Should RTU voltage drop be calculated from MCA or MOCP?

For a rooftop unit, start the voltage-drop check with the expected running load and use the nameplate MCA as a conservative design current. MOCP from NEC 440.22 is for short-circuit and ground-fault protection, not the normal load current.

What voltage drop target should I use for a commercial RTU feeder?

A common target is 3% for the feeder or branch circuit and 5% total feeder plus branch circuit, matching NEC 215.2(A)(1) Informational Note No. 2 and NEC 210.19(A)(1) Informational Note No. 4. Many HVAC designers hold 480 V RTU feeders near 2% when motor starting is sensitive.

How do I calculate voltage drop on a 480 V 3-phase RTU?

Use the 3-phase formula with 1.732 x current x distance x conductor impedance. For a simplified copper resistance check, 1 AWG copper at about 0.126 ohms per 1,000 ft over 230 ft one-way and 52 A is about 2.61 V, or 0.54% on 480 V.

Can a 60 A RTU breaker use conductors smaller than 60 A ampacity?

Sometimes, yes, if the unit nameplate MCA supports it and NEC 440 rules are followed. For example, an RTU with MCA 43 A and MOCP 60 A can often use conductors sized at or above 43 A ampacity while the 60 A MOCP protects compressor starting needs.

When should I upsize RTU feeder conductors for voltage drop?

Upsize when the run length, current, and starting behavior push delivered voltage outside the design target. A 208 V RTU with 48 A MCA and 180 ft one-way can exceed 3% on 6 AWG copper, while 4 AWG may bring the run closer to 2.5%.

Do IEC projects use the same RTU voltage drop limits as NEC projects?

Not exactly. IEC 60364-5-52 treats voltage drop as a design requirement tied to circuit purpose, installation method, and local project rules; 3% for lighting and 5% for other final circuits are common reference values, but local standards can be stricter.

Queda de tensão em alimentador de rooftop: MCA, MOCP, NEC 440 e trifásico

Em uma rooftop, o disjuntor não define sozinho o cabo. A MCA orienta a ampacidade do condutor, o MOCP define a proteção máxima e a queda de tensão mostra a tensão real no compressor.

Resumo

MCA: 43 A, 48 A, or 52 A is the normal conductor-sizing starting point.
MOCP: 60 A or 70 A can be correct even when conductor ampacity is lower.
NEC 215.2 and 210.19 design notes commonly use 3% and 5% voltage-drop targets.
IEC 60364-5-52 checks cable size, installation method, grouping, and voltage drop together.

Alimentador de RTU é o circuito que atende uma unidade HVAC compacta na cobertura. MCA é a ampacidade mínima do circuito; MOCP é a proteção máxima contra sobrecorrente. NEC and IEC references belong in the body of the calculation, not in the equipment schedule alone.

NEC 440 trata de ar-condicionado e refrigeração. As notas da NEC 215.2 e 210.19 usam 3%/5% como referência de projeto; em IEC, veja IEC 60364-5-52.

For rooftop units, separate MCA, MOCP, and voltage drop before choosing a conductor. A 60 A MOCP does not mean the equipment normally draws 60 A.
- Hommer Zhao, Technical Director

480 V example

Exemplo: RTU 480 V trifásica, MCA 52 A, 230 ft de ida, cobre 1 AWG a 0.126 ohm/kft: queda de cerca de 2.61 V, ou 0.54%.

Use the voltage drop calculator, the wire size calculator, and the NEC standards guide together so ampacity, overcurrent protection, and delivered voltage are all checked.

Comparação de alimentador longo para RTU 208 V

Conductor	ohm/kft	48 A, 180 ft	208 V %	Decision
8 AWG Cu	0.628	9.40 V	4.5%	Too high for many RTU feeders
6 AWG Cu	0.395	5.91 V	2.8%	Verify source voltage and starting
4 AWG Cu	0.249	3.73 V	1.8%	Good long-run choice
3 AWG Cu	0.197	2.95 V	1.4%	Extra margin for weak service voltage
2 AWG Al	0.321	4.80 V	2.3%	Use only with listed AL terminals

Em 208 V, a queda percentual pesa mais. Acima de 150 ft, trocar 6 AWG por 4 AWG pode resolver a margem de partida.

If an RTU has low-voltage trips, measure voltage at the panel and at the disconnect under load before changing any breaker or fuse.
- Hommer Zhao, Technical Director

Perguntas frequentes

MCA or MOCP for voltage drop?

Use MCA or expected running load. MOCP is protection, not normal current.

What target is typical?

3% on the feeder or branch circuit and 5% total is the common NEC design target.

Why are 208 V units harder?

The same 5 V loss is 2.4% at 208 V but only 1.0% at 480 V.

Can conductors be upsized without upsizing the breaker?

Yes. Keep overcurrent protection within the RTU nameplate MOCP and NEC 440 rules.

What must be documented?

Voltage, phase, MCA, MOCP, route length, conductor, material, and calculated percent drop.

Antes de comprar cabos e seccionadora, confira MCA, distância, condutor e queda de tensão na calculadora.

Voltage-drop review

Queda de tensão em alimentador de rooftop: MCA, MOCP, NEC 440 e trifásico

Resumo

480 V example

Comparação de alimentador longo para RTU 208 V

Perguntas frequentes

MCA or MOCP for voltage drop?

What target is typical?

Why are 208 V units harder?

Can conductors be upsized without upsizing the breaker?

What must be documented?

Começar a Calcular

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