Home wiring question

[futuristicmonkey]

Originally posted by: 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.

Many people here do not (nor need to, on a practical basis) understand the concept of real, reactive, and apparent power. If your typical single phase, 115VAC, 2 HP motor is operating at full load, it _will_ be drawing more than (2 * 746)/115 ~= 13 Amperes. This is because your typical household induction motor is not a pure resistor. It draws reactive current because the current is out of phase with the voltage.

These subjects are why people go to university for at least 4 years. Do NOT think that just because you can fiddle around with Ohm's laws that you know all that there is to know about electrical things. You will get yourself or others hurt.

Evadman: Motor nameplates (from any respectable manufacturer) most certainly will tell you what the full load current is. You cannot determine this from the voltage and power, unless you know the power factor and how to factor that into your math. The reason why you see 5HP vacuums and the like is because at start-up the motor works at about 5HP just to get going, at the instant you turn it on. The 5HP is just a marketing number. It's a peak rating.

 

[Elstupido]

futuristicmonkey, exactly, peak amps is typically used on vaccums, and sound systems,. Good post, nameplate is the only way to determine breaker size.

 

[BigJ]

Originally posted by: 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.

No offense, but you really do not know what you're talking about.

Do you or do you not know what a continuous load is?

210.19(A)(1) Specifically applies to having continuous loads (whether they be only or partial) on the line for the 80% rule. You are allowed to have a circuit made up of only noncontinuous loads up to the ampacity of the wire/CB for circuits under 600v. Example: I have a 20A circuit. If I have a noncontinuous load of 20A and a continuous load of 0A, that means I need a wire/OCPD rated for 100% of 20A and 125% of 0A. Which still leaves me with 20A.

If anything, you should've quoted 210.21, which at least gets into the 80% rule for receptacles. It does not restrict you from having a single cord-and-plug device using 80% of the ampacity and 5 non-cord-and-plug devices combining for the other 20%.

PS. Take a look at 210.23. Right there it's clear as day. Basically, you fell into the trap by trying to quote code showing it's against the NEC.

We also have 210.23(A)-(D), which simply means that on a multi-receptacle circuit, you are not allowed to have any one cord-and-plug device use more than 80% of the ampacity of the circuit. You are also allowed, on a single receptacle circuit, to have that single device use the full ampacity of the circuit, as the receptacle itself must be rated to the ampacity of the circuit (think 20A line with a single 20A receptacle vs. 20A line with multiple 15A receptacles) via 210.21 and 210.23.

 

[futuristicmonkey]

Originally posted by: BigJ

Originally posted by: 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.

No offense, but you really do not know what you're talking about.

Do you or do you not know what a continuous load is?

210.19(A)(1) Specifically applies to having continuous loads (whether they be only or partial) on the line for the 80% rule. You are allowed to have a circuit made up of only noncontinuous loads up to the ampacity of the wire/CB for circuits under 600v. Example: I have a 20A circuit. If I have a noncontinuous load of 20A and a continuous load of 0A, that means I need a wire/OCPD rated for 100% of 20A and 125% of 0A. Which still leaves me with 20A.

If anything, you should've quoted 210.21, which at least gets into the 80% rule for receptacles. It does not restrict you from having a single cord-and-plug device using 80% of the ampacity and 5 non-cord-and-plug devices combining for the other 20%.

PS. Take a look at 210.23. Right there it's clear as day. Basically, you fell into the trap by trying to quote code showing it's against the NEC.

We also have 210.23(A)-(D), which simply means that on a multi-receptacle circuit, you are not allowed to have any one cord-and-plug device use more than 80% of the ampacity of the circuit. You are also allowed, on a single receptacle circuit, to have that single device use the full ampacity of the circuit, as the receptacle itself must be rated to the ampacity of the circuit (think 20A line with a single 20A receptacle vs. 20A line with multiple 15A receptacles) via 210.21 and 210.23.

This doesn't look right. I'm a few thousand kilometers from my copy of the CEC (Canadian Electrical Code, very similar to the NEC) but that does not seem correct. You are allowed to load the cables in the wall up to 100% of their ampacity; the issue is the breaker. This is why circuits for compressors, baseboard heaters and other loads which are direct wired (where nothing else can be connected to the circuit) can have a 30A breaker protecting a 12-guage cable. If you look at any breaker's inverse-time curve you'll see that it will trip at 100% load after a (long) time.

 

[BigJ]

Originally posted by: futuristicmonkey

Originally posted by: BigJ

Originally posted by: 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.

No offense, but you really do not know what you're talking about.

Do you or do you not know what a continuous load is?

210.19(A)(1) Specifically applies to having continuous loads (whether they be only or partial) on the line for the 80% rule. You are allowed to have a circuit made up of only noncontinuous loads up to the ampacity of the wire/CB for circuits under 600v. Example: I have a 20A circuit. If I have a noncontinuous load of 20A and a continuous load of 0A, that means I need a wire/OCPD rated for 100% of 20A and 125% of 0A. Which still leaves me with 20A.

If anything, you should've quoted 210.21, which at least gets into the 80% rule for receptacles. It does not restrict you from having a single cord-and-plug device using 80% of the ampacity and 5 non-cord-and-plug devices combining for the other 20%.

PS. Take a look at 210.23. Right there it's clear as day. Basically, you fell into the trap by trying to quote code showing it's against the NEC.

We also have 210.23(A)-(D), which simply means that on a multi-receptacle circuit, you are not allowed to have any one cord-and-plug device use more than 80% of the ampacity of the circuit. You are also allowed, on a single receptacle circuit, to have that single device use the full ampacity of the circuit, as the receptacle itself must be rated to the ampacity of the circuit (think 20A line with a single 20A receptacle vs. 20A line with multiple 15A receptacles) via 210.21 and 210.23.

This doesn't look right. I'm a few thousand kilometers from my copy of the CEC (Canadian Electrical Code, very similar to the NEC) but that does not seem correct. You are allowed to load the cables in the wall up to 100% of their ampacity; the issue is the breaker. This is why circuits for compressors, baseboard heaters and other loads which are direct wired (where nothing else can be connected to the circuit) can have a 30A breaker protecting a 12-guage cable. If you look at any breaker's inverse-time curve you'll see that it will trip at 100% load after a (long) time.

Hence continuous loads requiring you to only load up to 80% of the OCPD rating with a standard circuit breaker. Non-continuous loads have no such rule. You're correct in that a standard CB will trip at 100% load over time. The NEC has deemed it safe to load 100% as long as the time the load is at 100% is less than 3 hours. Anything more and it is considered a continuous load and follows the 80% rule.

Here's a link to the National Electrical Code® Softbound 2005 Edition:
http://www.nfpa.org/freecodes/...d=7005SB&cookie_test=1

Click I agree, then hit the bullets once the popup window shows. Go to 210 Branch Circuits, more specifically 210.20(A)

Alternatively, you can go to EC&M's site here and read this article:
http://ecmweb.com/nec/electric_branch_circuits_part/
Go to overcurrent protection and conductor sizing.

Specifically:

Conductor sizing. You must size conductors no less than 125% of the continuous loads, plus 100% of the noncontinuous loads [210.19]. Base this on the terminal temperature rating ampacities as listed in Table 310.16 before any ampacity adjustment [110.14(C)].

In addition, conductors must have sufficient ampacity, after applying adjustment factors, to carry the load ? and they must be protected against overcurrent per their ampacity [210.19(A)(1), 210.20(A) and 240.4].

Overcurrent protection. Branch-circuit OCPDs must have an ampacity not less than 125% of the continuous loads, plus 100% of the noncontinuous loads [210.20(A)].

Permissible loads. An individual branch circuit can supply any load for which it's rated [210.23]. A multioutlet branch circuit can supply only lighting and/or equipment loads as summarized in Table 210.24 and as specified in 210.19, 210.20, and 210.21. If you're installing a multioutlet branch circuit, read those requirements carefully.

Cord-and-plug connected equipment not fastened in place, such as a drill press or table saw, for example, must not have an ampere rating more than 80% of the branch-circuit rating [210.23(A)(1)]. UL and other testing laboratories list portable equipment (such as hair dryers) up to 100% of the circuit rating. The NEC is an installation standard, not a product standard, so it cannot prohibit this practice. There really is no way to limit the load to 80% of the branch-circuit rating if testing laboratories permit equipment to be listed for 100% of the circuit rating.

Equipment fastened in place (not a luminaire) must not be rated more than 50% of the branch-circuit ampere rating if this circuit supplies luminaries, other receptacles, or both [210.23(A)(2).

To keep your mind clear when sizing conductors for branch circuits, remember that the OCPD defines the circuit. If a 20A circuit contains 8 AWG conductors because of voltage drop, it is still a 20A circuit ? the size of the OCPD determines the rating of the branch circuit. That's because the conductors must be able to handle the current supplied by the OCPD. Going forward, you should now be able to handle branch-circuit requirements.