Panel setup

[LarryFine]

When both legs show they are using 40 amps, the generator is not providing 80 amps...

I would provide a link to the recent discussion about this, but it's this thread.

 

[ggunn]

But as I stated, it isn't in the code book. When discussing and especially when instructing, I am a firm believer in referencing the code and the code language. Say what you mean, mean what you say.

As long as you get to a code compliant solution it makes no diff to me.

 

[winnie]

IMHO the truck analogy is spot on.

Two 1/2 ton trucks can carry together carry a ton of bricks, but can't carry a single 1 ton boulder without some sort of fancy load sharing system.

Similarly a US residential 200A 120/240V service can supply a nominal total of 48 kVA of load (which could be a large number of 20A 120V loads), but cannot supply 48 kVA as a single 400A 120V 120V load. (Again, without some sort of fancy load sharing system.)

When we say a circuit or a breaker is rated XX amps, that is always the value carried by each conductor or breaker leg. A two pole 20A 120/240V breaker will deliver 2x the kVA as a single pole 20A 120V breaker. But in both cases the trip rating is based on the current flow through any individual legs of the breaker.

IMHO in any situation where you want to know the 'total amps' supplied by a circuit, it is better to work in VA or kVA. I speak from experience working with 'high phase order' systems where we might have 5, 7, 15, 17, 18 or more individual phase legs feeding a motor. Far better to speak in terms of VA then try to explain how 18 phases with 40 'amps per phase' compares to 3 phases with 240 'amps per phase'.

-Jon

 

[ggunn]

IMHO the truck analogy is spot on.

Two 1/2 ton trucks can carry together carry a ton of bricks, but can't carry a single 1 ton boulder without some sort of fancy load sharing system.

Similarly a US residential 200A 120/240V service can supply a nominal total of 48 kVA of load (which could be a large number of 20A 120V loads), but cannot supply 48 kVA as a single 400A 120V 120V load. (Again, without some sort of fancy load sharing system.)

When we say a circuit or a breaker is rated XX amps, that is always the value carried by each conductor or breaker leg. A two pole 20A 120/240V breaker will deliver 2x the kVA as a single pole 20A 120V breaker. But in both cases the trip rating is based on the current flow through any individual legs of the breaker.

IMHO in any situation where you want to know the 'total amps' supplied by a circuit, it is better to work in VA or kVA. I speak from experience working with 'high phase order' systems where we might have 5, 7, 15, 17, 18 or more individual phase legs feeding a motor. Far better to speak in terms of VA then try to explain how 18 phases with 40 'amps per phase' compares to 3 phases with 240 'amps per phase'.

-Jon

I believe I said as much way back in post #25.

 

[winnie]

I believe I said as much way back in post #25.

Sometimes good ideas bear repeating

 

[Strathead]

As long as you get to a code compliant solution it makes no diff to me.

nor to me in casual conversation, but as a second year instructor, I want my students to refer to things as stated in the code book. After all they are going to need to take a Journeyman's test, and I am pretty sure you won't see 80% on the test. Kind of like saying you strap EMT at 10 feet.

 

[Strathead]

Never bought a transformer rated in watts

No, but you must size the transformer to the VA as modified by the expected power factor, which is watts.

 

[TwoBlocked]

As long as you get to a code compliant solution it makes no diff to me.

nor to me in casual conversation, but as a second year instructor, I want my students to refer to things as stated in the code book. After all they are going to need to take a Journeyman's test, and I am pretty sure you won't see 80% on the test. Kind of like saying you strap EMT at 10 feet.

Continuing this "casual conversation", Strathead. Are you teaching facts or concepts?

Makes me think of when Midshipmen would come on board the Merchant Marine ships and needed to fulfill their celestial navigation requirements. First question I would ask is whether the planets orbited the sun in a single plane like grooves on a record player, or if the orbits were at various angles like electrons are shown orbiting an atom's nucleus. A particular academy produced cadets that picked the latter, which is incorrect. But they could do the necessary math, until there was an error. Then they had no idea what the error might be because they did not understand the concepts. Like the sun's declination is never more than 23.5 degrees (approx).

 

[Strathead]

Continuing this "casual conversation", Strathead. Are you teaching facts or concepts?

Makes me think of when Midshipmen would come on board the Merchant Marine ships and needed to fulfill their celestial navigation requirements. First question I would ask is whether the planets orbited the sun in a single plane like grooves on a record player, or if the orbits were at various angles like electrons are shown orbiting an atom's nucleus. A particular academy produced cadets that picked the latter, which is incorrect. But they could do the necessary math, until there was an error. Then they had no idea what the error might be because they did not understand the concepts. Like the sun's declination is never more than 23.5 degrees (approx).

I don't see the analogy. I was recently on hold at our benefactors (Mike Holt's) business. There was a recording (I assume it was Mike) talking about the average "electricians" discussing the code and how they rarely have the code book in front of them when they should ALWAYS be referencing the code when discussing the code. If you paraphrase or modify the codes in your thoughts you will often miss the nuance of the situation you are using it for.

 

[TwoBlocked]

I don't see the analogy. I was recently on hold at our benefactors (Mike Holt's) business. There was a recording (I assume it was Mike) talking about the average "electricians" discussing the code and how they rarely have the code book in front of them when they should ALWAYS be referencing the code when discussing the code. If you paraphrase or modify the codes in your thoughts you will often miss the nuance of the situation you are using it for.

I'll put it this way. The code is made for the electricians, not the other way around.

 

[ggunn]

I don't see the analogy. I was recently on hold at our benefactors (Mike Holt's) business. There was a recording (I assume it was Mike) talking about the average "electricians" discussing the code and how they rarely have the code book in front of them when they should ALWAYS be referencing the code when discussing the code. If you paraphrase or modify the codes in your thoughts you will often miss the nuance of the situation you are using it for.

For continuous use in PV systems the code says that 1.25 times the maximum current must be less than the 75 degree ampacity of the conductors. That means that the maximum current must be less than 0.8 times the 75 degree ampacity. It's exactly the same thing, and for guys learning how this stuff works it's easier to understand since derating ampacity is how the conditions of use calculation is done. I haven't taught this stuff in a while, but when I did, I read them the code and then showed them the alternative way of getting to the same answer. YMMV.