupply.com/b...illsupply_archive.html

This will correct the formula above you used in an absurd manner: http://www.elec-toolbox.com/Fo...as/Useful/formulas.htm

 

[*kjm]

Mears do you only have room for the one breaker in your sub? If not I would leave the 50 amp in as your main and install three 20 amp breakers off that for your circuits.... to me it sounds like you may want to get some help on this one.... by a book at Home Depot and post what you plan to do before you do it here for help.

From what I'm reading it sounds like you need a sub with multiple circuits for your woodshop.

 

[Evadman]

Reading comprehension FTL. Take a basic electrical class alkemyst, I didn't pull the HP/watt formula out of thin air

 

[BigJ]

Originally posted by: Evadman
40 amp at 220v is insane for a compressor. 746 watts is one HP. At 40 amps and 220v, you are looking at a maximum of 440*220= 96800 watts or about 130 HP. Your compressor is not 130 HP.
.

Question for ya.

Where did the 440 come from?

It's 40A x 220v = 8800w. Using your formula, the rating in HP on a compressor would then be 11.8HP.

For a 30A 220v compressor, you'd be looking at 6600w. To HP, that'd be 8.85HP. 20A 220v would be 5.9HP.

 

[Elstupido]

Always go by the actual rating on the nameplate of a compressor, manufacturers like to play games with service factors. Although the HP to watts is good in theory, in real life the only thing that can be fact is what the manufacturer states as full load amps.

If you don't have a nameplate, the only thing you can do is run the compressor and put an ammeter on it to determine your amperage under a full load. Then size your breaker and wire size accordingly taking into consideration the 80% derating factor. Most often than not an appliance, or whatever, won't come anywhere near the nameplate rating under normal use. Something to keep in mind.

 

[BigJ]

Originally posted by: Elstupido
Always go by the actual rating on the nameplate of a compressor, manufacturers like to play games with service factors. Although the HP to watts is good in theory, in real life the only thing that can be fact is what the manufacturer states as full load amps.

If you don't have a nameplate, the only thing you can do is run the compressor and put an ammeter on it to determine your amperage under a full load. Then size your breaker and wire size accordingly taking into consideration the 80% derating factor. Most often than not an appliance, or whatever, won't come anywhere near the nameplate rating under normal use. Something to keep in mind.

Why are you telling people to apply the continuous load rule to everything?

 

[alkemyst]

Originally posted by: Evadman
Reading comprehension FTL. Take a basic electrical class alkemyst, I didn't pull the HP/watt formula out of thin air

You misused it absurdly like I said...perfect example of someone that really doesn't know the formulas they are using but could look them up with google just fine.

You may want to recheck your work.

Also you may want to go google some compressor manuals online and read their own specifications needed for power.

At 40 amps and 220v, you are looking at about 130 HP. Your compressor is not 130 HP

think about what you have said there.

 

[Elstupido]

BigJ, can you tell me, according to the NEC, when not to apply this?

 

[BigJ]

Originally posted by: Elstupido
BigJ, can you tell me, according to the NEC, when not to apply this?

That's the thing. The NEC is a permissive code book. Unless it is explicitly outlined that you cannot do it, you're allowed to. So it's actually the opposite of what you're asking me to do. You would have to find where it explicitly says in the NEC that you must only load up your circuits to 80% of their rating.

You are absolutely allowed to load up to the maximum amount of Amps the wire/CB combination is rated for as long as it does not qualify as a continuous load (and some other unique circumstances).

 

[alkemyst]

Last thing I'd want to do is put a tool on a circuit maxed out at it's nominal load. Any overload that the tool could power through will have your breaker stopping current at the worst time.

You never want to have a large power tool be at risk to pop the breaker *edit* under normal use.

 

[Elstupido]

BigJ, get out your NEC and read Article 210-19 (a). Self explanatory. You do not load a circuit to it's maximum amperage rating.

 

[IHateMyJob2004]

Originally posted by: Mears
Well, the original run is 8awg romex. I think I am going to replace it with 6awg, since 8awg probably isn't protected on the 50amp breaker.

8 awg is OK. I personally would want 6 AWG for 50 or 60 amp service though, especially if the run from the main panel is over 50 feet.

 

[IHateMyJob2004]

Originally posted by: Evadman

Originally posted by: alkemyst
5HP compressors are usually rated at 20-30 amps for 220v

What the hell are you talking about? A motor is not rated in amps. A motor is rated in HP or watts. 20 or 30 amps is a huge range at 240v; a manufacturer doesn't say 'this may draw 20 amps. Wait, it could draw 30 too'.

The only possible things I can think of are when you are reading the owners manual, it will say to use a 20 amp circuit or a 30 amp circuit. The other is that a motor will draw more amps at stall then running. But if either is what you mean, you have serious problems explaining what is going on in your head.

By the way, there are quite a few manufacturers that give a 5 HP rating to a regular 120v appliance, like a shop vac. the problem with this is that at 120v, the max you are going to get out of a regular outlet is 20 amps or slightly over 3 HP. It is flat out impossible to get more by the standard definition.

Originally posted by: Mears
I decided to replace the breaker in the subpanel with a 30 amp breaker. When I throw the (original) breaker in the subpanel, I get a reading of 0VAC on the breaker's outputs. However, when I throw the breaker in the main panel that feeds it and read the subpanels input voltage, it drops from 240VAC to just over 1VAC. Is the breaker in my main panel defective? I was hoping to work on the subpanel without completely disconnecting power, but now I'm not sure I'll be able to.

Sounds like internal corrosion if you are still getting a voltage difference.

Motors have a major current draw at start up. I wouldn't doubt it if some motors had some sort of current related rating.

 

[Elstupido]

Nameplates on motors most certainly do have amp ratings. We could get into a long discussion about Service factor, and how that affects HP ratings, and thus, amp ratings.
The start up current for a motor is already built into the circuit breaker. There are also various circuit breaker ratings which can accommodate more than normal startup currents.