Rules of Thumb

Status
Not open for further replies.
mshields

mshields

Senior Member
Location
Boston, MA
I'm always looking for rules of thumb to allow one to conceptually design on the fly. I'll give you one of my of my favorites and look forward to your inputs with others.

At 480V Current is approximately 1.2 times the KVA.

At 208V Current is approximately 3 times the KVA

To get fault current if the impedance is assumed to be 5%, you merely multiply the FLA times 20. Since impedance is more often than not 5.75, the number you come up with using the rule of thumb is always a safe one.
 
mshields said:
I'm always looking for rules of thumb to allow one to conceptually design on the fly. I'll give you one of my of my favorites and look forward to your inputs with others.

At 480V Current is approximately 1.2 times the KVA.

At 208V Current is approximately 3 times the KVA

To get fault current if the impedance is assumed to be 5%, you merely multiply the FLA times 20. Since impedance is more often than not 5.75, the number you come up with using the rule of thumb is always a safe one.
Click to expand...

One rule of thumb (that I learned on this site but never use) is that I can put as many receptacles as I want on a general use, residential branch circuit......

Some that I do use is:
Don't let the smoke out.
Don't touch it to see if it's hot.
Black wire goes on bronze screw.
White wire goes on silver screw.
Ground wire goes on green screw.
It's hard to trench thru solid rock.
Ground rods don't do much, but they're required just the same.
(Just trying to lighten the load on this holiday weekend)
steve


steve
 
Let's see . . .

Hot on the left, cold on the right, and shi . . . no, wait; wrong trade.

Hot on the left, cold on the right, ground up. Is that it? Could be.

Okay, last time: Hot on the left, hot on the right, very hot in the middle.
 
A person 6' tall is approximately 6' wide with their arms outstretched. This is handy when double-checking receptacle spacing.

Ten holes per 15A circuit ain't bad.

When in doubt: hesitate, deliberate, and then hang it out there for someone to smack down. :)
 
georgestolz said:
A person 6' tall is approximately 6' wide with their arms outstretched. This is handy when double-checking receptacle spacing.

Ten holes per 15A circuit ain't bad.

When in doubt: hesitate, deliberate, and then hang it out there for someone to smack down. :)
Click to expand...

George...It's funny you should mention the 6' rule. That's usually how I measure wire if I'm going to cut a piece to length, and it's pretty accurate. I also step off distance, and I'm pretty accurate at that too. So by stepping off distance and using arm's length to measure, I can just leave the tape measure in the truck. Less weight on my tool belt.
steve
 
hillbilly said:
George... That's usually how I measure wire if I'm going to cut a piece to length, and it's pretty accurate. I also step off distance, and I'm pretty accurate at that too.
Click to expand...

Same here. :)

I have a pretty accurate 3' step and when measuring wire I go by 5's without really stretching my arms.
 
iwire said:
Same here. :)

I have a pretty accurate 3' step and when measuring wire I go by 5's without really stretching my arms.
Click to expand...

I'm about 5'7" tall (or short, I mean), so I used about 5' with my arms strectched. Its funny the difference between residential and commercial guys. As soon as George talked about arm length, I immediatley thought about wire, and never would have thought about receptacle spacing.
 
iwire said:
Same here. :)

I have a pretty accurate 3' step and when measuring wire I go by 5's without really stretching my arms.
Click to expand...


I also use the 5' arm method, although with the cost of copper these days I might start to worry about the couple extra feet I always seem to end up with.
 
I'm long legged, but I step off to figure distances. Counting ceiling or floor tiles help too!

As far as "rule-of-thumb" I figure one 2x4 troffer per 100 -120 ft/2

I also figure about 100 amps per 1000 ft/2 (I adjust depending on whether it's total electric or has some gas appliances).

I use the 10 recpts on a 15 amp and 13 recpts on a 20 amp circuit as a rule-of-thumb as well.
 
One rule of foot I learned in boy scouts was how to pace off a distance. Walk 100 feet normally and see what your pace is. Mine is a five foot stride.
Being 6ft 3in I can just reach a 8 ft ceiling.
And the navy taught me Twinkle twinkle little star E=IxR. I was not an electrician.
 
LarryFine said:
I was in the Air Force, guy; we learned W=E x I.
Click to expand...


I never had a problem remembering PIE, I mean c'mon, who doesn't like pie. Atlho I am considering getting an Ohms Law tattoo, in case I get forgetful in my old age.

Like others, I use the 6' wingspan rule for measuring wire and cable for short distances. I'm 6'2" and it comes out about spot on every time.

If I put the nose of my Kleins in the back of a standard box, the handles = 6" of free wire, or close enough as to make no mind.

If I didn't measure a wire run myself, I tend to add about 5' per 100', I'd rather pay for "just in case" than pull it twice.

I'd rather tote a 40lb service bag once, than go to the truck 5 times.

For every $10,000 bid on a job, there is a 10% chance that at least one thing will go catastrophically wrong. For example, if a job bid out at $100,000, it's a surety that some piece of equipment listed at 40hp on the schedule will show up on the truck with a 100hp motor. This is why jobs costing over 500,000 might as well include lawyers fees in the bid.

And the most important rule of thumb for the independent sparky, the check isn't good until it clears. :D
 
ELI the ICE man: Voltage leads current in an inductive circuit/Current leads Voltage in a capacitive circuit

ROY G BIV: Color spectrum for resistors (Red, Orange, Yellow, Green, Blue, Indigo, Violet)

Right hand rule: Curve right hand with thumb up. Curvature of fingers indicate magnetic field. Thumb indicates current direction

Righty tighty, lefty loosey.
 
Last edited:
rr said:
ROY G BIV: Color spectrum for resistors (Red, Orange, Yellow, Green, Blue, Indigo, Violet)
Click to expand...
That is for rainbows, not resistors.

The only hint for remembering the resistor color code I know is probably not appropriate for the forum. :)
 
Resistor color code:

Bad Beer Rots Our Young Guts, But Vodka Goes Well.

Black Brown Red Orange Yellow Green Blue Violet Grey White
 
rr said:
Right hand rule: Curve right hand with thumb up. Curvature of fingers indicate magnetic field. Thumb indicates current direction
Click to expand...

The way I remember it from tech school in 1972 (wow I'm getting old, geesh), it was the left hand rule with the thumb indicating current direction and the curled fingers indicating the direction of the magnetic field.

I think one of us was taught hole flow and the other electron flow. Can anyone clarify which is which?
 
Status
Not open for further replies.
Top